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Characteristics and Consequences of Invasion
by Sweet Resin Bush into the Arid
Southwestern United States
Elizabeth A. Pierson and Joseph R. McAuliffe1
Abstract.-Eutyops multifidus (sweet resin bush), a shrubby composite
native to South Africa, was introduced to the arid southwestern United
States in 1935 by the USDA Soil Conservation Service. The spread of
this shrub represents one of the most serious threats to the ecological
integrity and economic value of several ecosystems in the semiarid
Southwest. In southern Arizona, this shrub readily invades vegetationally
intact, semi-arid grasslands and eventually forms virtually uninterrupted
monocultures from which native grasses, shrubs, and succulents are
almost completely excluded. Study of plant responses at the advancing
front of the areas occupied by sweet resin bush demonstrated that death
of native species is linked to the spread of this exotic shrub. These
dramatic vegetation alterations are persistent and lead to a variety of
additional detrimental changes, including marked increases in soil
erosion.
We have identified the climatic, vegetation, elevation, and soil
characteristics of sites that are susceptible to invasion by E. multifidus.
In southern Arizona sweet resin bush can occur in vegetation ranging
from Sonoran Desert at low elevation (ca. 850-1060 m) to grasslands,
chaparral, and woodland at higher elevation (ca. 1300+ m). In grassland,
this shrub can invade a wide variety of different soil types, ranging from
loamy calcic soils typically occupied by stands of black grama
(Bouteloua eriopoda) to heavy clay soils occupied by tobosa (Hilaria
mutica) and curly mesquite (H. belangeri). A significant amount of public
and private southwestern rangeland is within the range of this invasive
species. Our results demonstrate the catastrophic consequences of
invasion by sweet resin bush in the arid southwestern United States.
not been affected by invasions of exotic terrestrial
plants, and purposeful or accidental introduction
combined with urbanization and land usage have
been a major causal factor in these invasions.
At a global scale, grasslands are among the
most vulnerable ecosystems to extensive vegetation change due to plant invasions. According to
Mack (1989), in less than 300 years (and in most
cases, little more than 100 years) much of the temperate grassland outside Eurasia (a collective area
of 2.0 x 106 km 2 ) has been irreparably transformed by human settlement and the concomitant
introduction of alien plants. In contrast to temperate grasslands, semi-arid grasslands have been
viewed as less vulnerable to invasion. The perception is based on the notion that because of the
INTRODUCTION
Invasion of natural plant communities by introduced plants constitutes one of the most
serious threats to natural ecosystems worldwide.
The resulting transformations can permanently
decrease native diversity, and can produce permanent, self-perpetuating changes in ecosystem
properties including hydrology, biogeochemicalcycling, and disturbance regimes (especially fire).
There are few ecosystems in the world that have
1U.S.Geological Survey, 1675 W. Anklam Rd., Tucson, Arizona,
85745, USA and Desert Botanical Garden, 1201 N. Galvin Parkway,
Phoenix, Arizona, 85008, USA.
219
greater enyironmentallimitations placed on existence in arid and semi-arid climates, few species
are assumed capable of invading and the few that
are capable are unlikely to bring about extensive
vegetation change. The paucity of information
available on the consequences of plant invasions
in semi-arid grasslands has perpetuated the notion that these grasslands are less vulnerable to
invasion.
For the arid southwestern United States, this
notion may prove to be dangerously inaccurate.
The biseasonal rainfall and subtropical temperatures characteristic of the Sonoran Desert and
adjacent semi-arid grassland make them vulnerable to invasive species tolerant of both
Mediterranean and neotropical climate regimes
(Burgess, et al. 1991). Species of exotic plants tolerant of both climatic regimes have already
become naturalized in the semi-arid vegetation of
the Southwest; many were deliberately introduced by the United States Soil Conservation
Service (SCS) for their climate tolerance (Table 1).
The consequences of these invasions are just beginning to be recognized and further vegetation
change due to invasion is likely to be only a matter of time.
Examination of the characteristics of successful invading species and the consequences of their
spread for the invaded ecosystem provides an excellent opportunity to study the structure and
function of the native biological community and
assess the vulnerability of these com-!punities to
further invasion. In this paper we focus on E u-
Figure 1.-EuryOPS multifidus (Asteraceae), also known as sweet
resin bush, Is a perennial subshrub. It has ShOWY, yellow
Inflorescences which bloom in Arizona from December to
March.
ryops multifidus (Asteraceae), commonly known
as sweet resin bush (fig. 1). This woody subshrub
was introduced into the arid southwestern United
States from South Africa in 1935 by the SCS. The
ecosystem-level changes that have occurred as a
result of the range expansion of this species from
experimental introductions by the SCS are among
the most dramatic examples of the consequences
of plant invasions in the arid Southwest. Interestingly, relatively little has been written previously
about this serious ecological problem. In this paper, we summarize what is presently known
about the A) taxonomy and B) ecology of E. multifidus; C) describe the history of introduction of
sweet resin bush and other exotic species by the
SCS to the southwestern United States, D) describe some of the changes in the structure and
function of vegetation communities that have
been invaded by sweet resin bush, E) describe the
potential mechanisms that can account for these
changes, and F) delineate the geographic area in
North America at risk of future invasion by this
invasive species.
Table 1.-Examples of exotic plant species that have become
widely established in semi-arid and arid vegetation in
Arizona as a result of deliberate introduction for erosion
control and range improvement prior to 1942. The table
gives the family, genus, and species names, country of
origin, and source of introduction. An asterisk (*)
Indicates those species that were either imported and
introduced for the first time by the USDA Soil
Conservation Service (SCS) or widely distributed by the
SCS, having been initially introduced by other means.
Plant names
Asteraceae Euryops multifidus
Asteraceae Pentzia incana
Chenopodiaceae Atrip/ex semibaccata
Fabaceae Melilotus indicus
Fabaceae Melilotus officinalis
Geraniaceae Erodium cicutarium
Poaceae Agropyron cristatum
Poaceae Andropogon ischaem
Poaceae Avena fatua
Poaceae Bromus rubens
Poaceae Cynodon dactylon
Poaceae Eragrostis curvula
Poaceae Eragrostis lehmanniana
Poaceae Pennisetum ciliare
Poaceae Schismus barbatus
Origin Source
South Africa*
South Africa *
Australia*
Eurasia*
Eurasia'"
Europe*
Turkey*
Turkey*
Europe
Europe
Old world*
South Africa *
South Africa *
South Africa *
Old World*
220
TAXONOMY, PHYTOGEOGRAPHY, AND
PHYTOCHEMISTRY OF EURYOPS
MULTIFIDUS
Euryops multifidus belongs to the tribe
Senecioneae (Asteraceae), which includes the
prominent genera Senecio, Othonna, and Euryops
(Nordenstam, 1968). Representatives of the tribe
are most common in South Africa. There are 97
species of Euryops, all of which occur natively
only on the African continent; 96 of them occur
only in South Africa. Euryops multifidus has a
western Cape distribution in South Africa, ranging from the Malmesbury Flats in the south to
Lake Namaqualand in the north (range includes
the Cape Province, The Orange Free State, Basutoland, and Namaqualand States). It occurs in
transitional communities between arid fynbos and
karriod vegetation, in rhenosterveld and in succulent karroo. It often occurs on rocky outcrops
between sea level and 1500 m in elevation (Nordenstam, 1969). The distribution of E. multifidus
strongly overlaps that of E. tenuissim us. Interestingly, the latter species has been introduced as an
ornamental in the arid southwestern United
States, but has not yet become invasive.
The common name "resin bush" has been applied to all species of the genus Euryops (Smith,
1966). The common name is a literal translation of
the Dutch name Harpuis bosch, "hars" (resin)
"puisje" (a small pimple), referring to thel'esinous
secretion exuded from the stem and branches in
the form of small pimply drops on most Euryops
species. Apparently the resin which accumulates
under the bushes of most species was noted in
colonial South Africa for its alleged medicinal
value and was easily collected (Smith, 1966). Several of the species are known locally by distinctive
names. The names applied specifically to E. multifidus include Soetharpuis ("soet" meaning
"sweet"), Skaapbossie (meaning "sheep bush"),
and Kapokbossie (meaning "chicken pox bush")
(Smith, 1966; Nordenstam, 1968).
All members of the Senecioneae tribe are
known for their distinctive phytochemistry compared to other Asteraceae and this has been used
to taxonomically distinguish the genera of the
tribe and species within each genera (Hegnaur,
1977). Known phytochemicals common to the
three prominent genera of the tribe are diterpene
derivatives, sesquiterpene lactones, furanoelemophilones, and acetophenones. The production of
sesquiterpene lactones particularly is believed to
be involved in defense against herbivores and
parasites since most of them are intensely bitter
221
and several are toxic (Hegnaur, 1977). Almost all
members of the genus Euryops produce and secrete resin. The nature of the distinctive
phytochemistry of the entire tribe suggests that
many of the chemicals produced serve a role in
defense against herbivores and parasites, as
chemical inhibitors involved in allelopathy, or
both.
ECOLOGY OF EURYOPS MULTIFIDUS IN
ITS NATIVE RANGE
E'uryops multifidus is listed in A Catalogue of
Problem Plants in Southern Africa (Wells, et al. p
1966), as a ruderal, agrestal, and pastoral weed.
The undesirable characteristics of this weed include a) its ability to replace "preferred
vegetation" b) its unpalatability and c) its occurrence as a contaminant of seed (Wells, et al., 1966).
Sweet resin bush, like most members of the genus,
has showy, yellow flowers which bloom during
the winter and early spring. The 3-4 mm long, 1-2
mm wide achenes are covered by a wooly indumentum of 3-7 mm long white or brown hairs
and are easily transported on clothing or fur.
Three to seven achenes per inflorescence are commonly produced (fig. 2).
Information on the range ecology of E multifidus in South Africa, particularly its palatability,
is both sparse and contradictory. The problem
stems from its description in The Flowering Plants
of South Africa (Pole-Evans, 1928), one of the
most prominent early floras of South Africa. Here,
E. multifidus is described as being highly palatable to sheep, yet invasive of over-grazed and
over-stocked land. Another more recent source
Figure 2.-The fruits of sweet resin bush, 3-4 mm long, 1-2 mm wide
a(~henes, are covered by a wooly Indumentum and are easily
transported on clothing or fur. Three to seven fruits per
Innorescence are commonly produced.
Plant: Introduction Offices were closed or moved.
One site of introduction at Frye Mesa, Arizona has
been identified from the caption of a 1935 SCS
photograph of a test planting. Few other specific
records of test plantings and revegetation projects
have been identified, although SCS Annual Reports indicate that they occurred.
In order to determine the current distribution
of E. multifidus in the arid Southwest, we have
compiled a list of sightings recorded in local floras
and on specimens from herbarium collections in
Arizona, California, New Mexico and Texas. We
have found no records of the presence of sweet
resin bush outside of Arizona. We have recently
visited every documented report of sweet resin
bush to determine whether it is still present, the
extent of vegetation change that has taken place,
and, if possible, the source of the introduction. We
have determined that the current distribution of
sweet resin bush is localized around four epicenters where sweet resin bush was apparently
deliberately introduced in the late 1930's: Frye
Mesa, Marijilda Canyon, Upper Verde Valley, and
Sabino Canyon (fig. 3). Below we describe what is
known about the introduction of sweet resin bush
at each of these locations.
(Wells, et al., 1966) describes the palatability of
sweet resin bush as uncertain. The early reference
by Pole-Evans may have contributed to the selection of E multifidus by the SCS in the. 1~~O' s for
introduction to the southwest on the basis of its
forage value. The unresolved question of the plant's palatability in its native range is further
complicated by the distinctive phytochemistry of
the genus described previously.
HISTORY OF INTRODUCTION OF SWEET
RESIN BUSH BY THE SCS IN THE
SOUTHWESTERN UNITED STATES
According to SCS Annual Reports, Euryops
multifidus was among the first species collected
by Regional Director F. J. Crider in 1934 for introd uction to the arid Southwest. The main requisites
for the species selected by the SCS for introduction into Arizona and New Mexico, were a)
general climatic adaptation especially drought resistance, b) suitability for erosion control and
other economic uses, and c) ease of propagation"
(Crider, 1935). It should not be surprising given
these criteria that E. multifidus and other species
introduced by the SCS have become invasive and
now constitute serious threats to native vegetation
(Table 1). Each of the species was selected from
vegetation native to semiarid regions such as
western Asia, South Africa, and Australia. Euryops multifidus was selected for introduction
into the arid southwest because it was believed to
be extremely drought resistant; have good forage
value, especially for sheep; and to propagate readily from seed" (Crider, 1935).
Each species selected by the SCS for introduction to the arid southwest was observed and
increased for distribution in the Tucson Regional
Conservation Nursery and then distributed to
Area Nurseries in Safford, Arizona, and Shiprock
and Albuquerque, New Mexico. Test plantings of
exotics on public and private lands outside the
nurseries were carried out by each area nursery.
The Civilian Conservation Corps (CCC) was ultimately provided with stock from the nurseries
and this was used in range restoration projects
throughout the arid Southwest. Based on the recommendations of the SCS, seeds and young plants
were also made available to anyone wishing to
use them for range improvement. Unfortunately,
the SCS kept few records describing the exact locations, dates, and fates of test plantings, and
those that were kept have been lost as the SCS
Frye Mesa
Sweet resin bush was introduced in 1935 onto
Frye Mesa, 18.1 km southwest of Thatcher, Arizona, as part of an SCS experimental planting
program (fig. 4). Sweet resin bush has spread from
the abandoned 27 x 27 m fenced enclosure and has
become established on the mesa top, on depositional slopes and along washes below the mesa,
and at the base of the mesa. It occurs in vegetation
ranging from the creosote bush-dominated flats at
the base of the mesa (elev. 1060) to semi-arid
grassland and surrounding chaparral and woodland on the mesa top (elev. 1300+ m). At the foot
of the mesa, E. multifidus occurs on relatively
deep, loamy, and calcic soils that are occupied by
black grama (BouteJoua eriopoda) and a variety of
native shrubs. On terraces of relatively young alluvium along washes, it has displaced stands of
mesquite (Prosopis velutina)· and catclaw acacia
(Acacia greggii). On the mesa top, it occurs on
extremely clayey soils that support grasslands
dominated by curly mesquite (H belangeri).
Vegetation dominated by E. multifidus on the
mesa. top is centered approximately on the abandoned planting site and is spreading on the mesa
top from the point of introduction primarily as an
222
advancing front (Le., enlarging circle), radiating
out in all directions from the point of introduction
but advancing more quickly downslope and along
drainages. This pattern of spread has produced an
almost uninterrupted monoculture of E. m ultifidus over one ~uarter of the mesa top (ca. 1.3 km2
of the ca. 4 km area covered by semi-arid grassland). Interspersed within the semi-arid grassland
vegetation on the mesa top there exists a mosaic
of disj unct patches of vegetation dominated by E.
multifidus ranging in size from 2000 m2 to less
than 1 m 2 . These patches serve as invasion foci
(Mack, 1985) expanding in the same way as the
main population and eventually coalescing with
other patches or merging with the main population, The result is a patchwork of vegetation that
is clearly visible from the valley floor at most seasons because of the contrasting phenology of the
ARIZONA
Colorado
Plateau
o
Flagstaff
Figure 4.-Repeat photographs of the Soil Conservation Service
(SCS) test planting at Frye Mesa, Arizona: top, taken In 1935 by
the SCS; and bottom, taken In 1991 by R. M. Turner. Sweet resin
bush was Introduced Into the 27 x 27 m exe/osure pictured in
the 1935 photograph. Almost 60 years later, sweet resin bush
forms an almost unlnterupted monoeulture over one quarter of
the mesa top.
o
dominant grassland species and E. multifidus.
Sweet resin bush is a brilliant green throughout
the much of the year with showy yellow flowers
from late December to March, when much grassland vegetation is dormant. The situation is
reversed in summer.
100
km
fW arm,
· con t"
Reglon
alnlng
Marijilda Canyon
Semi-arid Grasslands
E'. multifidus was apparently introduced along
FSR 57 approximately 2.4 kilometers from its intersection with Swift Trail Road (State Route 366),
although no record of this introduction was found
in the SCS documentation available in Tucson,
Arizona. Circumstantial evidence for the role of
the SCS include the presence of a Civil Conservation Corp (CCC) work camp in Marijilda Canyon
Figure 3.-The current distribution of sweet resin bush Is localized
around four epicenters in Arizona where this exotic was
deliberately Introduced. The locations of these epicenters are
represented by solid triangles on this map of Arizona: A. Frye
Mesa, B. Marijilda Canyon, C. UpperYerde Yalley, and D. Sabino
Canyon. At risk of future Invasion are the warm, semi-arid
grassland and Sonoran desertscrub vegetation of Arizona. The
geographic range of these vegetation types Is Indicated on the
map.
223
11
during the period from 1934 to 1942 when E. multifidus was being used for revegetation work.
CCC labor was used frequently to set up test plots
and to revegetate eroded sites with plants recommended and provided by the SCS. The
co-occurrence of Pentzia incana (a South African
composite which was also introduced by the SCS
for revegetation during this period) and the presence of small spreader dams (used by the CCC for
erosion control) are also strong indications that E.
multifidus was introduced by the SCS using CCC
labor at this site.
E. multifidus occurs in two adjacent, yet currently distinct patches, one approximately 100 x
200 m and the other approximately 60 x 60 m in
size. The populations occur along the gentle slope
next to the roadway and extend into the adjacent
Marijilda Wash. Individuals can be found in Marijilda Wash and along its banks as much as one
quarter mile downstream from the introduction
site. Both areas have relatively clayey soils and
support a mixed species grassland in which
sideoats grama (Bouteloua curtipendula) is the
dominant species.
Sabino Canyon
At Sabino Canyon in the Coronado National
Forest in Tucson, Arizona, E. multifidus was apparently introduced at the Lowell Ranger Station.
It is unknown whether this introduction was part
of a SCS test planting or whether it escaped from
a garden at the Ranger Station. Sweet resin bush
has become naturalized on the grounds surrounding the Station and has spread into surrounding
Sonoran Desert vegetation along washes and
roadways.
CONSEQUENCES OF INVASION BY
EURYOPS MULTIFIDUS IN THE
SOUTHWEST
We sampled the vegetation in the four sites
discussed above where sweet resin bush was introduced and has become invasive (fig.3). The
sites represent an elevational and climatic continuum. The two sites at the upper elevational end
experience greater effective precipitation and are
characterized by vegetationally intact semiarid
grassland (Frye Mesa, elevation 1300 m; Marijilda
Canyon, elevation 1220 m). The intermediate site
is a former semiarid grassland converted to a
shrub-dominated community by a century of intense grazing (Upper Verde Valley, elevation 1065
m). The remaining site, characterized by Sonoran
desertscrub, occurs on the lower, drier end of the
continuum (Sabino Canyon, elevation 850 m).
Upper Verde Valley
E. multifidus was also apparently introduced
southeast of Cottonwood, Arizona along Camino
Real near its junction with FSR 359. Similar to the
Marijilda Canyon site, we have found nt> record of
this introduction in the SCS documentation available in Tucson, Arizona, however sufficient
circumstantial evidence exists to suggest this was
also an SCS site where E. multifidus was planted
for erosion control. The population occurs on a 10/ 0
slope and perpendicular to this slope are a series
of berms built for erosion control. E. multifidus
appears to have been planted in association with
berm construction in an effort to control erosion.
The native vegetation appears to have been
heavily disturbed and is currently dominated by
Gutierrezia sarothrae, Prosopis velutina, and
Bromus rubens, although it probably previously
supported a grassland dominated by tobosa
(Hilaria mutica). E. multifidus occurs on both
sides of Camino Real. It appears to have been introduced only on the uphill side of the road and
currently occupies an area of approximately 250 x
175 m. Downslope and north of the road E multifidus occurs in a series of disjunct patches in
approximately the same size area; however, individuals can be found as far as 300 m downslope
from the edge of the patch.
Vegetation Change Resulting From
Invasion By Sweet Resin Bush
At each of the four sites, we sampled vegetation within areas lacking sweet resin bush and
adjacent areas invaded by sweet resin bush in order to characterize differences in the number and
coverage of species occurring in each type of
vegetation. At each site, the areas used in these
comparisons were matched for slope, aspect, and
soils. At Marijilda Canyon, Upper Verde Valley,
and Sabino Canyon the coverage in each vegetation type was estimated in a total of fifty, 0.25 m 2
square plots spaced at 1 m intervals along two
parallel, 25 m transects. At Frye Mesa, where the
area invaded by sweet resin bush is much more
extensive, the plots were spaced at 6 m intervals
along two parallel, 150 m transects. The coverage
of grasses and forbs was estimated from basal
area, the coverage of shrubs and succulents was
224
estimated from canopy area, and the coverage of
bare soil was estimated to be the area of the plot
not covered by plant basal area or rocks. Coverage
is expressed as a percentage of the plot area and
was recorded in six classes (0-5, 5-12, 12-25, 25-50,
50-75, 75-100 %).
Additionally, at Frye Mesa, we quantified the
effect of sweet resin bush on the vigor of the small
tree Prosopis velutina which is not excluded by E.
multifidus, but exhibits significantly greater mortality of major branches in areas dominated by E.
multifidus than in intact grassland. Stem mortality was expressed as the percentage of the total
canopy composed of persistent, dead branches
and was recorded in one of six mortality classes
(0-5, 5-25, 25-50, 50-75, 75-95, 95-100%). We estimated the stem mortality for all trees in 5, 1000 m 2
circular plots spaced 40 m apart.
dramatic increase in exposure of bare soil (Figs.
5A, 5B) and increased soil erosion. In the intact
grassland vegetation, interdigitating bunches of
native perennial grasses, particularly Hilaria belangeri, form soil dikes which capture and hold
soil. Where the sweet resin bush has rep laced the
native species, not only is more bare soil exposed,
but the soil is more easily removed. Exposed roots
and soil pedestals around the bases of the remaining native grasses caused by soil erosion are
clearly evident in the transition zone and in vegetation dominated by sweet resin bush. This
situation is ironic since one of the goals of the SCS
Plant Introduction Program was to introduce species which would reduce soil erosion.
Upper Verde Valley, Altered Semi-arid
Grassland
Frye Mesa and Marijilda Canyon, Semi-arid
Grasslands
In the upper Verde Valley, where the grasslands have been altered by heavy grazing, the
native perennial grasses (Hilaria m utica, H belangeri, and Panicum obtusum) have been
excluded and replaced by woody plants (Gutierrezia sarothrae and Prosopis velutina). Inspite of
this transition to vegetation dominated by indigenous disturbance tolerators, we found that the
coverage and diversity of these species are significantly reduced in the presence of sweet resin bush
(Fig 5C). Coverage of the annual forb Plantago
insularis was also found to be dramatically different in invaded and uninvaded vegetation during
the spring of 1993. In the uninvaded vegetation,
this annual formed a nearly continuous carpet in
100% of the plots sampled (average density 3700
plants/m2, average biomass 48 g/m2), whereas in
vegetation dominated by sweet resin bush, P. insularis occurred in 90% of the plots sampled
(average density 1100 plants/m2, average biomass
12 g/m2).
In this altered grassland, more than 50% of the
soil surface is bare even in the absence of sweet
resin bush (as compared to less than 10% in vegetationally intact grassland). However, the
exposure of bare soil does not increase further following sweet resin bush establishment (fig. 5C).
Invasion by sweet resin bush has produced
dramatic and apparently persistent changes in the
structure and function of the semi-arid grasslands
sites we sampled. Sweet resin bush has spread extensively, forming near monocultures within both
of these semi-arid grassland sites. All native
grasses and most woody perennials including the
prevalent subshrubs Calliandra eriophylla, Eriogonum wrightii, and Gutierrezia' sarothrae are
completely excluded from areas now dominated
by sweet resin bush (Figs. 5A, 5B). The decrease in
the species richness of perennial plants on areas
occupied by sweet resin bush is dramatic (19 versus 2 species at Frye Mesa and 23 versus 6 species
at Marijilda Canyon). Most striking is the elimination of Gutierrezia sarothrae, a weedy native
which increases rapidly with disturbance. The
small tree Prosopis velutina is not excluded by E.
multifidus, but at Frye Mesa exhibits significantly
greater mortality of major branches in areas dominated by E. multifidus than in intact grassland (59
versus 15% of major branches, respectively). The
only woody perennial that is apparently unaffected by sweet resin" bush is the sub shrub
Krameria parvifolia which is equally prevalent
and vigorous in intact and invaded areas at Marijilda Canyon (fig. 5B). Interestingly, K parvifolia
is a facultative root parasite (MacDougal and Cannon, 1910). The significance of this exception is
discussed in the next section.
In both grassland sites, elimination of native
species, especially grasses, leads to significant and
I
Sabino Canyon, Sonoran Desertscrub
In contrast to the grassland sites, the species
richness of the desertscrub vegetation is not significantly reduced in the presence of E. multifidus
225
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Intact Grassland
Invaded by Euryops
50
'-
40
30
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i
60
Intact Grassland
Invaded by Euryops
so
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e
i
i~
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u.I
60
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Intact Grassland
40
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10
8
50
~ 20
20
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MARIJILDA CANYON
i;
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1:
60
Altered Grassland
Invaded by Euryops
50
'-
~
8
40
+-'
c:::
cu
~
cu
30
20
Intact Sonoran Desertscrub
Invaded by Euryops
40
30
20
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10
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..
0
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0
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A.
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10
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10
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Cover Type
Cover Type
Figure 5.-Mean percent cover of Euryops multifidus and native species in vagetationally intact and adjacent invaded areas. Coverage is given
for perennial grasses, prominent woody species, and all other perennial species combined at: upper lett. Frye Mesa, upper right, Marlillda
Canyon. lower lett. Uppw Verde Valley, lower right, Sabino Canyon. Coverage of bare soli, the area of the plot not covered by plant basal
area or rocks, Is also given.
226
(fig. 5D). However, some common perennial sub,hrubs (Encelia farinosa and Porophyllum gracile)
\Tere significantly less frequent in vegetation invaded by E. multifidus (fig. 5D). At this site,
<',weet resin bush seems to be invading bare
)round, rather than displacing the existing memoers of the community (notice the reduction in
coverage of bare soil where sweet resin bush has
nvaded, fig. 5D). Although the coverage of E.
nultifidus at this site is comparable to the coverage found in semi-arid grassland vegetation, a
;ignificant number of plots contained large, re:ently dead individuals of E. multifidus. The
inability of sweet resin bush to replace the native
species and the high turnover in the exotic popuiation suggests this site occurs near the lower,
drier limits of the range of this plant.
Transition from Intact Grassland to
Vegetation Dominated by Euryops
60
""'-Euryops
-e-Grasses
50
'-
~
40
8
c 30
~
CD
~
CD
Co
20
10
0
0
s
10
's
20
25
20
ABRUPT TRANSITION FROM NATIVE
VEGETATION TO VEGETATION
DOMINATED BY EURYOPS MULTIFIDUS
\..
-Gutierrezia
-e-Calliandra
15
~
8
~
C
CD
One of the most striking features of the grasslands that have been invaded by sweet resin bush
is the sharpness of the transition between native
vegetation and vegetation dominated by E. multifidus. We characterized this transition at Frye
Mesa by sampling vegetation along transects from
intact grassland to areas dominated by sweet resin
bush. The transects, 25 m in length, were oriented
such that the 10m point was located at the edge of
the area dominated by sweet resin bush, the start
of the transect (0 m) within desert grassland vegetation, and the end (25 m) within the area invaded
by sweet resin bush. In this way, vegetation sampling from 0 to 25 m along the transect
characterized the spatial transition from desert
grassland to zones dominated by E. multifidus.
The canopy coverage of vegetation was measured
as described above.
Our vegetation sampling demonstrates that
the transition from native vegetation to E. multifidus monoculture is extremely abrupt and is
characterized by the death and loss of dominant
native grasses and shrubs, concomitant with an
increase in sweet resin bush (fig. 6). An extreme
consequence of the loss of perennial grass cover is
an increase in the exposure of bare soil. Within 6
m along transects from desert grassland to vegetation invaded by E. multifidus the mean coverage
of E. multifidus and of bare ground increased
from 0 to 36% and 9.5 to 25 % , respectively; in the
same space the mean coverage of the dominant
bunchgrass species, Hilaria belangeri, dropped
10
~
~
5
0
0
5
10
lS
20
2S
Transect Position (' meter intervals)
Figure 6.-Mean percent cover of E. multifidus, native perennial
grasses, and the prominent subshrubs Gutierrezia sarothrae
and Call/andra erlophylla along five, 25 m transects from
grassland to sweet resin bush dominated vegetation. The
transects were oriented such that the 10m point was located at
the edge of the area dominated by sweet resin bush, the origin
within grassland vegetation, and the 25 m endpoint within the
area invaded by sweet resin bush. The Small Arrows Indicate
the presence of dead remains of perennial grasses.
from 12% to less than 4% and the frequency of
dead clumps of this grass increased (fig. 6). The
coverage of the small shrubs Gutierrezia sarothrae
and Calliandra eriophylla also declined as the
coverage of sweet resin bush increased. Isolated
patches of sweet resin bush representing more recent foci of establishment and occurring up to
hundreds of meters away from the central population exhibit the same sharp transition from
monoculture to grassland.
The sharpness of the transition is due in part
to limited recruitment of sweet resin bush away
from mature individuals. Although the achenes
can be dispersed by attachment to fur or clothing
or by water, most accumulate near the base of the
adults where they germinate. The increase in bare
227
soil at the base of adult plants may also aid the
seedlings of sweet resin bush in becoming established there. Whether recruitment away from the
patch is limited by seed dispersal or the availability of suitable sites (Le., areas with exposed soil) is
unknown.
creased seedling mortality due to charcoal particles adhering to and damaging cotyledons. Given
the extensive mortality of native species that occurs in proximity to established sweet resin bush
plants, we believe that this simple experiment was
adeq uate to identify persistent soil alterations if
they did exist. We now feel that persistent allelopathic soil alteration is not the primary
mechanism by which sweet resin bush excludes
native grassland species. However, there may be
other types of interference between sweet resin
bush and native species such as the root interactions that occur between Larrea tridenta and
Ambrosia dumosa as described by Mahall and
Callaway (1991).
Other field observations suggest that competitive exploitation of resources, especially water,
may be an important competitive mechanism. The
bright green appearance of sweet resin bush much
of the year, especially during the winter when
many of the dominant native perennials are dormant, suggests that sweet resin bush is capable of
effici~ntly acquiring and potentially exploiting
most of the available moisture within its proximity up to two months before native species
become active. We hypothesize that this exploitation of water is the mechanism responsible for
partial dieback (rather than complete elimination)
of some woody plants such as Prosopis velutina.
The only woody plant that thrives in both intact
grassland and monocultures of sweet resin bush is
Krameria parvifolia (fig. 5B) at Marijilda Canyon.
This shrub is a facultative root parasite which is
capable of obtaining water from the xylem of
other plants (MacDougal and Cannon, 1910).
Since Krameria parvifolia can potentially obtain
water parasitically from the roots of E, multifidus,
it may be immune to the depletion of soil water
by E. multifidus. We will be investigating further
the potential competition between sweet resin
bush and native grassland species for soil moisture in our future research.
A third category of mechanisms we are investigating involves indirect interactions between
sweet resin bush and the native vegetation. In future research, we will test the hypothesis that
changes in ecosystem level properties such as accelerated erosion or the elimination of members of
the community that make up different trophic levels (e.g., rhizosphere microorganisms) may in turn
accelerate the death of the native plant species.
Although our investigations of the potential
mechanisms to explain the success of sweet resin
bush in the Southwest are preliminary, they suggest that pre-adaptation of sweet resin bush to the
POTENTIAL MECHANISMS TO EXPLAIN
THE CHANGES IN STRUCTURE AND
FUNCTION OF VEGETATION
COMMUNITIES INVADED BY EURYOPS
MULTIFIDUS
We have demonstrated that sweet resin bush is
capable of changing the structure of semi-arid
grassland communities by forming sharply defined areas characterized by the elimination of
almost all native species and the concomitant exposure of soil to erosion. The substantial mortality
of native species in the transition zone suggests
that this dieback is the result of the proximity of
the older E. multifidus individuals. These changes
may result from either direct interaction between
exotic and native (such as interference or exploitation competition) or indirect interaction (such as
the alteration of an ecosystem level property by
the exotic which in turn affects the native vegetation). We are currently testing hypotheses
consistent with the possibility of both direct and
indirect interactions between exotics and natives
although much of our research is as yet preliminary.
The nearly complete exclusion of native flora
by E. multifidus and the formation of a zone of
bare soil and dead plants at the advancing front of
even small populations of sweet resin bush suggested that interference competition may be the
primary mechanism of interaction.With a simple
germination experiment, we tested the hypothesis
that this interaction may be due to persistent allelopathic soil alteration. We collected soil from
intact grasslands and areas dominated by E. m ultifidus. Quick-germinating radish seeds were
sown in both of these soils as well as in a mixture
of each soil with 20% (by volume) finely ground,
activated charcoal. The activated charcoal was
added to adsorb any potential allelopathic compounds. A total of 96 seeds per treatment were
sown, 4 seeds per 5 x 5 x 7 cm pot, in a completely
randomized design. Surprisingly, we found no
difference in germination (nearly 100% in all treatments) and no difference in seedling height
among soil treatments (data not shown). Both of
the charcoal amended treatments had slightly in-
228
climate of the arid Southwest has played a significant role in its success.
GEOGRAPHIC AREA IN NORTH AMERICA
AT RISK OF FUTURE INVASION BY
EURYOPS MULTIFIDUS
In southern Arizona, semi-arid grasslands occupy an elevational range between 900 and 1520
m. The three grassland sites where sweet resin
bush has become invasive range from 1060 to 1460
m elevation. Sweet resin bush becomes dominant
on many kinds of soil including 1) those with
thick clay-enriched horizons (Haplargids, Paleargids, Argiustolls), 2) deep, loamy calcic soils
(Calciorthids), and 3) young, sandy to gravelly
soils of recently formed alluvial terraces (Torrifluvents and Camborthids). These account for most
of the soils of semi-arid grasslands in southern
Arizona. Consequently, we believe that most of
the semi-arid grasslands of southern Arizona are
at risk (fig. 3). The considerably colder winter conditions of the temperate, semi-arid grasslands of
the Colorado Plateau in northeastern Arizona are
probably too severe for E. multifidus.
The occurrence of sweet resin bush in Sonoran
desertscrub at Sabino Canyon suggests that some
areas supporting this vegetation type may also be
at risk (fig. 3). However, at Sabino Canyon, E.
multifidus did not entirely exclude the,. native
vegetation. The abundance of dead E. multifidus
following a recent dry year suggests that habitats
supporting desertscrub may be at the drier limit
of the potential range of E. multifidus. Detailed
future analyses of the distribution of E. multifidus
with respect to climate in South Africa may provide a more detailed and predictive estimate of
potential range of spread in North America.
DISCUSSION
Between 1935 and 1942 the SCS introduced a
number of exotic species in the arid southwestern
United States for the purpose of range improvement and erosion control (Table 1). The
introduction of many of these species has subsequently been encouraged by public and private
groups, the end result being that many of the exotics introduced in the early days of the SCS have
become some of the Southwest's most serious ecological problems. The most notorious examples
are the drought-tolerant African grasses: bufflegrass (Pennisetum ciHare) and the lovegrasses
(Era~Jrostis lehmanniana and E. curvula). All three
of these species have been introduced extensively
in heavily grazed pastureland and, like sweet
resin bush, are capable of maintaining virtual
monocultures by competitively excluding native
species where they have been introduced. In one
of the few studies which has attempted to document the consequences of invasion by these exotic
grasses, Bock et al. (1986) demonstrated that the
native semi-arid grassland community in their
southeastern Arizona study site had a significantly greater variety and abundance of
indigenous grasses, herbs, shrubs, grasshoppers,
rodents and birds than the areas dominated by
African lovegrasses. Our preliminary observations suggest that monocultures of sweet resin
bush may be equally biologically sterile with regard to mammalian, avian, and insect species
(data not shown).
The consequences to the native ecosystem of
invasion by exotic plants can extend beyond the
loss of native diversity, resulting in ecosystem
level changes that perpetuate further reductions
in native diversity. For example, many of the exotic grasses which have established in the
Southwest, especially bufflegrass and red brome
(Bromus rubens), tolerate burning better than
most natives. Because they can alter the structure
of vegetation communities by producing continuous canopies of grass and can prod uce
substantially more biomass than the natives during favorable years, they seem to promote fires
that have a more adverse affect on the long-lived
native vegetation than the exotics. We do not
know whether sweet resin bush has altered disturbance regimes in the vegetation communities it
has invaded, but effects on hydrologic and biogeochemical-cycling regimes are likely to have
occurred. Further, we have observed substantially
enhanced erosion rates where sweet resin bush
has invaded semi-arid grassland. This ecosystem
level change is likely to have permanently altered
sites that have been invaded by sweet resin bush,
even if the exotic is removed.
The growing list of exotics which have become
natu1 alized in the Sonoran Desert and semi-arid
grassland vegetation of the southwestern United
States suggests that these vegetation types are indeed highly vulnerable to invasion by exotic
plants, particularly to those pre-adapted to the local climate and grazing regimes. The dramatic
consequences of invasion that have been documented for the few exotic species studied suggest
that these vegetation types are not only vulnerable, but have little resilience to invasion once
4
229
critical review of the manuscript. Finally, we
thank R. M. Turner for providing negatives for the
repeat photographs of Frye Mesa.
exotics become established and ecosystem
changes are initiated.
We suggest that the ecological danger posed
by sweet resin bush in the Southwest is significant. We believe that by examining further the
characteristics of successful invaders and the consequences of their spread, we can continue to
learn more about the structure and function of the
native vegetation communities and their vulnerability to invasion. We suggest future research on
sweet resin bush be focused on a) the life history
characteristics of sweet resin bush and the mechanisms by which native species are excluded, b) the
rate and mode of spread in different vegetation
communities where it has been introduced c) the
ecology of interactions between sweet resin bush
and other species in its native range, d) quantification of native ecosystem level changes as a
consequence of invasion by sweet resin bush (including soil erosion and the biodiversity of
microorganisms in the soil), and e) assessment of
the necessity and feasibility of an eradication program based on our know ledge of the ecological
consequences of invasion by sweet resin bush.
We hope this discussion of the dramatic consequences that have occurred as a result of invasion
by sweet resin bush and the other exotics mentioned above demonstrates the severity of this
ecological problem and illustrates the importance
of implementing management practices that minimize the future consequences of i!lvasion by
exotic species.
REFERENCES
Bock, C. E., J. H. Bock, K. L. Jepson, and J. C. Ortega. 1986 .
Ecological effects of planting African Lovegrasses in
Arizona. National Geographic Research 2(4):456-463.
Burgess, T. L.,J. E. Bowers, and R. M. Turner. 1991. Exotic
plants at the Desert Laboratory, Tucson, Arizona. Madrono38(2):96-114.
Crider, F. J. 1935. Annual Report of the Southwestern
Nurseries of the Soil Conservation Service. Tucson,
Arizona. Reporting period: Fiscal Year 19341935.
Hegnaur, R.1977. In: Heywood V. H.,]. B. Harborne, and
B. L. Turner, eds. The Biology and Chemistry of the
Composi tae Vol I. Academic Press, London, U.K.
MacDougal, D. T. and R. M. Cannon. 1910. Conditions of
parasitism in plants. Carnegie Institution of Washington, Publication No. 129.
Mack, R. N. 1985. Invading plants: their potential contribution to population biology. In: White, J., ed. Studies
on Plant Demography: A Festschrift for John L.Harper.
Academic Press, London, U. K.
Mack, R. N. 1989. Temperate Grasslands Vulnerable to
Plant Invasions: Characteristics and Consequences. In:
Drake,J. A., ed. Biological Invasions: a Global Perspecti ve .1989 SCOPE .John Wiley and Sons,
Mahall, B. E. and R. M. Callaway. 1991. Root communication among desert shrubs. Proceedings of the National
Academy of Sciences (USA) 88:874-876.
Nordenstam, B. 1968. The Genus Euryops, Part 1. Taxonomy. Opera Botanica, No. 20. C.W.K. Gleerup, Lund,
Sweden.409 pp.
Nordenstam, B. 1969. Phytogeography of the genus Euryops (Compositae) a contribution to the
phytogeography of Southern Africa. Opera Botanica,
Ko. 23.C.W.K.Gleerup,Lund, Sweden.
Pole··Evans, I. B. 1928. The Flowering Plants of South
Africa. Vol. III. The Specialty Press of South Africa,
LTD.pp281-320.
Smith, C. A. 1966. Common Names of South African
Plants. Memoirs of the Botanical Survey of South Africa No. 35. Government Printer, Pretoria. p 244.
Wells, M.J., A.A. Balsinhas, H. Joffe, V.M. Engelbrecht, G.
Harding, and C.H. Stirton.1986. A Catalogue ofProblem Plants in Southern Africa. Memoirs of the
Botanical Survey of South Africa No. 53. Government
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ACKNOWLEDGMENTS
We thank R. M. Turner for introducing E. A.
Pierson to the Frye Mesa site, M. St. John for introducing J. R. McAuliffe to the same site, and T. L.
Burgess for suggesting the collaboration. We also
thank B. Munda and M. Pater of the SCS Plant
Materials Center, Tucson, Arizona and D. Kerby
of the SCS Plant Materials Center, Las Lunas, New
Mexico for access to the archives of SCS Annual
Reports. We thank J. Ruyle, of the USDA Forest
Service for facilitating research at Sabino Canyon.
We also acknowledge T. L. Burgess, P. Warshall,
and M. P. McClaren for helpful discussions and J.
E. Bowers, R. M. Turner, and L. S. Pierson for their
230