AN ABSTRACT OF THE DISSERTATION OF
Claudia S. Ingham for the degree of Doctor of Philosophy in Rangeland
Ecology and Management presented on December 17, 2008.
Title: Himalayan blackberry (Rubus armeniacus) and English ivy (Hedera
helix) Response to High Intensity-Short Duration Goat Browsing
Abstract approved: _______________________________________
Michael M. Borman
The goal of this dissertation is to examine the effectiveness of high
intensity-short duration goat browsing for the control of Himalayan blackberry
(Rubus armeniacus) and English ivy (Hedera helix), two widespread noxious
weeds in the Pacific Northwest.
The effects of goat browsing on Himalayan blackberry vigor, as quantified
by densities of different age class stems, are compared to mowing and goat
browsing followed by mowing over a period of three years. Although total
stem density and node density declined, primocane density increased after all
types of treatment. This indicates that the population was still vigorous.
Recruitment of new plants was reflected by an increase in seedling density.
Changes in percent cover of vegetation functional groups were also examined.
All three treatments produced a decline in Himalayan blackberry cover and an
increase in perennial grass cover, though differences among treatment types
were not significant. Perennial forb cover increased as a result of all treatment
types.
English ivy was successfully controlled by goat browsing as quantified by
percent cover. Two years of browsing reduced percent cover more than one
year of browsing. Both levels of browsing produced results that were
significantly different when compared to the untreated control indicating that
repeated high intensity-short duration browsing is justified for control of this
invasive plant species.
Differences in response of these two invasive, creeping perennials are
attributed to differences in species growth rates, location of meristematic tissue
in stolons vs. rhizomes and shade conditions at the research sites.
©Copyright by Claudia S. Ingham
December 17, 2008
All Rights Reserved
Himalayan blackberry (Rubus armeniacus) and English ivy (Hedera helix)
Response to High Intensity-Short Duration Goat Browsing
by
Claudia S. Ingham
A DISSERTATION
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Presented December 17, 2008
Commencement June 2009
Doctor of Philosophy dissertation of Claudia S. Ingham presented on
December 17, 2008.
APPROVED:
_______________________________________________
Major Professor, representing Rangeland Ecology and Management
_______________________________________________
Head of the Department of Rangeland Ecology and Management
_______________________________________________
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of
Oregon State University libraries. My signature below authorizes release of
my thesis to any reader upon request.
_______________________________________________
Claudia S. Ingham, Author
ACKNOWLEDGEMENTS
I wish to thank the many people who supported my dream of earning a
PhD. Words of encouragement and many hours of shared labor inspired me to
begin the journey and to continue at every step along the way.
Initially, Dr. William C. Krueger listened to my ideas and facilitated
appointment of my major professor, Dr. Michael M. Borman. Dr. Borman
scouted the research sites at the Hawley Land & Cattle Co. and in Wilsonville
with me. Bill and Sharon Hoyt hosted me on their ranch in Divide, Oregon
and introduced me to their work for the City of Wilsonville. The Hoyts spent
many hours moving goats on and off the experimental plots at two research
sites as well as mowing at their ranch.
Dr. Jennifer Kling had significant influence on the design and approach to
statistical analysis. Dr. Steven Radosevich listened patiently to a litany of
questions about response variables and data collection and always showed
interest in the research. His kindness and enthusiasm will always be
remembered.
Len Rummel and Kim Townsend spent countless hours in the spring of
2006 helping me set-up plots in ‘nasty’ blackberry thickets and then helped me
count and measure stem diameters in the fall of that year. Len continued to
help me drive posts and set-up wire exclosures when I added a research site
that was two and a half hour drive from the original site.
Linda Rummel helped in the field in 2006 and kept many of us nourished
throughout the years. Lennie Rummel, Nicole Graham and Miriam Lynah also
assisted in plot set-up under very challenging weather conditions. Loren
Rummel worked through the entire 2008 field season and knows the whole
process like no one else.
Ruth Ingham, Kim Townsend and Lara Kossiakoff assisted with species
composition data collection. Julie Knurowski provided an invaluable review
of species identification and insight into invasive plants and their geographic
ranges. Bethany Vogeney counted and measured thousands of blackberry
canes with me in the summer of 2007 and was a terrific employee.
My husband, Conan Schleicher, supported me as I continued my
education. He managed our household while I spent many months away from
home to take classes in Corvallis and then to conduct my field work in Divide.
His was a continuous commitment over three years which cannot be
overstated.
CONTRIBUTIONS OF AUTHORS
Dr. Michael Borman discussed hypothesis formation, advised on research
design and data collection methods as well as reviewing the format, contents
and consistency of presentation of the entire thesis.
TABLE OF CONTENTS
1 INTRODUCTION ……………………………………….
1.1 The Challenge of Invasive Plants ………………...
1.2 Invasive Plants and Invasion Defined ……………
1.3 Invasiveness and the Potential for Invasion ..........
1.3.1 Plant characteristics ………………………..
1.3.2 Ecosystem characteristics ………………….
1.4 Two Invasive Creeping Perennials of the Pacific
Northwest……………………………................…
1.4.1 Physiology and ecology of Himalayan
blackberry (Rubus armeniacus) ……………
1.4.2 Physiology and ecology of English ivy
(Hedera helix) ..……………………..........
1.5 Domestic Livestock and Their Influence on Plant
Communities …………………………...
1.5.1 Grazing as disturbance …………………….
1.5.2 Targeted grazing and goat browsing for
invasive plant control …………………….
1.6 Developing a Model for Predicting Plant
Community Change ………………………...
1.7 Objectives ………………………………………..
2 HIMALAYAN BLACKBERRY (RUBUS ARMENIACUS)
RESPONSE TO HIGH INTENSITY-SHORT
DURATION GOAT BROWSING AND MOWING …
2.1 Abstract …………………………………………...
2.2 Introduction …………………………………........
2.3 Materials and Methods …………………………...
2.3.1 Site and plot description .…..........................
2.3.2 Treatments ……...……………………........
2.3.3 Application of treatments ………………….
2.3.4 Above ground biomass ..…………………..
2.3.5 Stem density .………………………………
2.3.6 Statistical model and analysis ……………...
2.4 Results and Discussion …………………………...
2.4.1 Above ground biomass …………………….
2.4.2 Stem density ……….……………………....
2.4.2.1 Total stem and floricane density. ……..
2.4.2.2 Seedling density ………………………
2.4.2.3 Primocane density .…………………...
2.4.2.4 Node density ………………………….
page
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TABLE OF CONTENTS (Continued)
page
2.4.3 Implications…...........………………………
2.5 Acknowledgements …………………………........
2.6 Literature Cited …………………………………...
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3 FUNCTIONAL GROUP AND SPECIES CHANGE IN
HIMALAYAN BLACKBERRY (RUBUS
ARMENIACUS)- AND EVERGREEN
BLACKBERRY (RUBUS LACINIATUS)- INVADED
PASTURE FOLLOWING GOAT BROWSING AND
MOWING ..…………………………………………....
3.1 Abstract …………………………………………..
3.2 Introduction ……………………………………...
3.3 Materials and Methods …………………………..
3.3.1 Site description and study design ………..
3.3.2 Treatments ..…..………………………….
3.3.3 Species composition and functional
groups ..…………………………………..
3.3.4 Statistical model and analysis …………...
3.4 Results …………………………………………...
3.5 Discussion ………………………………………..
3.6 Acknowledgements ……………………………...
3.7 References ……………………………………….
4 ENGLISH IVY (HEDERA HELIX) RESPONSE TO HIGH
INTENSITY-SHORT DURATION GOAT
BROWSING …………………………………………..
4.1 Abstract …………………………………………..
4.2 Introduction ……………………………………...
4.3 Materials and Methods …………………………..
4.3.1 Site description and study design ………….
4.3.2 Browsing treatment and levels …………….
4.3.3 Percent foliar cover ...……………………...
4.3.4 Statistical model and analysis ……………..
4.4 Results and Discussion ………………………….
4.5 Acknowledgements ……………………………...
4.6 Literature Cited ………………………………….
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TABLE OF CONTENTS (Continued)
page
5 SUMMARY OF RESULTS AND INSIGHTS FOR
FUTURE MANAGEMENT …………………………..
5.1 English ivy Control with Goat Browsing ….……..
5.2 Himalayan blackberry Control with Goat
Browsing …..…………………………………….
5.3 General Considerations for Invasive Plant
Management Using Goat Browsing ..…………….
94
BIBLIOGRAPHY ……………………………………………
102
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LIST OF FIGURES
Figure
Page
2.1
Mean Above Ground Biomass in 2008..………
33
2.2
Population Demographic by Treatment ....……
46
3.1
Mean % Cover Rubus for All Treatments ……
66
3.2
Change in Perennial Grass % Cover by
Treatment …………………………………......
68
Change in Perennial Forb % Cover by
Treatment ……………………………………..
69
Mean English ivy % Cover Before and After
Browsing Treatments …………………………
87
3.3
4.1
LIST OF TABLES
Table
Page
2.1
Treatment Descriptions ……………….
29
2.2
Mean Stem and Node Densities (per
m2) by Treatment ………………...…...
35
Mean Stem and Node Densities (per
m2) in Summer 2008 ..………………...
37
Seedling Density Pairwise
Comparisons for All Treatments .…...
39
Primocane Density Pairwise
Comparisons for All Treatments ……...
42
Node Density Pairwise Comparisons
for All Treatments …………………….
44
3.1
Species Inventory for Fall 2006 ...….…
63
3.2
Species by Functional Group….………
64
4.1
Browsing Levels Comparison and
Confidence Levels ................................
87
2.3
2.4
2.5
2.6
Himalayan blackberry (Rubus armeniacus) and English ivy (Hedera helix)
Response to High Intensity-Short Duration Goat Browsing
1. INTRODUCTION
1.1 The Challenge of Invasive Plants
The ecological and economic importance of invasive plants has captured
the attention of scientists and land managers for generations. Charles Darwin
(1937) commented on the hundreds of square miles made impenetrable by
stands of cardoon (Cynara cardunculus) in Chile and the Banda Oriental of
contemporary Uruguay and Brazil as he traveled the world aboard the Beagle.
According to Randall (1996), it was not until the publication of The Ecology of
Invasions by Animals and Plants (Elton, 1958), however, that concerns about
the effects of thousands of organisms from different parts of the world were
brought to the forefront of ecology. Many believe that invasion by non-native
organisms is responsible for greater losses of diversity worldwide than other
causes except for habitat loss and land use by humans (Pimm and Gilpin,
1989). The estimated economic cost of invasive organisms to American
agriculture, ranching, and fisheries is in the billions of dollars annually (Hill,
2003).
Simply stated, weeds are plants growing in places that humans do not
want them but invasive plants are those weeds that have the potential to spread
beyond their point of introduction (Radosevich et al., 2007) and challenge the
2
growth of other species or change ecological function. Although native plants
can be invasive, non-natives are the cause of the most damaging invasions
(Randall, 1996).
Despite control efforts, weeds are estimated to cause a 10% reduction in
world-wide crop and pasture production (Sindel, 2000) and they upset the
natural balance of ecosystems on all continents but Antarctica (Usher, 1988).
Hobbs and Huenneke (1992, p. 325) assert that “control of non-native plants
has become one of the most expensive and urgent tasks of managers in several
U.S. national parks, in island preserves such as the Galapagos, and elsewhere”.
A recent U.S. Forest Service survey in Oregon revealed that non-native plants
comprised 20 percent of the cover in one out of ten forested plots with
Himalayan blackberry being the most prevalent exotic species (Rapp, 2005).
Plant invasions cause concern because invading species can alter
ecosystem processes by displacing native species, by providing support for
other non-native animals, fungi or microbes, and hybridization can occur
which alters gene pools (Randall, 1996). Changes in ecological function and
economic losses drive managers to try to reduce the prevalence of weeds but
the removal of a weed produces feedback in a plant community that increases
the probability of invasion by new weeds (Radosevich et al., 2007). Thus,
careful consideration of site characteristics, physiology of the target weed, and
plant response to control measures is critical. Ideally the tools employed to
control weeds will reduce their vigor by manipulating the relative growth and
3
reproduction of the target species (Sheley et al., 1996) while having relatively
little impact on desirable species in the same plant community.
1.2 Invasive Plants and Invasion Defined
Invasive plants are weeds that demonstrate exceptional vigor and become
established, naturalized, and spread to new habitats without further human
intervention (Randall, 1997). It is important to note that invasive taxa can
spread where they are not native but they are a subset of naturalized taxa
(Rejmánek, 2000). Although non-native species are introduced to new
environments constantly through trade and human interest in new crops,
including ornamental plants, only a small sub-set of those plants become
weeds. Remarkably, weeds account for less that 0.1% of flowering plants
worldwide (Radosevich et al., 2007).
Many invasive plants have vegetative growth rates that exceed those of
native congeners and other species in the invaded community. Invasive plants
must produce reproductive offspring and have the potential to spread and
sustain a population without human intervention (Richardson et al., 2000). In
some cases, the invasive species becomes dominant in a plant community.
Dominance can be measured by percent cover (area occupied by one species as
a percentage of total area) or by density (number of plants per area).
During the colonization phase of an invasion process the number of
individuals in a population or the individuals per area increases (Cousens and
4
Mortimer, 1995). If, after taking into account deaths, this number is greater
than unity, as is the case during colonization, the population is expanding
(Radosevich et al., 2007). Invasive species have population growth rates of
greater than one until naturalization (Cousens and Mortimer, 1995) has
occurred at which time all niches are exploited and neither area, nor density,
changes.
For some invasive species, this state of naturalization describes the
population dynamic behind what we qualitatively see as a monoculture. The
primary species considered in this dissertation, Himalayan blackberry (Rubus
armeniacus) and English ivy (Hedera helix), have likely reached the
naturalization stage of invasion in many parts of their alien Pacific Northwest
range. In other locales, they may have just been introduced or are experiencing
colonization which likely is the time that managers become aware of their
presence.
1.3 Invasiveness and the Potential for Invasion
1.3.1 Plant characteristics
Whether an invasive plant is exotic or native, we need to understand its
physiology and ecology in order to design effective strategies for control or to
prevent future invasions (Baruch and Goldstein, 1999). Even low-density
weeds can pose challenges due to their extensive root systems or canopies.
Additionally, the presence of secondary compounds or morphologic
5
characteristics affect consumption of a plant by herbivores. For example, the
presence of thorns can limit access to an area thereby limiting control methods.
Many theories attempt to predict invasiveness based on characteristics of
the plant itself. These include seed dispersal characteristics (Harper, 1977),
growth analysis (Hunt, 1982), life-history characteristics (Sakai et al., 2001),
hybridization and polyploidy (Radosevich et al., 2007) among many others.
Williamson and Fitter (1996) report a number of characteristics of
invading plants derived from a review of the Ecological Flora Database and
other research. Abundant species make better invaders, though there are
exceptions. Williamson (1989) suggests that seed production may underlay the
observation of abundance. This author also notes that invasive species are
generally large, tall, and taller than wide as well as having large leaves but
postulates that these characteristics may be a sociological artifact; gardener’s
preference for large and showy plants. While this may hold true for the flora
examined in Britain, the annual grass invaders of the semi-arid west of North
America, notably cheatgrass (Bromus tectorum), red brome (Bromus rubens)
and medusahead (Taeniatherum caput-medusae), have very different
physiological characteristics but are still serious pests. Their invasiveness is
closely linked to disturbance regimes including grazing and fire (Whisenant,
1990). These species are also phenotypically plastic (Whisenant, 1990)
including the ability to grow in many types of soil (Stubbendieck et al., 2003).
They are also prolific seed producers (Pellant, 1990).
6
The plant strategy scheme of Grime (1977, 1979) using C, S and R
classifications for species is useful to describe the behavior of a phenotype in
many locations. The R, or ruderal, label for plants which respond vigorously
to disturbance, have short life spans, and make large reproductive effort is of
particular interest to those studying annuals that are invasive.
Hardy perennials which resist control efforts and dominate the plant
community of which they are a part seem best categorized as C, competitive,
plants in Grime’s scheme. While these descriptors are useful to compare
plants they do not account for the underlying mechanisms that make a plant
species invasive. Simply being a congener of an invasive species does not
predict invasiveness (Mack, 1996) though specific genes within the genotype
may ultimately explain why some plants are better invaders while others are
not. Sakai et al. (2001) provide ample justification to investigate life-history
and resource allocation as predictors of invasiveness. They suggest that
integrating population biology and weed science can lead to the identification
of life history stages at which management will be most effective.
The use of growth analysis is a powerful tool to explain why some species
are invasive while other, even closely related, species are not. Hunt (1982)
provides an exhaustive exploration of plant growth analysis. In brief, the
virtues of using metrics such as R (absolute or specific growth rate), E or NAR
(unit leaf rate), LAR (leaf area ratios) or RGR (relative growth rate) allow the
researcher to compare invasive plants to those they dominate in a plant
7
community. With this functional approach, the underlying mechanisms of why
invasive species are successful, or competitive in the terminology of Grime
(1977, 1979), can be elucidated.
There are plants which invade despite lower relative levels of seed
production and dispersal, lower growth rates, and shorter life spans than
species in the invaded plant community. In these cases, perhaps the
mechanisms for success lie in broader multi-factor interactions among plants,
animals and microbes in the ecosystem.
1.3.2 Ecosystem characteristics
Ecologists have developed a number of theories which attempt to predict
why some ecosystems are more prone to invasion than others. Although all
communities are invasible, most invasible communities have low average
levels of plant cover, are subject to frequent disturbance, or are close to sources
of alien plants (Crawley, 1987). Radosevich et al. (2007) concur that the
vulnerability to invasion varies among communities but emphasize that
disturbed habitats and agro-ecosystems, a sociological descriptor, are more
susceptible than plant systems that spend a long period at late successional
phases.
Disturbance, a mechanism which limits plant biomass by causing its
destruction (Grime, 1977), is frequently a precursor to the spread of weedy
plants. Numerous other factors within an ecosystem are also hypothesized to
8
play a role. These include the availability of seed dispersers, availability of
safe sites and absence of natural enemies (predators, competitors, disease
agents).
A commonly cited hypothesis to explain the varied degree of invasion
across ecosystems is that more diverse systems are less invasible. Levine and
D’Antonio (1999) add clarity to this idea by stating that diversity does not
render a community completely resistant to invasion but it influences the
probability that a propagule will be successful in that community. In contrast,
an analysis of 184 sites throughout the world found that communities richer in
native species had more, not fewer, exotics (Lonsdale, 1999).
Harper (1977) focused on the availability of safe sites as a primary
predictor of seedling success with four components including stimuli for
breaking seed dormancy, conditions supporting germination, resource
availability for growth, and the absence of hazards. In effect, the safe site
construct combines aspects of environment and plant biology to predict plant
propagule success.
1.4 Two Invasive Creeping Perennials of the Pacific Northwest
The research in this dissertation focuses primarily on Himalayan blackberry
(Rubus armeniacus Focke) and English ivy (Hedera helix L.) for very specific
reasons. Himalayan blackberry is the most widespread and economically
disruptive of all the noxious weed species in western Oregon (Wilson et al.,
9
2006). English ivy dominates large areas of forested land by displacing native
vegetation and becoming intertwined with shrubs which makes its removal
very challenging (Oregon Department of Agriculture, 2008). Though their life
histories and specific physiology vary, they are both highly successful invaders
due in large part to their creeping vegetative growth and seed dispersal by birds
(Butaye et al., 2001; Tirmenstein, 1989).
1.4.1 Physiology and ecology of Himalayan blackberry (Rubus
armeniacus)
Himalayan blackberry is an invasive shrub in Family Rosaceae, native to
Asia and introduced to North America via England (Pojar & MacKinnon,
1994; Mathews, 1992). It is increasingly a problem in high rainfall areas of the
Pacific Northwest. In a survey of riparian sites along 145 km of the
Willamette River in Oregon, Fierke and Kauffman (2006) found that 89% of
28 sites had Himalayan blackberry present. They conclude that the native
thimbleberry (Rubus parviflorus) or salmonberry (Rubus spectabilis) might be
displaced over time due to the ability of Himalayan blackberry to form
homogeneous cover and inhibit the germination and establishment of native
species.
Himalayan blackberry typically grows at low densities possessing one
crown with many canes (Radosevich, personal communication) that produce
large canopies which then shade an area of large circumference around the
10
plant. Like other cane fruit, it reproduces by seed and grows vegetatively
through ‘daughter plants’ at cane tips, rhizomes, and from severed pieces of
canes and roots (Amor, 1974b; Kigel and Koller, 1985).
The ability of Himalayan blackberry to refoliate after disturbance, and to
grow at rates faster than other native shrubs in the plant communities it
invades, places it in the stress-tolerant competitor scheme in Grime’s C-S-R
model. In a comparison of four Rubus species, McDowell (2002b) found that
the non-natives, Himalayan blackberry and evergreen blackberry (Rubus
laciniatus), have higher photosynthetic capacities than two natives, black
raspberry (Rubus leucodermis) and trailing blackberry (Rubus ursinus), which
allows the invasive species to fix significantly more carbon. Additionally, the
higher average maximum photosynthetic rate (A max) of the two non-native
invasive species is maintained throughout the year. The non-natives also had
a larger pool of available carbon which may be translocated to perennial roots
following cane senescence and stored for future cane production. The leaves
of Himalayan blackberry remain on the canes longer than non-invasive species
thus allowing them a longer period of carbon gain than congeners (McDowell,
2002b).
Himalayan blackberry is like other plants which have rapid rates of
vegetative growth, especially after top removal. It is a colonizer in disturbed
forests because it tends to allocate a large portion of its resources to stems and
branches (Radosevich et al., 2007). This can easily be observed in the large
11
thickets of Himalayan blackberry that can climb into tree canopies and
compete with those trees for incident light.
Bud dormancy, which can be induced by herbicides, complicates control
of this and other rhizomatous weeds (Amor, 1974b). It is possible to prevent
the establishment of new daughter plants by mowing or by herbicide treatment
(Bruzzese and Lane, 1996) but the effect is variable. Amor (1974b) reports
that an herbicide trial in Victoria, Australia resulted in thickets of Rubus
fruticosus (re-classified as R. armeniacus) which produce more suckers when
sprayed with 2,4,5-T than thickets left unsprayed (Amor 1974a). Plant age is a
major source of the variable response to herbicide (Amor, 1974b).
Factors other than the ecology of a species can contribute to its persistence
as a weed. Cane fruit production is a thriving industry in western Oregon and,
although Himalayan blackberry is not commercially cultivated for fruit, it
frequently grows in association with evergreen blackberry (Rubus laciniatus)
which is commercially important in Oregon (Oregon Raspberry and
Blackberry Commission, 2008). Control methods that may be effective
against Himalayan blackberry are of concern to this sector of the horticulture
industry due to the potential impacts on other species in the Rubus genus.
Recent confirmation of Phragmidium violaceum, a fungal foliar pathogen,
in both Himalayan and evergreen blackberry in Oregon (Aime and Rossman,
2007) is a concern to this producer group. Phragmidium violaceum was
intentionally released in Australia as a control agent (Sindel, 2000) under the
12
Commonwealth Biological Control Act of 1984. The act allows for the
release of a biological control agent to control invasive species and protects
people who release a biological control agent from legal action as a result of
unintended effects of that agent.
The quantity and size of thorns on Himalayan blackberry protect it from
herbivory by many grazing animals. Sites invaded by this blackberry are
likely to increase in shrub cover or density as herbivory is limited (Edwards
and Gilman, 1987).
1.4.2 Physiology and ecology of English ivy (Hedera helix)
English ivy (Hedera helix) is a temperate liana in Family Araliaceae
which has been raised for centuries as a ground cover. It was introduced to
North America from Europe in the colonial era (Clarke et al., 2006). It is now
naturalized in the mid-Atlantic and Western United States (Randall, 1996).
Recent sampling of invasive populations in British Columbia, Washington, and
Oregon revealed that 83% of 58 populations were actually Hedera hibernica
(Kirchner) Bean rather than Hedera helix L. (Clarke et al. 2006). Hedera
helix is used in this dissertation because invasive populations are managed as
H. helix. This is also how naturalists and horticulturalists refer to this
naturalized plant.
English ivy is classified as a B list noxious weed in Oregon (ODA
Plant Division, 2008), yet some cultivars may still be sold until December
13
2009. English ivy is a serious pest of forests in and around urban areas in the
Pacific Northwest. It poses challenges where trees have been cut and the
resultant open canopy allows new source populations to arise from seed
dispersal by bird endozoochory. In 1999, more than 55,000 hours of mostly
volunteer labor were expended to remove this plant from parks and open
spaces in the Portland metropolitan area (Anonymous, 2000).
English ivy grows by vegetative structures including long stems which
produce stolons. Prolific stoloniferous growth and subsequent clonal growth
contribute to its invasiveness with densities of 10,000 plants/acre (Reichard,
2007). The vine climbs with the aid of aerial roots and as it reaches higher
light intensities in forest canopies leaves become ovate (Okerman, 2006). The
plant is then referred to as an adult or mature individual. Vertical forms of the
plant partition new dry matter preferentially to increase leaf laminar area at the
“expense” of shoot axis dry weight (Frey and Frick, 1987).
In Grime’s C-S-R classification system (1977, 1979), English ivy can be
classified as both competitive and stress tolerant species. It is competitive
because it produces a canopy of fast-growing leaves that exclude other species
in plant communities. Cordero et al. (1985) found that juvenile plants had a
plastochron (time per production of leaf) of 4.23 days whether grown under
short or long day conditions. Short days promoted vegetative growth and long
days initiated flower production in mature plants.
14
The stress tolerant classification is appropriate because English ivy is
tolerant of Mediterranean drought and deep shade though it can become
stressed by extreme conditions Sack (2004). Specifically, English ivy is more
tolerant of drought under 3% rather than 30% daylight conditions (Sack, 2004).
Oberhuber and Bauer (1991) observed loss of chlorophyll in east and southfacing leaves exposed to strong light and severe frost during winter. They note
that all leaves showed remarkable recovery and returned to pre-stress
photosynthetic levels within seven days after the stress. These researchers
conclude that adult ivy leaves possess an enormous capacity to repair lightinduced damage to the photosynthetic apparatus.
Seed dispersal by bird endozoochory is common. In forest plantations
with a history of logging in Belgium, many forest patches contain English ivy
dispersed by bird endozoochory (Butaye et al., 2001). Patches with English
ivy had significant (.001 ≤ p < .01) reductions in species richness, with an
average of 5 fewer species, compared to those with other species. Laskurain
et al. (2004) examined seedling dynamics in a temperate forest in southern
Spain with similar climatic conditions to the Pacific Northwest (165cm annual
precipitation, mean annual temperatures of 11°C, 43° N latitude) and found
that 98% of emergent seedlings survived to six months and 96% of those
survived to one year. High seed survival and seedling establishment were
attributed to the heavier weight of English ivy seeds compared to the heath and
Rubus species and the species’ drought and shade tolerance (Laskurain et al.,
15
2004). The dominant eigenvalue or finite rate of increase, for English ivy is
1.28 (calculated from Laskurain et al., 2004). It is likely that the eigenvalue
declines as the density of this plant increases because the primary means of
expansion is by vegetative growth. Most dispersed seed falls into the ivy
ground cover below the host trees where germination is limited. The
availability of shrubs or trees to climb and the amount of incident light
throughout the year have substantial effects on rates of sexual reproduction.
Published research focuses on vegetative growth or the effect of English ivy on
wildlife abundance and density rather than population dynamics of the ivy
itself.
English ivy forms dense ground cover which prevents emergence of
herbaceous plants. However, it did not alter the presence of seeds of other
species in a secondary Piedmont forest in Georgia (Biggerstaff & Beck, 2006).
Species richness and Shannon-Weaver indices for sites with and without ivy
were not significantly different. Additionally, no evidence for allelopathy was
found as seeds of numerous others species germinated equally well in pots
with and without ivy (Biggerstaff & Beck, 2006).
Moore (2004) notes that English ivy biomass doubled and stem length
increased by 137% when CO2 levels were raised from 365 ppm to 660ppm in a
forested site in Switzerland. Zotz et al. (2006) report similar results when CO2
levels were raised to 500 and 650 ppm and total biomass of new shoots
increased by more than 80% and leaf biomass increased 100%. Zotz et al.
16
(2006) conclude that English ivy lianas will reach the canopy more quickly as
carbon dioxide levels rise and thus compete more with the foliage of host trees.
English ivy is tolerant of pre-emergent herbicides (Derr, 1992) but will
respond to glyphosate application in the early spring if applied at 1.4 kg/ha (2.7
lbs/ac) (Reichard, 2007). The addition of surfactants aids in penetration of the
waxy leaves by herbicides (Soll, 2005) but neighbor plants are also affected.
The most effective control measure for this plant is hand-pulling (Reichard,
2007).
1.5 Domestic Livestock and Their Influence on Plant Communities
Humans have interacted with grazing animals, primarily as a source of
food, for centuries. The earliest known domesticated herbivores are sheep
believed to have been domesticated in the Zagros Mountains of northern Iraq
about 10,700 years ago (Reed, 1980). Since that time humans have herded
livestock of many species and transported them to new continents. This
migration has altered relationships between plants and animals by changing the
pressure from disturbance that plants experience from herbivory.
European immigration to the Americas and Australia led to large herds of
domesticated cattle, sheep and other classes of livestock in locales where they
had not previously existed. In the United States, the concept of rangelands was
born and scientists endeavored to quantify the effects of domestic livestock on
plant communities. Dyksterhuis (1948, 1949) used the terms decreaser,
17
increaser and invader to describe trends in plant species response to grazing.
Dyksterhuis noted that invaders were not present in climax communities,
following the convention of Clements (1936), but were present in disturbed
areas such as mounds of burrowing animals. He also observed that
overgrazing with domestic livestock could produce areas where invaders
occupy entire landscapes.
Vesk and Westoby (2001) attempted to categorize 829 species on
Australian rangelands as increasers, decreasers or neutral under grazing but
analyses were challenged in a number of ways. For example, 505 of the
species occurred at only one geographical location, weather differentially
affected plant growth, plants known to be palatable may not have been selected
when other more desirable food sources were available, and the intensity of
grazing was variable. Despite these challenges, efforts to quantify response
undertaken by these researchers provide a backdrop for the application of
targeted grazing.
1.5.1 Grazing as disturbance
Archer and Stokes (2000) have fined-tuned the widely accepted definition
of disturbance by elaborating that disturbance regimes are the sums of types,
frequencies and intensities of disturbance through time in a landscape.
Grazing management seeks to maximize livestock production but all wild and
domestic herbivores selectively utilize plant species in any ecosystem (Briske
18
and Heitschmidt, 1991) which leads to differential growth following the
grazing disturbance. Additionally, the season of disturbance can modify the
intensity of the disturbance, the availability of propagules, and conditions
which favor establishment of one species over another (Noble and Slatyer,
1980). Frequency, magnitude, season of tissue removal, plant parts lost,
resource availability for re-growth, and stage in the plant life cycle all
influence plant response to herbivory (Archer and Smeins, 1991). If plants are
unable to survive under a disturbance regime, and niches become available to
invaders, plant species composition will change.
Archer et al. (1988) and D’Antonio and Vitousek (1992) discuss the
transitions that occur when ‘keystone species’ establish and initiate positive
feedbacks that redirect plant succession. If plant composition on a landscape is
substantially altered, the underlying ecological processes will also change.
Targeted grazing is a management tool that seeks to alter these trends and stop
the feedback so that, with integrated pest management (IPM) techniques,
restoration may be possible.
1.5.2 Targeted grazing and goat browsing for invasive plant control
The application of specific livestock at a determined season, duration, and
intensity to accomplish defined vegetation or landscape goals is known as
targeted grazing (Frost and Launchbaugh, 2003). Targeted grazing has the
potential to reduce plant vigor and ultimately contribute to weed management.
19
The use of livestock to manage certain types of vegetation is not new but close
adjustment of timing, duration, and intensity of the herbivory and
quantification of the plant response are newer in management applications.
Because plant community composition reflects its history of herbivory, plant
species which are intolerant of a change in herbivore regimen will be
eliminated (Edwards and Gilman, 1987). An industry based on the pairing of
plant with herbivore is receiving closer scrutiny as managers seek alternatives
to chemical and mechanical control methods.
Livestock vary in the way they consume vegetation. In particular, goats
are browsers and well suited for managing woody plants because they consume
more browse than cattle or sheep (Campbell and Taylor, 2006). They have
been used for brush control and weed management throughout the western
United States for generations (Scifres, 1980). Because goats tend to avoid
clover, and show a greater preference for fibrous plants than do sheep and
cattle (Hodgson, 1990), they are often chosen to improve pasture quality. Use
of goats to control invasive plants in a forested understory can be more
complicated if desirable species are also shrubs or trees. Thus Scifres (1980)
warns that techniques must be designed specifically to the situation.
Because precipitation in western Oregon falls primarily between October
and May, most browsing for weed control is done June through September.
This timing avoids soil compaction and the creation of depressions from stock
hooves (pugs) which favor seedling establishment by creating safe sites
20
(Harper, 1965). To be most effective against the target plant species, browsing
is timed with the onset of flowering when plants are believed to be most
vulnerable. For Himalayan blackberry, primocane and shoot growth are
greatest in spring and summer and fortunately this coincides with the best
timing to protect soil resources. The portions of English ivy plants consumed
by goats are vegetative so there is potentially more flexibility in timing
application of browsing treatment on this species. The flowers and fruits of
this ivy exist primarily in sunlit portions of tree canopies so hand-pulling to
remove vines from trees will always be necessary to control English ivy.
Duration is an important component to consider in a targeted grazing plan.
Scifres (1980) claims short-term, high stocking rates are more effective than
grazing on a year-long basis but this is likely to vary depending on the vital
attributes (Noble and Slatyer, 1980) of the target species.
1.6 Developing a Model for Predicting Plant Community Change
In order to form meaningful generalizations, or models, of how plants will
respond to disturbance Lavorel et al. (1997) propose that we fine tune the C-SR plant strategy scheme of Grime (1977, 1979) to include morphology and
regeneration strategy. One might also include plant demographic traits which
contribute to the success of individual species. However, a study of 18
herbaceous species found that contributions of growth, fecundity, and survival
to finite rates of population increase (λ) had no significant correlation with
21
CSR status (Silvertown et al., 1992). Rejmánek (2000) advocates the use of
habitat specific predictions to guide management of the most troubling
invasive species whether to prevent new introductions or to control species that
are already problematic. Edwards and Gillman (1987) believe the best way to
study the role of herbivory is to perturb the system by introducing, removing,
or somehow controlling herbivore activity. The work of these ecologists
points to the need for sophisticated constructs (White and Gunstone, 1992) to
group plants so that one can predict plant response to changes, including
disturbance through herbivory.
1.7 Objectives
This dissertation includes three studies. Chapters 2 and 3 address the
response of Himalayan blackberry to high intensity-short duration goat
browsing, mowing, and a combination of these two methods for three levels of
treatment. Himalayan blackberry vigor is examined in Chapter 2 and questions
addressed include: Is stem density altered under the treatment types? How is
the population dynamic of the blackberry population (including seedlings,
primocanes, floricanes, and nodes) altered? Which treatment provides the
most effective control of Himalayan blackberry? Chapter 3 includes the
invasive evergreen blackberry because it grows in association with Himalayan
blackberry and functions similarly in the ecosystems it invades. Specific
research questions include: How does plant composition change under each of
22
the treatments? Are species and functional group changes different among the
treatments? How will these changes affect restoration?
In Chapter 4, the efficacy of high intensity-short duration goat browsing is
examined for the control of English ivy. Specifically: Does goat browsing
reduce the percent cover of English ivy? Is there a difference in plant response
for one vs. two years of the treatment? Does species composition in the plant
community change?
23
2 HIMALAYAN BLACKBERRY (RUBUS ARMENIACUS) RESPONSE
TO HIGH INTENSITY-SHORT DURATION GOAT BROWSING
AND MOWING
2.1 Abstract
Throughout the Pacific Northwest, Himalayan blackberry (Rubus
armeniacus) is a noxious invader which dominates sites with its persistent
canopy and large underground crowns. The resultant shading and occupation
of root space inhibit the growth of desired species in pasture, forest and
agronomic settings where this non-native blackberry species is present.
Control is complicated by vigorous re-growth after mechanical control and
variable response to herbicides. Recent interest in the use of domestic
livestock to control invasive plants through targeted grazing methods has led
land managers to use a variety of long and short term methods to control
unwanted species through disturbance by herbivory.
This study examines the efficacy of high intensity-short duration goat
browsing, mowing and a combination of the two methods to control
Himalayan blackberry. Experimental plots were monitored from spring 2006
through fall 2008.
Residual above ground biomass in June 2008 was significantly less in
plots mowed in two years than in untreated plots. Mean total stem density
increased after two annual repetitions of goat browsing, mowing and goat
browsing followed by mowing but was not significantly different among the
24
treatment types. Mean total stem density then declined from 2007 to 2008
after a mowing treatment was applied to all experimental plots. Fluctuations in
densities of seedlings, primocanes, floricanes and nodes are discussed.
Nomenclature: Himalayan blackberry, Rubus armeniacus Focke
Key Words: Himalayan blackberry (Rubus armeniacus Focke); plant vigor,
seedlings, primocanes, floricanes, nodes, biomass, high intensity-short
duration browsing, goats, mowing.
2.2 Introduction
Disturbance is frequently a precursor to the presence and spread of weedy
plants (Rejmánek, 1989). Common sources of disturbance in pasture plant
communities are cultural methods such as soil tillage, seeding with forage
species and subsequent grazing by one class of livestock. Where cattle have
been the predominant herbivore, grasses are disturbed by repeated grazing
while forbs and shrubs are not consumed to the same degree. To control an
invasive species through alteration of the grazing regime, both the type and
intensity of disturbance are changed (Archer and Stokes, 2000). Disturbance
can be an opportunity to alter species composition in a plant community to a
more desired state (Westoby et al., 1989).
25
Response to disturbance is determined by attributes of the component
plant species (Hobbs and Huenneke, 1992) and the resource environment in
which those species exist. However, it has been noted that basal meristem
monocots respond differently than dicot herbs and woody plants (Hawkes and
Sullivan, 2001). In this study, patches in a pasture dominated by Himalayan
blackberry were browsed by goats (defoliation and consumption of young
canes) or mowed (removal of most aboveground biomass) to determine the
resulting vigor of the target species and possible emergence of new plant
species. Changes in species composition, the functional groups of those
species, and implications for restoration are discussed in Chapter 3.
Control of Himalayan blackberry is complicated by a number of factors.
The maximum relative growth rate of 0.104 g/g/day for primocanes is more
like herbaceous than woody plants (Amor, 1974a). Plants that have rapid rates
of vegetative growth, especially after top removal, are often colonizers in
disturbed forests (Radosevich et al., 2007) and adjacent pasture plant
communities. Survival of daughter plants which originate from adventitious
buds on roots is often higher than that of seedlings (Amor, 1974a).
Bud dormancy, which can be induced by herbicides, also complicates
control of this and other rhizomatous weeds (Amor, 1974b). Although
Bruzzese and Lane (1996) reported that establishment of new ramets can be
prevented by mowing or by herbicide treatment, Amor (1974a) found that
thickets of Rubus fruticosus produced more suckers when sprayed with 2,4,5-
26
T than thickets left unsprayed. Amor (1974b) notes that plant age is a major
source of variation in response of this species to herbicide. Thus the age of
the Himalayan blackberry ramets in this study will likely be a factor in their
response to control methods.
All organisms are subject to controllers and resources in their
environments (Bazzaz, 1987). While altering resources in this pasture plant
community beyond the increase in available nutrients from decomposing cane
litter is not practical, we can influence vegetative and reproductive growth
with browsing or mowing which impose substantial disturbance. Both
methods are expected to reduce individual aboveground biomass while also
stimulating re-growth that may draw on root reserves and ultimately cause a
decline in vigor of the target species.
Managers seek to drive plant community dynamics by manipulating
relative growth and reproduction of plant species (Sheley et al., 1996). The
application of high intensity-short duration goat browsing damages the
Himalayan blackberry with a kind and level of disturbance that limits net
primary production of the plant in the long term (Hendrickson and Olson,
2006). This control method may be particularly effective in a degraded plant
community with a monoculture that resists herbivory by the usual grazing
species, cattle. Changing the type of disturbance from grazing to browsing,
intensity from low to high, and timing from long to short duration is a
substantial shift in disturbance regime (Archer and Stokes, 2000). It is
27
expected that changes in plant cover, density and other measures of vigor, will
be observed.
The objectives of this study were to determine the effect of high intensityshort duration goat browsing, mowing and a combination of goat browsing and
mowing on the vigor of Himalayan blackberry. Stem density of various ages
of stems (including seedlings, primocanes and floricanes), total stem density,
node density and biomass per area were the response variables measured. The
methods chosen were based on control protocols used by commercial graziers.
2.3 Materials and Methods
2.3.1 Site and plot description
This study was conducted south of Cottage Grove, Oregon in the
Willamette Valley from May 2006 through August 2008. The research plots
were located in a north aspect pasture on Nekia silty clay loam soils with 30 to
50 percent slopes at 100 to 135 meters (330 to 445 feet) elevation (Soil Data
Mart, 2008). The pasture was planted with subterranean clover (Trifolium
subterraneum) and tall fescue (Schedonorus phoenix) in the mid-1970s after
excavation and installation of a natural gas pipeline. Since that time, neither
pasture improvement, nor weed management, has been undertaken. The site
has been grazed by cattle since the early 1900s but was not grazed by goats
prior to this study.
28
Location of the plots was determined by visual estimate of dense (95100%) Himalayan blackberry cover. Using a completely randomized design
(CRD), treatments were randomly assigned to each of the plots. There were
four replicates of each of the seven treatments for a total of 28 plots. Each plot
was 36 m2 in area with a 2m buffer extending out from the perimeter of each
plot to ensure that no interactions occurred between plots. The buffer also
allowed for the installation of temporary electric fencing to confine the goats to
the browsed plots. Because many plots had Himalayan blackberry canopies
that were 2 meters tall in the spring of 2006 metal t-posts capped with white
PVC pipe were used to delineate the four corners of each plot.
2.3.2 Treatments
The different treatments (Table 2.1) included browsing, mowing and
browsing plus mowing at three levels; 0, 1 or 2 years. Four untreated plots
served as controls and were neither browsed nor mowed. To obtain a final
assessment of above ground biomass all plots were mowed in late June 2008
(Section 2.3.5). In effect, the biomass harvest conducted in 2008 produced
new treatment types and so the 2008 stem density data are compared separately
to the 2006 and 2007 data.
29
Table 2.1
Treatment Descriptions
Year
Treatment
1g
Treatment
2g
Treatment
1m
Treatment
2m
Treatment
1gm
Treatment
2gm
Treatment
c
2006
not treated
browsed
not treated
mowed
not treated
browsed &
mowed
not treated
2007
browsed
browsed
mowed
mowed
browsed &
mowed
browsed &
mowed
not treated
2008
mowed
mowed
mowed
mowed
mowed
mowed
mowed
1g = goat browsing in one year, 2g = goat browsing in two years, 1m = mowing in one year
2m = mowing in two years, 1gm = goat browsing & mowing in one year
2gm = goat browsing & mowing in two years, c = control (untreated)
2.3.3 Application of treatments
Mowing was conducted in July of 2006 and 2007 after the onset of
flowering of the Himalayan blackberry. A Bobcat® single blade deck mower
was used to cut the canes to approximately 5-10cm from the soil surface.
Temporary electric fencing was installed around each plot prior to browsing.
Goat does and their kids were herded onto the appropriate plots and allowed to
browse for approximately one day. This resulted in a stocking rate equivalent
to 988 head/hectare (400 head/acre). Animals were removed from the plots
when plants were defoliated, young canes eaten, and the herd showed little
interest in continued browsing. For the browsing followed by mowing
30
treatment (Table 2.1), goat browsing was applied as described above and those
same plots were mowed within one week after the animals were removed.
2.3.4 Above ground biomass
In late June 2008 all plots were mowed from the upper to lower edge with
one pass of a 1.5m (5ft) wide Bobcat® mower. This produced a sampled area
of 1.5m x 6m (9m2). The cane from this area was raked, non-blackberry debris
removed, and left for 3 days to air dry. Each pile was turned and allowed to
dry for an additional four hours and weighed to determine the biomass of the
cane debris from each plot.
2.3.5 Stem density
In late August and September of 2006-2008, five to seven weeks
following each treatment (Table 2.1), each plot was sampled for stem density.
A grid of 1m2 sub-plots numbered from left to right and top to bottom was used
to delineate the sub-plots and orange-topped wooden stakes marked the four
corners of each sub-plot. Nine sub-plots were randomly selected and stem
density determined. This resulted in 25% (9 of 36 m2) of each plot being
sampled. The response variables included density of seedlings, primocanes,
floricanes, total stems and nodes. Seedlings are plants that have sprouted from
seed in the current year, primocanes are new canes which have germinated
from a crown in the current growing season, and floricanes are canes one year
31
or older. Total canes were calculated as the sum of seedlings, primocanes, and
floricanes. Nodes are defined as a group of canes which grow from the same
area on a crown. Nodes were determined by brushing away soil from the
crown or grasping an individual cane to determine how it moved relative to
other nearby canes. If a cane was attached to the crown near other canes all the
canes were counted as part of the same node.
2.3.6 Statistical model and analysis
Differences in residual above ground biomass were compared using the
following model:
Уij = ų + χi + єij
Уij = residual biomass/m2 in 2008,
i = treatment (1…7)
j = replicate (plot 1, 2, 3, 4)
ų = over-all mean biomass/m2
χi = effect of treatment on biomass /m2
єij = random variation of ith trt, jth plot that adds variability to the value of
Уij, єij N(0, δ2 ), єij and єi’j’ are independent.
The SAS 9.1.3 Statistical Package (2003) was used to analyze the data.
Tukey’s HSD was used to make pairwise comparisons for all treatments for the
response variable of biomass/m2.
32
The following General Linear Model (GLM) was fit to the stem density
data:
Уij = ų + χi + єij
Уij = observed change in stem density from 2006 to 2007 (or 2007 to 2008)
i = trt type (browsing once, browsing twice, mowing once, mowing twice,
browsing plus mowing once, browsing plus mowing twice)
j = replicate (plot 1, 2, 3, 4)
Ų = over-all mean of change in blackberry stems/ m2
χi = difference in stem density for the ith treatment type relative to the overall mean
єij = random variation of ith trt, jth plot that adds variability to the value of Уij,
єij N(0, δ2 ), єij and єi’j’ are independent
Changes in stem density from 2007 to 2008 are for plots treated once or
twice with the three treatments followed by mowing in 2008 (Table 2.2).
The SAS 9.1.3 Statistical Package (2003) was used to analyze the data.
Tukey’s HSD Test was used to make pairwise comparisons for all treatments
for the response variables of biomass/m2, seedlings/m2, primocanes/m2,
floricanes/m2, total stems/m2 and nodes/m2. This test was also used to
compare mean treatment biomass results.
33
2.4 Results and Discussion
2.4.1 Above ground biomass
Residual above ground biomass was examined in June 2008 to asses the
robustness of Himalayan blackberry plants. The abilities to form ramets and
rapidly capture nutrients to grow new leaves (Grime and Hodgson, 1987) are
characteristics of competitive species thus biomass harvest provides the benefit
of gauging vigor using an indirect measure of relative growth rate. Mean
aboveground biomass ranged from 409 g/m2 for plots mowed twice to 1018
g/m2 for untreated plots (Figure 2.1).
Figure 2.1 Mean Aboveground Biomass in 2008
1400
1200
g/m2
1000
1007
1018
950
775
800
835
812
600
409
400
200
0
Treatment
goats 1 yr
goats 2 yrs
goats & mow 1 yr
goats & mow 2 yrs
mow 1 yr
mow 2 yrs
untreated
Error bars = 1 SE
34
Mean residual biomass for plots mowed in two years was significantly less
than untreated plots, 409 g/ m2 compared to 1018 g/ m2 at the 90% confidence
level. Mean residual biomass for plots mowed in two years was also
significantly less than plots browsed by goats for one year, and plots browsed
by goats for two years at the 80% confidence level. This result was expected
as the mowing created a high level of disturbance, particularly compared to
plots that remained untreated.
Removal of Himalayan blackberry above ground biomass by goat
browsing, mowing or a combination of the two was an attempt to direct the
plant community by reducing the vigor of the target plant. Hobbs and
Huenneke (1992) suggest that mowing allows the persistence of less
competitive species which would be desirable particularly if these included
grasses of high palatability to livestock. The change in grazing regime (Hobbs
& Huenneke, 1992) from low intensity by cattle, which primarily select grasses
in their diet, to high intensity-short duration browsing by goats which consume
parts of woody plants, did reduce Himalayan blackberry biomass. However,
repeated mowing is more effective than goat browsing to achieve the goal of
reducing above ground biomass.
2.4.2 Stem Density
Himalayan blackberry responds to disturbance in many ways including
growing primocanes, tip-rooting, sprouting new branches from floricanes,
35
sprouting from stem remnants and growing new ramets from adventitious roots
(Amor, 1974a). Thus control of this species can be difficult and repetition of
control methods may be necessary as was done in this study. Stem densities
by treatment, after two years of the study, are presented in Table 2.2.
Table 2.2
Mean Stem and Node Densities (per m2) by Treatment
Goat Browsing x 1 *
Goat Browsing x 2 **
Goat Browsing and
Mowing x 1 *
Goat Browsing and
Mowing x 2 **
Mowing x 1 *
Mowing x 2 **
Seedlings
Primocanes
Floricanes
Total
Canes
Nodes
0.1
3.8
0.2
1.2
32.8
46.9
33.1
51.9
10.6
18.5
0.2
3.0
31.1
33.5
16.2
10.6
0.1
21.2
7.8
4.5
3.1
45.4
32.6
41.0
63.7
37.2
65.3
30.5
15.5
34.9
* Means for pooled data from plots treated once, data collected in 2006 and in 2007
** Means for plots treated twice, data collected in 2007
2.4.2.1 Total stem and floricane density
Total stem density increased from 2006 to 2007 in plots of all treatment
types by an average of 21 stems/m2 (CL = 8.2, 34.3, df = 1, p < 0.08).
However, the decline of 10 stems/m2 from 2007 to 2008 was not statistically
significant.
Two years of mowing produced 32 more stems/m2 than one year of
treatment with goat browsing (CL = 3.2, 61.1, df = 6, p = 0.003) at the 95%
confidence level. As well, two years of mowing produced 32 more stems/m2
36
than one year of treatment with goat browsing followed by mowing (CL = 2.8,
60.8, df = 6, p = 0.003) at the 95% confidence level. Mowing in two years also
resulted in greater density than mowing in one year with 28 more total
stems/m2 (CL = 2.0, 54.1, df = 6, p = 0.003) at the 90% confidence level.
Two years of goat browsing followed by mowing also produced an
increase in total stem density with 30 more canes than plots treated once the
same treatment (CL = 1.2, 59.2, df = 6, p = 0.003) and 31 more canes than
plots treated once with goat browsing (CL = 1.6, 60.0, df = 6, p = 0.003), both
results reported at the 95% confidence level.
Mowing stimulates the growth of new primocanes and allows for
seedlings to germinate so the increase in density is expected. It is possible
that plants disturbed twice in one season show a reduction in density.
Examining the population demographic of that density change will provide a
more thorough understanding of how the Himalayan blackberry responds.
Although a high degree of disturbance was imposed, the ability of
Himalayan blackberry to re-sprout from its crowns allowed it to compensate
for the removal of above ground biomass. It is possible from the 2008 data
(Table 2.3) that a downward trend in total stem density was beginning but
continued monitoring in subsequent years would be necessary to confirm this
trajectory.
37
Table 2.3
Mean Stem and Node Densities (per m2) in Summer 2008
Goat Browsing x 1
Goat Browsing x 2
Goat Browsing and
Mowing x 1
Goat Browsing and
Mowing x 2
Mowing x 1
Mowing x 2
Untreated
Seedlings
Primocanes
Floricanes
Total
Canes
Nodes
0.3
0.3
9.5
13.4
24.8
25.8
34.6
39.4
12.3
15.3
11.1
10.8
20.3
42.3
24.6
1.1
1.2
8.9
0.3
9.6
8.5
11.0
9.6
29.6
24.8
28.5
31.1
40.3
34.4
48.4
41.0
15.6
14.0
25.2
15.1
Note: All data above determined five to seven weeks after mowing in 2008 per Table 2.1.
Mean floricane density increased for all treatments from 2006 to 2007 by
12 floricanes/m2 (CL = 1.0, 22.4, df = 6, p = 0.59) at the 80% confidence level.
Changes in floricane density from 2007 to 2008 were highly variable and the
low level of statistical certainty inherent in the 2006 to 2007 change point to
the need to examine density of other phenological stages of Himalayan
blackberry. This approach is also validated by manner in which Himalayan
blackberry responds to disturbance.
38
2.4.2.2 Seedling density
Mean seedling density for all treatments increased from 2006 to 2007 by 6
seedlings/m2 (CL = 0.2, 11.6, df = 6, p < 0.0001). Changes in seedling density
were highly variable from 2007 to 2008 with a mean of 6 fewer seedlings/m2,
though the results were not statistically significant. However, the biological
significance of this result is that seedling numbers increased initially and then
declined to essentially the same level that existed after one year of treatment.
Seedling survival can be attributed to many factors but the presence of other
species and the shade they create are important factors. These factors are
discussed in Chapter 3.
Plots mowed twice were most different from other treatments. Table 2.4
shows a comparison treatment means at the 95% confidence level.
39
Table 2.4 Seedling Density Pairwise Comparisons for All Treatments
Treatment
Comparison
Difference
Between
Means
2m - 2gm
2m - 2g
2m - c
2m - 1gm
2m - 1m
2m - 1g
2gm - 2g
2gm - c
2gm - 1gm
2gm - 1m
2gm - 1g
2g - c
2g - 1gm
2g - 1m
2g - 1g
c - 1gm
c - 1m
c - 1g
1gm - 1m
1gm - 1g
1m - 1g
10.6
17.4
20.9
21.1
21.1
21.1
6.8
10.3
10.4
10.5
10.5
3.5
3.7
3.7
3.8
0.2
0.2
0.3
0.1
0.1
0.0
Simultaneous 95%
Confidence Limits
-3.9
2.8
6.3
8.4
8.5
8.6
-7.8
-4.3
-2.2
-2.1
-2.1
-11.1
-9.0
-8.9
-8.9
-12.4
-12.4
-12.3
-10.3
-10.2
-10.3
25.2
32.0
35.5
33.7
33.7
33.8
21.4
24.8
23.1
23.1
23.2
18.1
16.3
16.3
16.4
12.8
12.9
13.0
10.4
10.4
10.4
*
*
*
*
*
1g = goat browsing in one year, 2g = goat browsing in two years,
1m = mowing in one year, 2m = mowing in two years
1gm = goat browsing & mowing in one year, 2gm = goat browsing & mowing in
two years
c = control (untreated)
* = significant at 95% confidence level
Mowing in two years clearly produces conditions which favor seedling
germination. In contrast, goats defoliate the Himalayan blackberry and readily
40
consume the berries at the same time which may contribute to the decline in
seedling density. The lack of statistical significance between the plots mowed
twice and those browsed and then mowed twice is due to the range of resultant
data. More replicates of this treatment would assist researchers in confirming
the strength of the trend toward an increase in seedling density with mowing.
If left undisturbed the Himalayan blackberry would likely have relatively
constant seed rain which contributes to its ability to disperse to new sites
(Lambrecht-McDowell and Radosevich, 2005). This potential is not expressed
near the parent plant due to shading from the dense canopy. Once the canopy
is reduced the seedlings are able to germinate further challenging restoration of
the site (Radosevich et al., 2007). Himalayan blackberry produces
significantly (p< 0.0001) more fruits/cane than the native trailing blackberry
(Rubus ursinus), 720.3 ± 123.9 vs. 23.5 ± 4.4 (McDowell, 2002a) which
illustrates its reproductive potential. Annual control measures could limit the
seed bank in the long term because canes must be two years or older to flower
but invasion potential is strongly influenced by vegetative re-growth.
41
2.4.2.3 Primocane density
Mean primocane density for all treatments accelerated over the three years
of the study with an increase of 3 primocanes/m2 (CL = 1.5, 4.8, df = 1, p =
.0003) from 2006 to 2007 to an increase of 6 primocanes/m2 (CL = 2.9, 8.0, df
= 1, p = .0003) at the 95% confidence level from 2007 to 2008. These results
indicate that the Himalayan blackberry plant population is vigorous and
becoming younger. Comparisons between treatments are important because
they will reveal which treatments are most effective at suppressing vegetative
growth of primocanes (Table 2.5).
42
Table 2.5 Primocane Density Pairwise Comparisons for All Treatments
Trt
Comparison
Difference
Between
Means
c - 2gm
c - 1m
c - 2m
c - 1gm
c - 2g
c - 1g
2gm - 1m
2gm - 2m
2gm - 1gm
2gm - 2g
2gm - 1g
1gm - 1m
1gm - 2m
1gm - 2g
1gm - 1g
2m - 1m
2m - 2g
2m - 1g
1m - 2g
1m - 1g
2g - 1g
1.8
5.0
6.5
6.6
8.4
9.3
3.3
4.7
4.8
6.6
7.5
-1.5
-0.1
1.8
2.7
-1.4
1.9
2.8
3.3
4.2
0.9
Simultaneous 95%
Confidence Limits
-2.5
1.3
2.2
2.9
4.1
5.6
-0.4
0.4
1.1
2.3
3.8
-4.5
-3.8
-1.9
-0.3
-5.1
-2.4
-0.9
-0.4
1.2
-2.8
6.1
8.8
10.8
10.3
12.7
13.1
7.0
9.0
8.5
10.9
11.2
1.6
3.6
5.5
5.8
2.4
6.3
6.6
7.0
7.3
4.7
*
*
*
*
*
*
*
*
*
*
1g = goat browsing in one year, 2g = goat browsing in two years
1m = mowing in one year, 2m = mowing in two years
1gm = goat browsing & mowing in one year, 2gm = goat browsing & mowing in two years
c = control (untreated)
* = significant at the 95% confidence level
Increases in primocane density for control plots, relative to most other
treatments, are likely an artifact due to the mowing conducted in June 2008 to
obtain biomass measurements. Sampling primocanes in plots that had not been
treated at all would have provided more useful information but this was not
possible due to the dense thickets.
43
It is noteworthy that two years of goat browsing followed by mowing
produced significant increases in primocane density (Table 2.5). Plots treated
in this manner experienced two disturbances per year and thus primocane
growth was stimulated. This result differs from mowing where all floricanes
are damaged. Although some floricanes persist, re-growth of more primocanes
in plots treated with mowing was expected. Destruction of floricanes is a
stimulus for the sprouting of new primocanes which are associated with
regeneration buds on perennating structures, rhizomes or roots (Kigel and
Koller, 1985).
The over-all trend in the pasture toward an increase in primocane density
indicates that the Himalayan blackberry plants are not dying but are allocating
resources differently by sprouting new primocanes after the destruction of
above ground biomass. An increase in primocane density after three years of
the study indicates that the blackberry ramets are adjusting to the treatment by
re-growth of many young canes rather than elongation of existing floricanes.
2.4.2.4 Node density
Changes in node density may indicate what is happening to underground
meristematic tissue in the crowns. This tissue is a vital source of vegetative regrowth (Amor, 1974a) particularly under the treatments investigated.
Mean node density increased for all treatments from 2006 to 2007 by 12
nodes/m2 (CL = 6.5, 17.5, df = 1, p < 0.001) at the 95% confidence level.
44
From 2007 to 2008, the trend was reversed with a decline of 8 nodes/m2 (CL =
-14.8, -0.3, df = 1, p < 0.01) at the 90% confidence level.
The trend in node density for mowing in two years is similar to the trend
in seedling density for the same treatment (Table 2.6).
Table 2.6 Node Density Pairwise Comparisons for All Treatments
Treatment
Comparison
Difference
Between Simultaneous 95%
Means
Confidence Limits
2m - 2gm
2m - 2g
2m - 1gm
2m - 1m
2m - c
2m - 1g
2gm - 2g
2gm - 1gm
2gm - 1m
2gm - c
2gm - 1g
2g - 1gm
2g - 1m
2g - c
2g - 1g
1gm - 1m
1gm - c
1gm - 1g
1m - c
1m - 1g
c - 1g
4.3
16.3
18.7
19.4
19.7
24.3
12.0
14.3
15.1
15.4
19.9
2.3
3.1
3.4
7.9
0.7
1.1
5.6
0.3
4.9
4.6
-9.7
2.3
6.5
7.2
5.6
12.1
-2.1
2.2
2.9
1.3
7.8
-9.9
-9.1
-10.7
-4.2
-9.2
-11.1
-4.3
-11.9
-5.1
-7.6
18.4
30.4 *
30.9 *
31.6 *
33.8 *
36.5 *
26.1
26.5 *
27.3 *
29.5 *
32.1 *
14.5
15.2
17.5
20.1
10.7
13.2
15.6
12.5
14.8
16.7
1g = goat browsing in one year, 2g = goat browsing in two years,
1m = mowing in one year, 2m = mowing in two years
1gm = goat browsing & mowing in one year, 2gm = goat browsing & mowing in two years
c = control (untreated)
* = significant at 95% confidence level
45
Two years of mowing produced substantial increases in node density
compared to five of six other treatments; ranging from 16-24 more nodes/m2.
Two years of goat browsing followed by mowing produced significant
increases in node density as well (Table 2.6). In the case of mowing or
browsing followed by mowing, mowing close the soil surface is the stimulus
for vegetative growth from rhizomes or crowns.
One cannot be certain that these node measurements are correlated with
below ground competition. This uncertainty is important to investigate as
Aerts et al. (1991) and Wilson (1988) found that below ground competition has
a greater effect on the balance between species than above ground competition.
Figure 2.2 summarizes the change in the population demographic of
Himalayan blackberry under the different treatments.
46
Figure 2.2 Population Demographic By Treatment
Density (stems/m2)
70
60
50
40
30
20
10
Br
o
Br
ow
sin
g
x1
Br
ow
ws
sin
in
g
g
x2
&
Br
M
ow
ow
sin
in
g
g
x1
&
M
ow
in
g
x2
M
ow
in
g
x1
M
ow
in
g
x2
U
nt
re
at
ed
0
Seedlings
Treatment
Primocanes
Floricanes
Total Canes
Examination of population data reveals patterns which are a critical part of
prediction as well as description in ecology (Weiner, 1995). Though
primocanes and floricanes are portions of ramets, and the seedlings are clearly
new genets in the population, there is value to this approach for examining the
growth of clonal organisms, or modular growth vs. new genets (Harper, 1980).
Clearly, the blackberry is phenotypically plastic and changes in both vegetative
and reproductive growth are responses to changes in grazing or mowing.
47
2.4.3 Implications
Many landowners report that pastures are improved by goat browsing,
including reducing Himalayan blackberry dominance. Effective control of a
target species includes matching the herbivore with the target plant by
palatability and browsing at a frequency and intensity that will be most
detrimental (Olson and Launchbaugh, 2006). Thus summer browsing of
Himalayan blackberry should be maintained for two reasons. First, browsing
after the onset of flowering will damage the plant by removing stems that
contain stored energy and nutrients (Hendrickson and Olson, 2006). Second,
mid-summer treatments occur when soil is at its driest of the year. However,
repeated browsing, or more continuous browsing through the summer season,
would likely yield greater benefit than short duration browsing.
Goat browsing may hold potential for Himalayan blackberry control if it is
applied repeatedly in the same growing season (Hendrickson and Olson, 2006)
and for more than two years. Goats select the highly palatable primocane regrowth which could ultimately prevent flowering and consume important
storage and meristematic tissue of the plants. The greatest benefit in using
short term-high intensity goat browsing would likely be use of the animals
after mowing. In this sequence, reversed from that investigated, mowing
would remove most of the above ground biomass, stimulate the growth of
primocanes which in turn would be consumed by the goats because they are
48
highly palatable. When goat browsing is applied to unmowed plots substantial
biomass, primarily floricanes, is left standing.
Despite a uniform soil series and adequate soil moisture for growth of the
Himalayan blackberry, large variation in the data collected make quantifying
change due to goat browsing and mowing difficult. It was not possible to
assess density prior to treatment since initial biomass measurements would
have necessitated destructive sampling. If pre-treatment data for density or
biomass were available, differences among treatments may have been
quantifiable. The short period of three years is also inadequate to fully
examine the impact of a change in herbivory regime on a long-lived,
competitive perennial such as Himalayan blackberry.
Harper (1980) challenged ecologists’ assumption that organisms are
peculiarly fitted to their environment. Harper’s idea has validity for native
plants that have evolved in an ecosystem over many generations. However,
the fact that many exotic invasive plants, including Himalayan blackberry,
thrive in alien habitats contradicts the theory. Like other invasive plants, this
exotic species has exploited many niches to the point of becoming naturalized
(Cousens and Mortimer, 1995) throughout the Pacific Northwest.
Himalayan blackberry is highly successful in its alien environment and is
therefore fit to exploit resources for maximum growth. If its relatively large
underground root mass confers the advantage that the native dwarf bramble
49
(Rubus lasiococcus) has for survival in drier sites (Antos, 1988), managers will
be further challenged to control invasions by this species.
Control of Himalayan blackberry is essential because it forms
monocultures which reduce species richness and functional diversity indicating
alteration of ecological function (Whisenant, 1999). In the case of the pasture,
Himalayan blackberry presence limits productivity of grasses which are the
primary forage for cattle. The thickets of Himalayan blackberry also alter
patterns of use by herbivores of the areas which do contain palatable
vegetation.
The control methods employed in this study may have nudged this plant
community along a pathway toward more desirable vegetation. There certainly
has not been a change to new stable state (Bestelmeyer at. al., 2003), however,
because the underground meristematic tissue has not been sufficiently stressed.
The shift in the population dynamic toward younger individual plants within
the study indicates that, although the canopy has been removed, re-invasion is
likely.
50
2.5 Acknowledgements
Funding for this research was provided in part by a grant from NRCS
(Natural Resources Conservation Service). The author thanks B. and S. Hoyt
of the Hawley Land and Cattle Co. for hosting the research on their ranch, for
herding the goats for browsing treatments and for mowing at the research site.
L. Rummel and K. Townsend assisted with installation of the plots. B.
Vogeney, K. Townsend, L. Rummel, L. G. Rummel, and L. R. Rummel
assisted with stem counts and diameter measurements.
2.6 Literature Cited
Aerts, R., R. G. A. Boot and P. J. M. van der Aart. 1991. The relation
between above- and belowground biomass allocation patterns and
competitive ability. Oecologia 87: 551-559
Amor, R. L. 1974(a). Ecology and control of blackberry (Rubus fruticosus L.
agg.) II: Reproduction. Weed Research 14: 231-238.
Amor, R. L. 1974(b). Ecology and control of blackberry (Rubus fruticosus L.
agg.) III: Response of R. procerus to mechanical removal of top growth
and to foliage applied herbicides. Weed Research 14: 239-243.
Antos, J. A. 1988. Underground morphology and habitat relationships of
three pairs of forest herbs. American Journal of Botany 75(1): 106-113.
Archer, A. and C. Stokes. 2000. Stress, disturbance and change in rangelands.
Pages 17-38 in: O. Arnolds and S. Archer, eds., Rangeland
Desertification. Boston, MA: Kluwer Academic Press.
Bazzaz, F. A. 1987. Experimental studies on the evolution of niche in
successional plant populations. Pages 245-272 in: A.J. Gray, M.J.
51
Crawley and P.J. Edwards, eds. Colonization, Succession and Stability.
Oxford, England: Blackwell Scientific Publications.
Bestelmeyer, B. T., J. R. Brown, K. M. Havstad, R. Alexander, G. Chavez and
J. E. Herrick. 2003. Development and use of state-and-transition models
for rangelands. Journal of Range Management 56: 114-126.
Bruzzese, E. and M. Lane. 1996. The Blackberry Management Handbook.
East Melbourne, Australia: Department of Conservation and Natural
Resources. 50 p.
Cousens, R. and M. Mortimer. 1995. Dynamics of Weed Populations. New
York: Cambridge University Press. 332 p.
Grime, J. P. and J. G. Hodgson. 1987. Botanical contributions to
contemporary ecological theory. Pages 283-296 in: I. H. Rorison, J. P.
Grime, R. Hunt, G.A.F. Hendry, D. H. Lewis, eds. Frontiers of
Comparative Plant Ecology. New Phytologist 106.
Harper, J. L. 1980. Plant demography and ecological theory. Oikos 35: 244253.
Hawkes, C. V. and J. J. Sullivan. 2001. The impact of herbivory on plants in
different resource conditions: a meta-analysis. Ecology 82(7): 2045-2058.
Hendrickson, J. and B. Olson. 2006. Understanding Plant Response to
Grazing. Pages 32-39 in K. Launchbaugh, ed., Targeted Grazing: A
natural approach to vegetation management and landscape enhancement.
Centennial, CO: American Sheep Industry Association.
Hobbs, R. J. and L. F. Huenneke. 1992. Disturbance, diversity, and invasion:
implications for conservation. Conservation Biology 6(3): 324-337.
Kigel, J. and D. Koller. 1985. Asexual reproduction of weeds. Pages 65-100
in S. O. Duke, Weed Physiology (vol 1): Reproduction and
Ecophysiology. Boca Raton, FL: CRC Press.
Lambrecht-McDowell, S. and S. Radosevich. 2005. Population
demographics and trade-offs to reproduction of an invasive and
noninvasive species of Rubus. Biological Invasions 7:281-295.
McDowell, S. L. 2002a. Life-history and Physiological Trade-offs to
Reproduction of Invasive and Noninvasive Rubus. PhD. Dissertation.
Oregon State University.
52
McDowell, S.C.L. 2002b. Photosynthetic characteristics of invasive and
noninvasive species of Rubus (Rosaceae). American Journal of Botany
89(9): 1431-1438.
Olson, B. and K. Launchbaugh. 2006. Managing herbaceous broadleaf weeds
with targeted grazing. Pages 58-67 in K. Launchbaugh, ed., Targeted
Grazing: A natural approach to vegetation management and landscape
enhancement. Centennial, CO: American Sheep Industry Association.
Radosevich, S. R., J. S. Holt and C. M. Ghersa. 2007. Ecology of Weeds and
Invasive Plants: Relationship to Agriculture and Natural Resource
Management. Hoboken, N.J: Wiley-Interscience.
Rejmánek, M. 1989. Invasibility of plant communities. Pages 369-388 in J.
A. Drake, H. A. Mooney, F. di Castri, R. H. Groves, F. F. Kruger, M.
Rejmánek and M. Williamson, eds. Biological Invasions: a global
perspective. Chichester, England: Wiley.
SAS 9.1.3 Service Pack 4. 2003. SAS Institute Inc. Cary, NC.
Sheley, R., T. J. Svejcar and B. M. Maxwell. 1996. A theoretical framework
for developing successional weed management strategies on rangeland.
Weed Technology 10: 766-773.
Soil Data Mart. 2008. Natural Resources Conservation Service.
http://soildatamart.nrcs.usda.gov/Survey.aspx?County=OR039. Accessed
October 7, 2008
Weiner, J. 1995. On the practice of ecology. Journal of Ecology 83: 153-158.
Westoby, M., B. Walker and I. Noy-Meir. 1989. Range management on the
basis of a model which does not seek to establish equilibrium. Journal of
Arid Environments 17: 235-239.
Wilson, J. B. 1988. Shoot competition and root competition. Journal of
Applied Ecology 25: 279-296.
53
3. FUNCTIONAL GROUP AND SPECIES CHANGE IN HIMALAYAN
BLACKBERRY (RUBUS ARMENIACUS) AND
EVERGREEN BLACKBERRY (RUBUS LACINIATUS)-INVADED
PASTURE FOLLOWING GOAT BROWSING AND MOWING
3.1 Abstract
Himalayan blackberry (Rubus armeniacus Focke) is a non-native shrub
which has invaded sites throughout the Pacific Northwest since its
introduction to North America. Its persistent canopy and large underground
crowns create a competitive environment which prevents desirable species
from germinating and/or establishing. Evergreen blackberry (Rubus
laciniatus Willd.), an exotic plant introduced from Europe, grows in
association with Himalayan blackberry and control efforts frequently target
both species. Control of Himalayan blackberry is affected by vigorous
vegetative re-growth after mechanical control and variable response to
chemical methods. Recent interest in goat browsing to control these and other
invasive plants has led to a variety of long- and short-term methods to control
unwanted plants through disturbance by herbivory.
This study examines changes in plant functional group percent cover in a
perennial grass pasture invaded by Himalayan blackberry and evergreen
blackberry in the southern Willamette Valley of Oregon. The appearance of
species, and their functional group membership, following three treatment
54
protocols were evaluated as indicators of the need for active vs. passive
restoration. Changes in the percent cover of Himalayan and evergreen
blackberry, annual grasses, perennial grasses, annual forbs and perennial forbs
were examined after two annual treatments with; 1) high intensity-short
duration goat browsing, 2) mowing and 3) high intensity-short duration goat
browsing followed by mowing. These data were assessed to examine trends
in re-vegetation of the pasture.
All treatments caused a significant decline in percent cover of the invasive
blackberries (p < 0.0001) but differences among treatments were not detected.
All treatments produced increases in perennial grass cover but differences
among treatments were not different. All treatments also resulted in increases
in the percent cover of perennial forb. Plots treated with goat browsing
followed by mowing showed significant increases relative to other treatments
(p = 0.008). Changes in percent cover of other functional groups were not
different with browsing or mowing treatments. The presence of individual
species within the perennial grass and perennial forb groups and their
potential influence on restoration are discussed.
Key Words: Himalayan blackberry (Rubus armeniacus Focke), evergreen
blackberry (Rubus laciniatus Willd.), functional group change, perennial
pasture, high intensity-short duration browsing, goats, mowing, restoration
55
3.2 Introduction
Himalayan blackberry is an invasive shrub, native to Asia and introduced
to North America via England (Mathews, 1992; Pojar and MacKinnon, 1994).
This species is increasingly problematic in high rainfall areas of the Pacific
Northwest where its prolific growth in pastures, forests and riparian areas
suppresses the growth of other plants. Himalayan blackberry and evergreen
blackberry grow in association. Both have escaped from horticultural
production (Gilkey and Dennis, 1980). They are considered together in this
study although Himalayan blackberry is the predominant pest species at the
study site.
In Oregon, Himalayan blackberry is designated as a “B” noxious weed
(Oregon Department of Agriculture, 2006) and was the most prevalent exotic
species in forested plots recently surveyed by the U.S. Forest Service (Rapp,
2005). Another survey of riparian sites along 145 km (87 mi) of the
Willamette River revealed that 89% of 28 sites had Himalayan blackberry
present (Fierke and Kauffman, 2006). Rapid dry matter production, rapid
stem extension and leaf production of many shrubs and trees, including
members of the Rubus genus, pose challenges to forest regeneration in
northwestern North America (Radosevich et al., 2007) and influence invasion
of pastures.
Physiological traits of Himalayan blackberry which contribute to its
success include fitness homeostasis (Hoffman and Parsons, 1991), high leaf
56
area ratio and a short juvenile period (Rejmanek, 2000). Himalayan
blackberry reproduces by seed and grows vegetatively by clones at cane tips,
sprouting of adventitious roots on lateral roots (rhizomes) and regeneration
from severed pieces of canes and roots (Amor, 1974).
Resource use also contributes to species function within ecosystems
(Hobbs, 1992). McDowell (2002) reports that non-native Rubus species,
including Himalayan blackberry, have higher photosynthetic capacities than
native Rubus species which allows the non-native species to fix significantly
more carbon than the native species. However, Himalayan blackberry is a
poor functional replacement for a diverse native plant understory in open
forests or meadows (Soll, 2004). Its ability to form homogeneous cover,
inhibit germination, and reduce the establishment of native species can initiate
alternative successional pathways (Fierke and Kauffman, 2006) on invaded
sites.
In order to design successful control strategies, consideration of how a
species functions in its environment is critical. In addition, site specific
knowledge allows managers to adjust management to enhance desirable
species survival and choose species that will fill the niche vacated when the
target species cover or density is reduced. Species inventories, and
subsequent functional group classifications, should assist in meeting these
goals. Westoby et al. (1989) emphasize that priority in competition is a
mechanism influencing the presence of alternate stable states. In the case of
57
invaded pastures, the abundance of blackberry propagules after treatment
carries the risk that only competitive plants (Grime, 1977) will survive in the
long-term.
In pastures where forage production is an obvious goal, managers need to
consider how grasses function when undertaking weed control efforts.
Control efforts stimulate feedback within the plant community which
increases the probability of invasion by new weeds (Radosevich et al., 2007).
If a target plant is suppressed but not eliminated and ecological processes are
not altered by physical or biological changes to the site, reinvasion and/or
invasion by another undesirable species is likely (Sheley and KruegerMangold, 2003).
Challenges to traditional control methods have led managers to seek
alternatives such as the use of biological agents. For example, blackberries
are so problematic in Australia that the Commonwealth Biological Control
Act of 1984 was invoked to allow release of a biological control agent (Sindel,
2000). Commercial berry production is an important part of Oregon’s
agricultural economy, however, and an intentional release of a biocontrol
agent in Oregon is unlikely. Phragmidium violaceum is a fungal pathogen of
many species of Rubus and this pathogen now infects both Himalayan and
evergreen blackberry in Oregon (Aime and Rossman, 2007). The presence of
Phragmidium violaceum may influence management of these pest plants in
the future but it was not a factor in this study.
58
Himalayan blackberry, with morphological features like thorns that
prevent herbivory (Bruzzese and Lane, 1996), is especially challenging in
cattle pastures. This dilemma enhances the competitive capacity of
Himalayan blackberry to dominate an area and spread to adjacent land
(Sindel, 2000). Goats are a viable management alternative due to their
preference for browse and prehensile tongues which allow efficient
consumption of young stems and leaves (Campbell and Taylor, 2006).
Plant community response to high intensity-short duration goat browsing,
mowing and a combination of the two treatment methods are examined in this
study. It was conducted from 2006 to 2008 in the southern Willamette Valley
of Oregon. If plant functional group diversity rebounds after treatment it is
possible that augmenting such diversity will assist restoration because the
change reflects ecosystem processes (Pokorny et al., 2005). However, strict
interpretation of plant community health by the presence of numerous
functional groups ignores the risks created by individual invasive species
presence. To avoid the potential for invasion with a different plant, careful
monitoring of the site to indentify emergent species and their evaluation based
on biological and ecological characteristics should lead to more successful
management of plant communities along a desired trajectory (Bestelmeyer et
al., 2003). Applied functional types (Woodward et al., 1997) may emerge as
the best approach for evaluating responses of Himalayan blackberry to
herbivory and mechanical methods. Species composition within functional
59
groups could then be used to design other control efforts or the addition of
desirable species propagules.
3.3 Materials and Methods
3.3.1 Site description and study design
This study was conducted south of Cottage Grove in Oregon’s Willamette
Valley from May 2006 through September 2008. The research site was
located in a north aspect pasture with Nekia silty clay loam soil with 30 to 50
percent slopes at elevations of 100 to 135 meters (330 to 445 feet). The
annual mean daily temperature is 17.2˚C (63.0˚). December is the coldest and
wettest month with a daily average temperature of 4.2˚C (39.8˚F) and 211mm
(8.29 in) of precipitation. July is the driest month with only 16mm (0.64 in)
of rainfall and August is the warmest month with average temperatures of
19.1˚C (66.4˚F) (National Climate Data Center, 2008).
The pasture has been grazed by cattle since the early 1900s but was not
grazed by goats prior to this study. A portion of the pasture was planted with
subterranean clover (Trifolium subterraneum) and tall fescue (Schedonorus
phoenix) in the mid-1970s, after excavation and installation of a natural gas
pipeline. Since that time, neither pasture improvement nor weed management
has been undertaken.
Location of the research plots was based on a visual estimate of 95-100%
cover and a height of at least 1m of Himalayan blackberry in the spring of
60
2006. Using a completely randomized design, treatments were assigned to
each of the plots.
3.3.2 Treatments
The three treatments included browsing, mowing and browsing plus
mowing for two years. Each treatment had four replicates. Thus, three
treatments with four replicates of each meant that 12 plots were considered in
the analyses.
Each plot was 36 m2 area with a 2m buffer extending out from the
perimeter of each plot to ensure that no interactions occurred between plots.
The buffer also allowed for the installation of temporary electric fencing to
confine goats to browsed plots.
The mowing treatment was applied in July of 2006 and 2007 after the
onset of flowering of the Himalayan blackberry. A Bobcat® single blade
deck mower was used to cut the canes to 5-10 cm from the soil surface. For
the browsing treatment, temporary electric fencing was installed around each
plot. Goat does and their kids were then herded onto the appropriate plots and
allowed to browse the area for approximately one day. This stocking rate was
equivalent to 988 head/hectare (400 head/acre). Animals were removed from
the plots after plants were defoliated and younger canes eaten resulting in the
herd showing little interest in continued browsing at that location. For the
third treatment type, browsing and mowing, goat browsing was applied as
61
described above but the same plots were mowed within one week of
browsing.
3.3.3 Species composition and functional groups
In June of 2007 and 2008 plant species composition was recorded for nine
randomly-selected 1m2 sub-plots. Metal t-posts delineated the four corners of
each plot and wooden stakes marked the corners of the sub-plots. A grid
numbered from left to right and top to bottom was used to delineate the 36
1m2 sub-plots. All species present and their percent cover were recorded.
After data collection in June 2007, functional groups were determined based
on the functional characteristics of the species present. The eight functional
groups were: 1) invasive non-native Rubus shrubs including Himalayan
blackberry and evergreen blackberry, 2) other shrubs and trees, 3) annual
grasses, 4) perennial grasses, 5) annual and biennial forbs, 6) perennial forbs,
7) ferns and 8) mosses. Additionally, cane litter and bare ground were
recorded.
62
3.3.4 Statistical model and analysis
The following General Linear Model (GLM) was fit to the data to test
differences among treatments in changing the percent cover of plant
functional groups.
Уij = ų + αi + єij
Уij = change in % cover of a functional group from 2007 to 2008
i = trt type (browsing in 2 years, mowing in 2 years, browsing
followed by mowing in 2 years)
j = plot (replicate 1, 2, 3, 4)
Ų = grand mean of % change in functional group percent cover
αi = difference in functional group % cover for the ith treatment relative to
the grand mean
єij = random variation of ith trt, jth plot that adds variability to the value of Уij,
єij N(0, δ2 ), єij and єi’j’ are independent.
The primary predictor was treatment; goat browsing, mowing or goat
browsing followed by mowing. The Tukey-Kramer adjustment for multiple
comparisons was used to compare percent change among the three treatment
types for the different plant functional groups.
The SAS 9.1.3 Statistical Package was used for all data analyses.
63
3.4 Results
The species inventory for the fall of 2006 (Table 3.1) includes all species
present after one application of the three treatments. Other than the invasive
blackberries, species included two native shrubs, four native fern and fern-like
species and the perennial forb, Canada thistle (Cirsium arvense).
Table 3.1
Species Inventory for Fall 2006
Rubus armeniacus (Himalayan blackberry)
Rubus laciniatus (evergreen blackberry)
Prunus emarginata (cherry)
Rosa nutkana (wild rose)
Cirsium arvense var. horridum (Canada thistle)
Blechnum spicant (deer fern)
Equisetum telmateia subsp. braunii (giant horsetail)
Polystichum munitum (sword fern)
Pteridium aquilinum subsp. pubescens (bracken fern)
All species present in June of 2008, after two applications of treatments,
are organized by functional group and listed in Table 3.2. Increases in species
diversity are particularly evident in the perennial grasses, annual forbs and
perennial forbs.
64
Table 3.2
Species by Functional Group
Scientific Name
Common Name
Status
and
Growth
N = native
or I =
introduced
r = rhizome
or s = stolon
Invasive shrubs
Rubus armeniacus
Rubus laciniatus
Himalayan blackberry
evergreen blackberry
I, r
I
Shrubs & Trees
Prunus emarginata
Rosa nutkana
Rubus ursinus
Symphoricarpos mollis
Acer macrophyllum
Pseudotsuga menziesii
wild cherry
nootka rose
trailing blackberry
creeping snowberry
bigleaf maple
Douglas fir
N
N, r
N
N, r
N
N
Annual Grasses
Cynosurus echinatus
dogtail chess
I
Perennial Grasses
Agrostis spp.
Briza minor
Dactylis glomerata
Elymus glaucus
Holcus lanatus
Phleum pratense
Poa pratensis
Schedonorus phoenix
bent grass
little quaking grass
orchardgrass
wild rye
velvet grass
timothy
Kentucky bluegrass
tall fescue
N
I
I
N
I
I
I
I
Ferns
Blechnum spicant
Pteridium aquilinum subsp. pubescens
Polystichum munitum
Equisetum telmateia subsp. braunii
deer fern
bracken fern
sword fern
giant horsetail
N
N
N
N
65
Table 3.2
Species by Functional Group (cont.)
Scientific Name
Common Name
Status
and
Growth
N = native
or I =
introduced
r = rhizome
or s = stolon
Annual (and Biennial) Forbs
Brassica nigra
Cirsium vulgare
Daucus carota
Dipsacus sylvestris
Epilobium spp.
Galium aparine
Geranium dissectum
Geranium molle
Medicago polymorpha
Myosotis arvensis
Nemophila parviflora
Parentucellia viscosa
Solanum spp.
Sonchus oleraceus
Stellaria media
Perennial Forbs
Cirsium arvense var. horridum
Heracleum maximum
Hypericum perforatum
Leucanthemum vulgare
Lomatium spp.
Hypochaeris radicata
Ranunculus occidentalis var. occidentalis
Ranunculs repens
Rumex crispus
Senecio jacobaea
Taraxacum officinale
Tellima grandiflora
Trifolium pratense
Trifolium repens
Urtica dioica subsp. gracilis
Vancouveria hexandra
Veronica spp.
Vicia spp.
black mustard
bull thistle
Queen Anne's lace
common teasel
willow herbs
cleavers
cutleaf geranium
dove's-foot geranium
bur clover
common forget-me-not
small –flowered
nemophila
yellow glandweed
nightshade
common sow thistle
common chickweed
Canada thistle
cow parsnip
St. John's-wort
ox-eye daisy
lomatium
false dandelion
Western buttercup
creeping buttercup
curly-leaved dock
tansy ragwort
common dandelion
fringecup
red clover
white clover
American nettle
inside out flower
speedwell
Vetch
I
I
I
I
N
N
I
I
I
I
N
I
I
I
I
I, r
N
I
I
?
I
N
I, s
I
I
I
N, r
I
I
N
N, r
?
?
66
Results of the analysis of percent cover of the various functional groups
are reported at the 95% (α = .05) confidence level.
The mean percent cover of invasive Rubus species for all three treatments
declined significantly from approximately 95% in 2006 to 24% (CL = 18.9,
29.2, df = 2, p < 0.0001) in 2007. Mean percent cover of invasive Rubus
species was 46% (CL = 31.8, 59.1, df = 2, p < 0.0001) in 2008 which was a
significant increase from the previous year (Figure 3.1).
Figure 3.1 Mean % Cover Rubus for All Treatments
110
100
90
% Cover Rubus
80
70
60
50
40
30
20
10
0
2006
Error Bars = 1 SE
2007
2008
67
Differences among responses to the three treatments were highly variable
yet plots mowed twice did show a decline of 28% (CL = -54, - 1, df = 5, p =
.09, α = .95) Himalayan blackberry cover relative to plots treated once with
mowing. In 2007, the percent cover of Rubus in twice mowed plots was only
26% compared to 55% Rubus cover in plots mowed once.
All changes in cover for functional groups other than the invasive Rubus
were examined from 2007 to 2008, but not 2006 to 2007 because non-Rubus
groups were not evident in the tall, dense Himalayan blackberry thickets
present in spring 2006.
No annual grasses were identified in 2007 but the presence of the annual
dogtail chess (Cynosurus echinatus) in 2008 is discussed in Section 3.5.
Perennial grass cover increased for all treatments but the level of increase
from year to year was not statistically different among treatments (Figure 3.2).
68
40
Figure 3.2 Change in Perennial Grass % Cover
by Treatment
35
30
% Cover
25
20
2007
2008
15
10
5
0
Goat Brow sing x 2
Goats & Mow ing x 2
Mow ing x 2
Error Bars = 1 SE
Species present in the perennial grass functional group and their potential
to compete with the invasive blackberries are discussed in Section 3.5.
Changes in annual forb percent cover were not significant among the three
treatment types.
All treatments applied in two years resulted in an increase in percent cover
of perennial forbs (Figure 3.3). Goat browsing and mowing in two years
showed the most dramatic increase. This change was significantly greater
than the other two treatments at the 95% confidence level (p = .008). Species
composition of this functional group is discussed in Section 3.5
69
Figure 3.3
Change in Perennial Forb % Cover by
Treatment
30
% Cover
25
20
15
10
5
0
Goat Browsing x Goat Browsing
2
+ Mowing x 2
Treatment
Mowing x 2
2007
2008
Error Bars = 1SE
The shrub and tree group (not including the invasive species of Rubus) and
the fern and moss group had low abundance and thus were not evaluated using
a General Linear Model. Changes in the percent cover of bare ground were
not significant among the three treatments.
70
3.5 Discussion
Conditions favoring exotic plant invasion were likely present for decades
prior to the start of this study in 2006. Although excavation for and
installation of a pipeline in the mid-1970s was followed by seeding of tall
fescue and subterranean clover, a disturbance of this magnitude (Grime, 1977)
likely created safe sites for propagules of many ruderal species. Because all
vegetation was removed in the short term, the availability of limiting
resources was altered which increased vulnerability to invasion (Davis et al.,
2000). There was obviously a good fit between Himalayan blackberry
propagules and niches in this particular habitat (Alpert et al., 2000) as seeds
were dispersed by birds, and to a lesser extent by deer, and cattle (personal
observation) that consumed the berries. Once a source population was
established, satellites appeared in the vicinity progressing to the point where
thickets were present on a landscape scale as is common throughout the
Pacific Northwest.
Continuous cattle grazing in the pasture produced frequent and intense
disturbance (Archer and Stokes, 2000) that allowed Himalayan blackberry to
further colonize (Cousens and Mortimer, 1995) because grasses are the
primary plant functional group consumed by cattle. The stresses of low water
availability and low nutrient levels do not appear to be large factors at this
site. Ferns are present in many of the plots and the growth of agronomic grass
71
species, including orchardgrass (Dactylis glomerata), timothy (Phleum
pretense) and Kentucky bluegrass (Poa pratensis), is vigorous.
The removal of the Himalayan blackberry canopy by mowing or goat
browsing can produce immediate benefits for other species as light becomes
available. With the increase in light reaching the soil surface, particularly in
the far-red wave lengths, seeds are stimulated to germinate (Radosevich et al.,
2007). This was evident in this study wherein all three treatments resulted in
reduced blackberry canopy and the appearance of species not present in the
spring of 2006. The total number of species present changed from nine in
2006, to 25 in 2007, and 46 in 2008.
Three functional groups were evident in 2006 which increased to seven in
2008. Though functional group diversity has been confirmed as a factor in
resistance to invasion (Pokorny et al., 2005), degraded landscapes may behave
in different ways and close attention to the characteristics of species that
emerge is critical for directing succession (Wright et al., 2006).
Canada thistle was the dominant species within the perennial forbs. Its
abundance confirms movement along a trajectory toward invasion by another
weed when ecological processes are not addressed (Sheley and KruegerMangold, 2003). Management must address control of this species or it will
simply replace the invasive blackberries. If not addressed, Canada thistle will
function like the blackberries with its rhizomatous growth, prolific seed
production and resistance to herbivory by cattle.
72
The cane litter and bare ground categories represent unfilled niches which
can provide safe sites (Harper, 1977). In essence, the area occupied by these
two categories represents opportunities for seed germination and
establishment of recorded species or species not currently present. With
removal of the Himalayan blackberry canopy, the abundance and
characteristics of the safe sites within the seedbed are altered (Whisenant,
1999). The bare ground present in the plots was most often due to burrowing
rodents with no vegetation present even when upper layers of soil were moved
with a hand tool.
Because blackberry plants are not necessarily killed by the treatments, the
occupation of soil space by large crowns remains a concern for the
establishment of individual plants of desirable species. The emergence of
orchardgrass in the plots after treatment is of interest because the
establishment of competitive grasses may reduce encroachment by noxious
weeds (Sheley et al., 2002). According to the ranch owners, orchardgrass was
not intentionally seeded but its presence is not unusual. This perennial grass
is frequently seeded in pastures and in wildland re-seedings (Stubbendieck et
al., 2003). It was probably transported to the pasture by wind, wildlife or
possibly the browsing goats. It is the dominant grass in the perennial grass
functional group. Its shade tolerance (Stubbendieck et al., 2003) and
temperature dependent flowering (Evans, 1958) may allow it to persist despite
the competitive environment with Himalayan blackberry and Canada thistle.
73
Orchardgrass is not rhizomatous but its presence is likely supported by the
available nitrogen from the blackberry cane litter.
Borman et al. (1991) identified the need for perennial grass species to
suppress introduced and undesirable annual grasses in southwest Oregon.
Although the research site here is not plagued by annual grasses, it is in a
climatic transition zone and the presence of dogtail chess points to the need
for competitive grass species for restoration. In addition to early and
continued growth into winter, as identified by Borman (1989) and Borman et
al. (1990), grasses seeded in this pasture should have rapid root elongation and
rhizomatous growth to occupy root space.
Although many analyses reveal the limited utility of a priori classification
systems for functional groups (Wright et al., 2006), the post hoc grouping of
emergent species still uses assumptions common to traditional ecological
groupings. These include plant longevity (annual vs. perennial) and growth
form (shrub, forb, grass). This classification is slightly improved by
consideration of individual species and their invasion potential as is done in
this dissertation.
Managers seek predictability of plant community response. Spatial
replication of experiments like this one may allow development of needed
habitat specific predictions (Rejmánek, 2000) by evaluating emergent species
at a site. Assessing the availability of desired species propagules to fill niches
vacated by the target of control and resources at that site are also important.
74
Perhaps a quantification of nutrient availability and propagule survival due to
micro-topography would reveal differences that could further inform
management practices. Without attention to the trajectory of the plant
community, including functional group and characteristics of member species,
one cannot be certain that changes will occur which support new and desirable
stable plant communities.
3.6 Acknowledgements
Funding for this research was provided in part by a grant from NRCS
(Natural Resources Conservation Service). The author thanks B. and S. Hoyt
of the Hawley Land and Cattle Co. for hosting the research on their ranch in
Cottage Grove, Oregon and for herding the goats for browsing treatments and
for mowing at the research site. L. Rummel and K. Townsend assisted with
installation of the plots. B. Vogeney, K. Towns9end, L. Rummel, L.
Kossiakoff and R. Ingham assisted with species composition data collection. J.
Knurowski, Botanist at the BLM Roseburg District Office, verified species
identification.
75
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agg.) III: Response of R. procerus to mechanical removal of top growth
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Archer, A. and C. Stokes. 2000. Stress, disturbance and change in
rangelands. Pages in: O. Arnolds and S. Archer, Rangeland
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Bestelmeyer, B. T., J. R. Brown, K. M. Havstad, R. Alexander, G. Chavez and
J. E. Herrick. 2003. Development and use of state-and-transition models
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Borman, M. M. 1989. Growth characteristics and site potentials of perennial
grass species. PhD Thesis. Oregon State University, Corvallis.
Borman, M. M., W. C. Krueger and D. E. Johnson. 1990. Growth patterns of
perennial grasses in the annual grassland type of Southwest Oregon.
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Borman, M. M., W. C. Krueger and D. E. Johnson. 1991. Effects of
established perennial grasses on yields of associated annual weeds.
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Bruzzese, E. and M. Lane. 1996. The Blackberry Management
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Natural Resources. 50 p.
Campbell, E. and C. A. Taylor. 2006. Targeted grazing to manage weedy
brush and trees. Pages 77-87 in K. Launchbaugh, ed., Targeted Grazing:
A natural approach to vegetation management and landscape
enhancement. Centennial, CO: American Sheep Industry Association
Cousens, R. and M. Mortimer. 1995. Dynamics of Weed Populations.
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Davis, M. A., J. P. Grime and K. Thompson. 2000. Fluctuating resources in
plant communities; a general theory of invasibility. Journal of Ecology
88: 528-534.
Evans, M. W. 1958. Growth and development in certain economic
grasses. Agronomy Series No. 147. Wooster, OH: Ohio Agricultural
Experiment Station. 112 p.
Fierke, M.K. and J.B. Kauffman. 2006. Invasive species influence riparian
plant diversity along a successional gradient, Willamette River, Oregon.
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Gilkey, H. M. and L. R. Dennis. 1980. Handbook of Northwestern Plants.
Corvallis, OR: Oregon State University. 507 p.
Grime, J. P. 1977. Evidence for the existence of three primary strategies in
plants and its relevance to ecological and evolutionary theory. The
American Naturalist. vol. 111(982): 1169-1194.
Harper, J. L. 1977. Population Biology of Plants. Academic Press, New
York. 892 p.
Hobbs, R. J. and L. F. Huenneke. 1992. Disturbance, diversity, and invasion:
implications for conservation. Conservation Biology 6(3): 324-337.
Hoffman, A. A. and P. A. Parsons. 1991. Evolutionary Genetics and
Environmental Stress. Oxford: Oxford University Press.
Mathews, D. 1992. Cascade-Olympic Natural History. Hong Kong:
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noninvasive species of Rubus (Rosaceae). American Journal of Botany
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Pojar, J. and A. MacKinnon (eds). 1994. Plants of the Pacific Northwest
Coast. Lone Pine Publishing, Vancouver.
Pokorny, M. L., R. L. Sheley, C. A. Zabinski, R. E. Engel, T. J. Svejcar and J.
J. Borkowski. 2005. Plant functional group diversity as a mechanism for
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79
4. ENGLISH IVY (HEDERA HELIX) RESPONSE TO HIGH
INTENSITY-SHORT DURATION GOAT BROWSING
4.1 Abstract
English ivy (Hedera helix) is an exotic liana which invades disturbed
forest sites in North America and Europe. It forms dense monocultures on the
forest floor as well as climbing trees as a vine. This study evaluates the
effectiveness of high intensity-short duration goat browsing to control English
ivy in an old field site in the Willamette Valley of Oregon. Species
composition and percent cover were determined in August 2006 prior to the
first browsing treatment. Experimental plots were monitored before and after
treatment in 2006 and 2007 and a final assessment was made in July 2008.
The effects of browsing were evaluated using comparison of multiple means
statistical methods. Plots browsed once or twice showed a significant decline
in percent cover of English ivy compared to untreated plots. The difference
between levels of browsing was also significant indicating that the repetition of
browsing for a second year is warranted. Change in species composition was
minimal with the appearance of sword fern and removal of Himalaya
blackberry occurring over the three years of the study.
Nomenclature: English ivy, Hedera helix L., goats
80
Key words: English ivy, goats, high intensity-short duration browsing, old
field plant community, mixed conifer-broadleaf tree canopy, invasive species
control
4.2 Introduction
English ivy (Hedera helix) is a temperate liana native to Eurasia which
was introduced to North America in the colonial era as a ground cover. It is
now naturalized in both the Eastern and Western United States (Reichard,
2007). Invasion of forest fragments and disturbed forests by English ivy is a
problem in the Mid-Atlantic and Pacific Northwest of the United States
(Randall, 1996) as well as in Europe (Butaye et al., 2001 and Laskurain et al.,
2004). English ivy is classified as a B list noxious weed in Oregon (ODA
Plant Division, 2008) because it is of economic importance but is only
regionally abundant, specifically west of the Cascade Mountains.
Sampling of invasive populations in British Columbia, Washington and
Oregon revealed that 83% of 58 populations were actually Hedera hibernica
(Kirchner) Bean rather than Hedera helix (Clarke et al. 2006). Though the
specific epithet of most of Oregon’s English ivy is not known, these invasive
plants are referred to as English ivy (Hedera helix) by horticulturalists and
naturalists.
Control of English ivy is challenging in forests where trees have been cut
and the open canopy allows new source populations to arise from bird
endozoochory (Butaye et al., 2001). High seed survival and seedling
81
establishment have been attributed to the heavier weight of Hedera helix seeds
compared to native species as well as drought and shade tolerance (Laskurain
et al., 2004). A study of seedling dynamics in southern Spain, with similar
climatic conditions to the Pacific Northwest (165 cm annual precipitation,
mean annual temperatures of 11°C at 43° N latitude), revealed that 98% of
emergent seedlings survive to six months and 96% of those survive to one year
(Laskurain et al., 2004). Zotz et al. (2006) assert that ivy suppresses
regeneration of native herbs and tree seedlings. Growth is primarily
vegetative, including stolons and subsequent clonal production, which
contribute to its invasiveness with densities up to 25,000 plants/ha (10,000
plants/ac) (USDA Plants Database, 2008). Forest patches with a history of
logging and plantation where H. helix was present had significant (.001 ≤ p ≥
.01) reductions in species richness (Butaye et al., 2001).
Several methods have been used to control English ivy including herbicide
application, mechanical methods and livestock browsing. Dicamba,
glyphosate, glycine, triclopyr amine, acetic acid and 2,4-D amine were tested
as control agents by Derr (1993). He finds that only glyphosate reduced new
shoot growth and that English ivy is tolerant of pre-emergent herbicides (Derr,
1992). Spring application of glyphosate at a rate of 3.1 kg/ha (2.7 lbs/ac) can
control the juvenile growth form of this species (Reichard, 2007). However,
Spring is generally when rainfall is high and desirable herbaceous plants are
also vulnerable. The waxy leaves of English ivy also challenge the practicality
82
of herbicide use but Soll (2005) found that addition of surfactant aids
penetration. To control vines, a much higher rate and two applications of
glyphosate at 4.5 kg/ha (4 lb/ac) are needed (Soll, 2005).
Although chemical control methods have been widely used, there are
locations where their use is not practical due to susceptibility of desirable
species and public sensitivity to the use of herbicides. Mechanical methods,
including hand-pulling, are also used. In 1999, more than 55,000 hours of
mostly volunteer labor was expended to remove this plant from parks and open
spaces in the Portland metropolitan area (Anonymous, 2000).
The objective of this research was to determine the efficacy of high
intensity-short duration goat browsing for the control of juvenile, ground
cover, English ivy. Specifically, this study examines the claim of commercial
graziers that two years of high intensity-short duration goat browsing will
control English ivy in Willamette Valley old field plant communities.
4.3 Materials and Methods
4.3.1 Site description and study design
The research site is located in Memorial Park in Wilsonville, Oregon north
of the Willamette River. The plant community is an old field with a dense
mixed conifer-broadleaf tree canopy. Douglas fir (Pseudotsuga menziesii),
Western red cedar (Thuja plicata) and Bigleaf maple (Acer macrophyllum) are
83
the most abundant tree species. Historically, this vegetation was logged and
cleared for agricultural uses including nut orchards. Currently English ivy is
the dominant plant species at ground level. Herbaceous species in the research
area include waterleaf (Hydrophyllum tenuipes), western trillium (Trillium
ovatum), stinging nettle (Urtica dioica) and sword fern (Polystichum
munitum). All shrubs present are members of Family Rosaceae including the
non-native Himalayan blackberry (Rubus armeniacus), and two native
brambles, thimbleberry (Rubus parviflorus) and trailing blackberry (Rubus
ursinus).
Rainfall in the area averages 991mm/yr (39 in/yr) with July being the
driest month. July is also the warmest month with average high and low
temperatures of 27ºC (80ºF) and 12ºC (53ºF), respectively. December is the
coldest month when average high temperatures reach 8ºC (46ºF) and average
lows are 1ºC (33ºF) (National Weather Service, 2008). The soils primarily
Chehalis silt loam with a small strip of Cloquato silt loam on the southern
boundary, closest to the Willamette River (Soil Survey Staff, 2008).
This study was initiated during the summer of 2006. It is a completely
randomized design (CRD) with 32 1m2 plots located under forest canopy
where English ivy was the dominant plant species. English ivy was removed
from the trunks of some trees by clipping and hand-pulling but no control of
the ground cover has been undertaken in the research area.
84
4.3.2 Browsing treatment and levels
Treatment consisted of browsing by experienced doe-kid pairs for one
day during the last week of August 2006 and 2007. Does had prior experience
eating English ivy. This is important because the presence of Hederin, a
saponin secondary plant compound, in the English ivy tissue suppresses intake
by livestock (Cheeke, 1998). Experience ingesting secondary compounds
early in life can increase intake later in life (Distel and Provenza, 1991),
however, and training is important to ensure that the animals consumed
English ivy in this study.
The stocking rate was approximately 100 goats/1,000 m2 (400 goats/ac).
Goats were herded into the research area in the morning and kept there by
temporary electric fencing until most of the English ivy plants were defoliated
and the goats ceased to consume the vegetation. Behavioral indicators of
satiety were animals laying down to ruminate and walking within the enclosure
without browsing. The goats were not returned to the plots until the following
year.
Plots treated twice were browsed in 2006 and 2007 and those treated once
were browsed only in 2007. Unbrowsed plots were excluded from browsing
by 1.2m (4ft) high wire mesh fencing for the duration of the study. The
unbrowsed plots serve as controls, or treatment level 0. Treatments were
randomly assigned to each of the 32 plots. Eleven plots were treated with high
intensity-short duration goat browsing in two consecutive years (2006 and
85
2007), 11 plots were treated with browsing by goats for one year (2007) and 10
plots were not browsed.
Each m2 plot was separated from other plots and walking paths of the park
by at least 75 cm. This separation provided a buffer and reduced the
possibility that interactions occurred between the plots. Wooden stakes
marked the four corners of each plot.
4.3.3 Percent foliar cover
Percent foliar cover of all plant species was determined by visual estimate
using a m2 quadrat frame placed at a height of 5-20 cm above ground level.
Bare ground and detritus were also estimated. Pre-treatment data were
recorded two weeks prior to treatment in August 2006 and four weeks prior to
treatment in 2007. Post-treatment data were recorded two weeks after
treatment in September 2006 and 2007 and again in July 2008. The last data
set, from July 2008, represents re-growth over an 11-month period from the
time of the last browsing treatment.
86
4.3.4 Statistical model and analysis
The following General Linear Model (GLM) was fit to the data to test the
effectiveness of goat browsing as a control method for English ivy
Уij = ų + αi + βχij + εij
Уij = percent change in English ivy cover from 2006 to 2008
for the ith treatment, jth plot
ų = grand mean of English ivy percent cover in 2008
αi = effect of treatment group i
β = relationship between baseline % cover of English ivy and treatment
χij = over-all mean of H. helix cover in 2006 prior to start of study
εij = error term ~ normal (0, σ2)
The primary predictor was treatment with goat browsing at three levels: 0,
1 and 2 years. A term for baseline English ivy, βχij , was included because
mean percent cover English ivy differed significantly by treatment level (F =
9.15, p = .0002) prior to the start of the study. The Tukey-Kramer adjustment
for multiple comparisons was used to compare percent change among the three
levels of treatment.
All analyses were performed using the SAS 9.1.3 Statistical Package.
87
4.4 Results and Discussion
High intensity-short duration goat browsing applied in the summer for one
or two years reduced the percent cover of English ivy. The mean percent
change in percent cover of English ivy for plots treated twice with goat
browsing was 95% (CI = -1.33, -0.90, df = 2, p < 0.0001) from 2006 to 2008.
Change in English ivy percent cover for plots treated only once was 71% (CI =
-1.14, -.70, df =2, 10, p = 0.029) over the same period. Unbrowsed plots
increased by a mean of 13% (CI = .70, 1.33, df= 2, p < 0.0001) in English
ivy cover from 2006 to 2008. Changes presented in Figure 4.1 use absolute
values of percent cover in 2006 and 2008.
Fig. 4.1 Mean English Ivy % Cover Before and After
Browsing Treatments
100
90
80
Percent Cover
70
60
77
50
40
82
75
70
30
23
20
10
4
0
0 years
1 year
2 years
Browsing Level
Error Bars = 1 SE
2006
2008
88
Whether or not the second application of browsing produces a significant
result to warrant its time and expense is also of interest. Plots treated twice
were significantly different from those treated only once with 20% less English
ivy cover (CI = -0.41, 0.01, df = 2, p < 0.03). Multiple comparisons are
presented in Table 4.1.
Table 4.1
Browsing Level Comparisons and Confidence Levels
Browsing
Levels
compared
Difference between
Means
1&0
1&2
2&0
-0.89
0.22
-1.12
Simultaneous Confidence
Limits
-1.16
0.02
-1.38
-0.63
0.42
-0.85
Confidence Level
0.01
0.05
0.01
The success of goat browsing to control English ivy is due to several
factors including plant physiology and growth rates, environmental conditions
and livestock feeding preferences. English ivy grows vegetatively by
producing stolons and this meristematic tissue is clearly vulnerable to the
disturbance of goat browsing. English ivy does not have particularly high
relative growth rates with 0.199 ± 0.019 for the horizontal (ground cover) form
and 0.162 ± 0.013 for the vertical (tree-climbing vines) (Frey & Frick, 1987).
Thus, repeated removal of aboveground biomass should be an effective control
strategy. Despite the relatively low rainfall prior to the browsing treatment in
89
late August, the ivy would be actively growing because it tolerates low soil
matric pressure under low light conditions (Sack, 2004).
The goats had prior experience with the target species and readily
consumed it despite the secondary compound, hederin. Additionally, use of
the goats at a time when plants are fast growing yet soil is relatively dry means
that plants are less subject to the indirect affects of compaction (Heitschmidt,
1990). This is important to maintain soil function.
The primary objectives of this study were to examine the efficacy of goat
browsing to control English ivy and to examine the differences between levels
of the browsing treatment. For the land manager, however, the goal of moving
the plant community toward a set of more desirable, native species is also of
interest. It is noteworthy that only three herbaceous species (waterleaf, trillium
and stinging nettle) were present in the research area prior to treatment. In
2008, these same herbaceous species were present. The only new species to
appear was sword fern which emerged in only one of the plots. The only
species present in 2006 which did not persist into the summer of 2008 was
Himalayan blackberry. This may be due to goats’ browsing preference for
this shrub because this same herd is used in commercial browsing to control
Himalayan blackberry. Future control efforts with browsing might include the
provision of a dietary source of tannins to the goats because pairing them with
saponins increases intake of saponins (Rogosic et al., 2006), the secondary
compound present in English ivy.
90
The monoculture stands of English ivy at Memorial Park exist under a
dense tree canopy where sunlight reaches the herbaceous layer intermittently.
During the summer growing season sunlight reaches the forest floor for only a
few hours each day. This shade likely inhibits germination and emergence of
other species (Radosevich et al., 2007) which may be present in the seed bank.
English ivy is well-adapted to shade (Schnitzler & Heuzé, 2006), however,
and as long as seed and stolon propagules remain the English ivy will persist
and re-invade the area. Removal of some tree canopy might allow species in
the seed bank to emerge and compete with the English ivy because a ten-fold
increase in irradiance causes a decline in ivy growth (Sack, 2004).
Fortunately, there is no evidence of allelopathy from English ivy (Biggerstaff
and Beck, 2006) as seeds of numerous other species germinated equally well in
pots with and without ivy. Continued monitoring of the site would reveal the
extent to which active restoration, including the transplantation of desired
plants, is needed to meet land manager goals.
91
4.5 Acknowledgements
Funding for this research was provided in part by a grant from NRCS
(Natural Resources Conservation Service). Goat browsing services were
provided by the Hawley Land and Cattle Co. of Cottage Grove, Oregon as part
of an invasive plant control project within Memorial Park at Wilsonville,
Oregon. Kerry Rappold, Natural Resources Program Manager for the City of
Wilsonville, supported research activities at the research site.
The author thanks Leonard Rummel for assistance with plot installation
and grazing exclosure construction and Nina Schleicher for assistance with
statistical modeling.
4.6 Literature Cited
Anonymous. 2000. “Wanted: Dead English ivy!” Environmental Services,
City of Portland.
Biggerstaff, M. S. and C. W. Beck. 2006. Effects of English ivy (Hedera
helix) on seed bank formation and germination. American Midland
Naturalist 157: 250-257.
Butaye, J., H. Jacquemyn and M. Hermy. 2001. Differential colonization
causing non-random forest plant community structure in a fragmented
agricultural landscape. Ecography 24: 369-380.
Cheeke, P. R. 1998. Natural Toxicants in Feeds, Forages, and Poisonous
Plants. 2nd ed. Interstate Publishers Inc. Danville, IL. 479 p.
92
Clarke, M. M., S. H. Reichard and C. W. Hamilton. 2006. Prevalence of
different horticultural taxa of ivy (Hedera spp., Araliaceae) in invading
populations. Biological Invasions 8: 149-157.
Derr, J. F. 1992. Weed control in nursery crops. Pages 105-116 in: Pest
Management Guide for Horticultural and Forest Crops. Blacksburg, VA:
Virginia Cooperative Extension Pub. 456-017
Derr, J. F. 1993. English ivy (Hedera helix) response to post emergence
herbicides. Journal of Environmental Horticulture 11(2): 45-48.
Distel, R. A. and F. D. Provenza. 1991. Experience early in life affects
voluntary intake of blackbrush by goats. Journal of Chemical Ecology
17(2): 431-450.
Frey, D. and H. Frick. 1987. Altered partitioning of new dry matter in Hedera
helix L. (Araliaceae) induced by altered orientation. Bulletin of
the
Torrey Botanical Club 114(4): 407-411.
Heitschmidt, R. K. 1990. The role of livestock and other herbivores in
improving rangeland vegetation. Rangelands 12(2): 112-115.
Laskurain, N.A., A. Escudero, J.M. Olano and J. Loidi. 2004. Seedling
dynamics of shrubs in a fully closed temperate forest: greater than
expected. Ecography 27:650-658.
National Weather Service. 2008.
http://www.wrh.noaa.gov/pqr/climate/pdx_clisummary.php
Oberhuber, W. and H. Bauer. 1991. Photoinhibition of photosynthesis under
natural conditions in ivy (Hedera helix L.) growing in an understory of
deciduous trees. Planta 185: 545-553.
Oregon Department of Agriculture Plant Division. 2008.
http://oregon.gov/ODA/PLANT/WEEDS/profile_englishivy.shtml
Accessed November 19, 2008
Radosevich, S.R., J.S. Holt and C.M. Ghersa. 2007. Ecology of Weeds and
Invasive Plants: Relationship to Agriculture and Natural Resource
Management. Hoboken, N.J: Wiley-Interscience.
Randall, J. M. 1996. Weed control for the preservation of biological
diversity. Weed Technology 10: 370-383.
93
Reichard, S. 2007. Hedera helix. University of California, Davis.
http://ucce.ucdavis.edu/datastore/detailreport.cfm?usernumber=55&survey
number=182. Accessed October 20, 2007.
Rogosic, J., R.E. Estell, D. Skobic, A.Martinovic and S. Maric. 2006. Role of
species diversity and secondary compound complementarity on diet
selection of Mediterranean shrubs by goats. Journal of Chemical Ecology
32:1279-1287.
Sack, L. 2004. Response of temperate woody seedlings to shade and
drought: Do trade-offs limit potential niche differentiation? Oikos 107:
110-127.
SAS 9.1.3 Service Pack 4. 2003. SAS Institute Inc. Cary, NC.
Schnitzler, A. and P. Heuzé. 2006. Ivy (Hedera helix L.) dynamics in riverine
forests: Effects of river regulation and forest disturbance. Forest Ecology
and Management 236: 12-17
Soil Survey Staff. 2008. Official Soil Series Descriptions. Natural Resources
Conservation Service, United States Department of Agriculture.
http://soils.usda.gov/technical/classification/osd/index.html
Soll, J. 2005. Controlling English ivy (Hedera helix) in the Pacific
Northwest. http://tncweeds.ucdavis.edu/moredocs//hedhe102.pdf.
Zotz, G., N. Cueni and C. KÖrner. 2006. In situ growth stimulation of a
temperate zone liana (Hedera helix) in elevated CO2. Functional Ecology
20: 763-769.
94
5 SUMMARY OF RESULTS AND INSIGHTS FOR FUTURE
MANAGEMENT
This dissertation examined the effectiveness of high intensity-short
duration goat browsing to control Himalayan blackberry and English ivy in the
Pacific Northwest. The conditions at each site and the characteristics of the
target plants themselves were factors in the varied responses to control.
5.1 English ivy Control with Goat Browsing
Substantial reductions in percent cover of English ivy were observed after
one repetition of goat browsing. A further reduction after a second application
of the goat browsing was also statistically significant. The success of this
treatment is due to the manner in which goats browse including defoliating the
plants, consuming large portions of the stems, and pulling up the shallow roots
(Stein and Fosket, 1969). English ivy is a stoloniferous plant and is therefore
highly susceptible to the disturbance of the high intensity browsing. Despite
the presence of the secondary plant compound Hederin, the herd readily
consumed the plant.
In many instances, the presence of secondary compounds in a particular
plant species will suppress intake or make that plant entirely unpalatable. To
ensure that animals will consume such plant species, managers have at least
two options. First, animals can be introduced to plant species which contain
toxins with conspecifics who will teach them to eat the plants. For example,
95
the herd used for this research had prior experience with English ivy. Each
year the does of the does-kid pairs were chosen because they had eaten the
plant in the previous year. In this way, target plant consumption is increased
by using animals whose herd mates teach them to consume the target plant.
Second, the provision of a high protein feed prior to browsing the target plant
will increase intake. Managers could feed a protein supplement early in the
day before releasing animals onto the infested plant community and thereby
increase efficacy of treatment for plants which contain secondary compounds.
Although English ivy cover in the forested site was reduced by the goat
browsing, a lack of plant species recruitment indicates that new species are not
supported in the ecosystem. The extensive shade at the site probably prevented
the establishment of other competitive, non-native species which are abundant
in adjacent areas of the park. The shade probably prevented recruitment of
desirable native species as well. If managers want a diverse and native plant
community, further manipulation is needed which may require removal of
large dominant trees (Parker, 2000) to reduce shade and allow early
successional species to establish.
If propagules of desired species are not present they must be introduced
through active restoration. Propagules could be seeded or planted after
managers determine that ecological processes of soil stability, hydrology and
nutrient cycling are functioning (Whisenant, 1999) and are stable.
96
5.2 Himalayan blackberry Control with Goat Browsing
Control of Himalayan blackberry produced mixed results. Mowing in two
years was more effective at reducing above ground Himalayan blackberry
biomass than goat browsing. This result was expected as mowing removes all
canes just above the soil surface while browsing is a selective behavior
exhibited by goats. This result does not mean that goats are not a useful tool
for control of shrubs such as Himalayan blackberry, however. Though the
biomass was reduced in the short term, the underlying potential for re-growth,
and its component population demographic, is critical to understanding how
the species responds and how future treatments might yield more reliable
results.
The age of Himalayan blackberry plants that remain after treatment is an
important indicator of what will occur in the years following treatment, and
thus the limited duration of this study. Total cane density increased after one
year of treatment but then declined from 2007 to 2008. In particular, plots
mowed for two years increased in Himalayan blackberry density relative to
other treatments, especially goat browsing for one or two years. This result
confirms the need to closely examine components of growth that allow this
species to maintain its vigor despite substantial control efforts.
At this pasture research site, there is little shade inhibition of the
Himalayan blackberry but the canopy of the blackberry itself produces shade
inhibition of grasses and forbs. Removal of the Himalayan blackberry canopy
97
by the various disturbance treatments changed light conditions at the soil
surface and a release from dormancy (Radosevich et al., 2007) necessary for
germination and establishment of plant species. The number of species present
increased from nine in 2006, to 25 in 2007, and finally to 46 in 2008. The
membership of these species in functional groups is important to ecological
function and because the primary goal for the site is to host forage species.
Each of the treatments; goat browsing, mowing, and goat browsing
followed by mowing, reduced the cover of Himalayan blackberry and
increased the cover of perennial grasses. These are positive outcomes. None
of the treatments was significantly different from another, however, so the
added expense of goat browsing followed by mowing in the same plots is
difficult to justify solely based on reduction of Himalayan blackberry cover
and perennial grass emergence.
Perennial forb cover increased significantly in all plots treated for two
years. However, only plots treated with goat browsing followed by mowing
were significantly different from the other treatments applied for two years.
This may indicate that seeds of perennial forbs respond positively to higher
levels of mechanical soil disturbance which is a by-product of movement of
the goat herd and mowing machinery.
Some species in the perennial forb functional group have the potential to
become as problematic as blackberry if not addressed. In particular, Canada
thistle is a rhizomatous forb which has the potential to displace early seral
98
vegetation (Radosevich et al., 2007) and is less palatable to cattle than the
Himalayan blackberry particularly after its spines develop. One application of
browsing to both Himalayan blackberry and Canada thistle can reduce seed
production but rhizomatous plants are often more difficult to suppress
(Hendrickson and Olson, 2006). Greater reductions in Himalayan blackberry
stem density likely would be achieved by repeated browsing within one season
or for multiple seasons to deplete root reserves as has been observed for
Canada thistle control (Wilson et al., 2006). Additionally, repeated treatment
in one growing season is justified because the critical time for survival of
ramets is during active growth immediately following new shoot initiation
(Radosevich et al., 2007). The defoliation of goat browsing would stimulate
this shoot initiation. A second browsing would then result in disturbance of
the re-growth thus stressing the plants.
Examination of cane density provides some indication of trends. New
ramets, in the form of primocanes, increased from 2006-2008 indicating that
the population was becoming younger. Total stem density and node density
declined, however, so it is possible that exhaustion of carbohydrate reserves in
crowns was beginning in the last year of the study. Close timing of browsing
treatments with times when carbohydrate reserves are lowest in the roots might
ultimately ‘exhaust’ the plants (Davis et al., 1975).
Considering the response in biomass and the population dynamics of
Himalayan blackberry under these treatment protocols, future research should
99
be directed to the application of mowing first followed by goat browsing (the
reverse sequence to what was examined in this dissertation). The advantages
to this approach are many. First, the canopy will be removed by the most
effective means available. This will allow for germination of species in the
seed bank, desired and undesired. After monitoring of emergent species, one
can direct further management efforts to promote or suppress those new
species. Second, continued control of the Himalayan blackberry at life stages,
seedling and primocane, when it is most palatable to goats can be undertaken.
If problematic species emerge, such as Canada thistle, goat browsing will
address that concern as well. Third, it is likely that grasses and other forbs will
emerge that are selected as a smaller portion of the goat diet. Properly timed
goat browsing can protect the desired species as they establish and target the
palatable undesired species at the same time.
5.3 General Considerations for Invasive Plant Management Using Goat
Browsing
If the target of control is naturalized (Cousens and Mortimer, 1995), a
situation wherein individual plants have occupied all the niches and density
will not increase; it is possible to conclude that a threshold has been crossed
from one state to another (Westoby et al., 1989). In the case of the English
ivy, the percent cover was reduced indicating that the plant community can
more easily be moved along a trajectory toward a more desirable state
100
(Bestelmeyer et al., 2003) than a Himalayan blackberry-infested plant
community.
The removal of one weed species often produces ideal conditions for
another invader (Hobbs and Humphries, 1995). The existence of patches of
each of the target species in areas adjacent to both research sites necessitates
that management address the target species on a broader scale.
Little is known about the survival of seeds through the digestion process
for most plant species being targeted for control (Frost and Launchbaugh,
2003). However, we do know that fewer viable seeds are recovered from
domestic herbivores with longer rates of gut passage (Lacey et al., 1992).
Once more is known about passage time and seed survival through the
digestion process, there is potential for goats to be used to increase the
immigration of seeds of desired species (Archer and Pyke, 1991). This could
be a complementary practice for active restoration of degraded sites.
Of course, monitoring must be an integral part of any management plan so
that one is fully aware of all species present and this information can be used to
adjust management to reflect the iterative nature of successional management
(Sheley et al., 2006).
The successful use of goats for vegetation management relies on a
thorough knowledge of the biology and phenology of the target plant species,
its susceptibility to browsing disturbance, and its palatability to goats. If
managers are willing to plan for timing, intensity and repeated application of
101
this tool with adjustments based of careful quantitative monitoring of
outcomes, we may be able to devise predictive models for vegetation response.
It is possible to harvest the potential of goat browsing as part of an integrated
pest management plan when the plant community is known both quantitatively
and qualitatively and response to various control methods is incorporated into
management actions.
102
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