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ECOLOGICAL SIGNIFICANCE OF SEED
BANKS WITH SPECIAL REFERENCE
TO ALIEN ANNUALS
David A. Pyke
ABSTRACT
seed production and germination; other annuals, like pepperweed (Lepidium perfoliatum), may be abundant some
years and extremely sparse in other years (Duba 1976).
Species that are less predictable components of the community often have a narrow range of conditions for successful reproduction, germination, or survival, yet persist
in the seed bank.
The presence of an annual species in a community may
be regulated by its seed bank dynamics. Once the seeds
develop, dispersal may occur immediately or it may be
delayed until later in the season. These two types of dispersal mechanisms result in combinations of risks and benefits. Immediate dispersal allows seeds to reach the soil
and to move by secondary dispersal to safe sites for germination. However, immediate dispersal places the seeds at
risk of predation from ground foraging granivores, or of
pathogens.
Delayed dispersal can be caused by the retention of the
seed in the inflorescence after seeds mature. Medusahead
(Taeniatherum caput-medusae) maintains mature seeds in
inflorescences for up to a month or until the spikes are disturbed (McKell and others 1962). Sufficient disturbance can
be rendered by high winds or by animals moving through
a stand. The long awns associated with medusahead seeds
may cause the seeds to remain aboveground in the litter.
Aboveground seed banks do not provide seeds with adequate conditions for germination, but allow a persistent
seed bank (note this is a broader definition than Parker and
others 1989). The aboveground seed bank on semiarid sites
of the Intermountain West is susceptible to mortality by
fire, in contrast to the aboveground seed banks of conifer
forests and chaparral shrublands, which require fire for
germination (Archibold 1989; Parker and Kelly 1989). The
species found in the aboveground seed banks of the Intermountain West generally require disturbances to move the
seed into adequate contact with the soil, thus moving the
seed into the soil seed bank where imbibition, germination,
and establishment are improved.
Any description of a planfs life history is incomplete if
it does not include a description of the seed bank dynamics.
Such descriptions must include quantification of the temporal variability in the numbers of seeds in the soil and of the
spatial variability of the seed bank. Knowledge of the seed
bank dynamics can then be related to natural or induced
disturbances. These concepts are discussed relative to the
management of lands dominated by exotic annuals.
INTRODUCTION
Reproductive strategies of plants can differ widely among
species that coexist within the same community. Some may
rarely produce viable seeds, yet persist through vegetative
propagation. Others may rely exclusively on the yearly production of viable seeds. Another group may only occur as
vegetation during years when environmental conditions are
conducive to germination yet remain in the seed banks during years when germination conditions are poor. The seed
bank is the storage of viable seeds in either litter or soil until
conditions for germination are achieved. An understanding
of the patterns and processes that regulate seed bank dynamics of desirable and undesirable species is necessary
for the development of ecologically sound management strategies to maintain and restore desirable ecosystems.
In this paper, I will provide a brief overview of seed bank
ecology. I will address seed banks as being spatially and
temporally dynamic. I will discuss seed bank classification
and will relate current vegetation on a landscape to its current seed bank. I will conclude with some sugg~stions of
future directions for research on see~ banks pertaining to
exotic species found in the Intermountain West.
SEED BANK ECOLOGY
As a population of annual plants reaches the end of its
growing season, the culmination of its life is the production
of seeds that will make up the next generation. For some
exotic species like cheatgrass (Bromus tectorum), individuals are annually a consistent part of the community. These
species have a wide range of environmental conditions for
SEED BANK CLASSIFICATION AND
PERSISTENCE
Before 1970 many studies investigated the physiological
mechanisms associated with germination (see review by
Baskin and Baskin 1989), but attempts to incorporate these
mechanisms into a general classification of the seed production, persistence, and germination have been few (Grime
1981; Roberts 1981, 1986; Thompson and Grime 1979). For
the purposes of this paper, I will use the Thompson and
Grime (1979) classification to relate the dynamics of annual seeds in soils of the Intermountain West.
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
David A. Pyke is Senior Rangeland Ecologist, Bureau of Land Management, Cooperative Research Unit, and Associate Professor, Department of
Rangeland Resources, Oregon State University, Corvallis, OR 97331.
197
Table 1-8eed bank classification of Thompson and Grime (1979)
based on germination time and on persistence of the seed
bank
Seed bank type
Germination time
Persistence time
I
II
Ill
IV
Autumn
Spring
When dispersed
Continuous, but
gradual
Summer only
Winter only
Small and persistent
Large and persistent
Annuals with Type IV seed banks are more difficult to
control, because large portions of the population remain
dormant. They will require more complex prescriptions
involving multiple treatments and application times to
attack different life stages of the plant. For example, prescribed fire during early summer may kill recently produced seeds while herbicides after germination kill seedlings that emerged from seeds that escaped the fire.
DORMANCY
The Thompson and Grime (1979) classification is a combination of seed dispersal time, the period of seed bank
persistence, and the time of germination (table 1). Exotic
annuals with Type I through Type m seed banks are susceptible to management strategies intended to reduce the
seed bank population size by reducing the current year's
seed crop (for example, prescribed burns after seed maturation and before germination).
Medusahead would be considered a hybrid of Type I and
seed banks. It requires a short after-ripening period
before the embryo is capable of germinating (Murphy and
Turner 1959; Young and others 1968). This period of afterripening maintains a short summer dormancy. About 10
percent of the seed is capable of persisting in litter or soil for
more than 1 year (Sharp and others 1957). An additional
short-lived dormancy is thought to exist that can be broken
by removing medusahead awns (Nelson and Wilson 1969);
however, the exact mechanism of this dormancy is unknown.
Cheatgrass is a true Type
species. It is capable of germinating when dispersed, yet it maintains a small persistent
seed bank (see Pyke and Novak, these proceedings).
For many alien annuals, we are uncertain how long they
may persist in the seed bank. Cheatgrass can be stored
under laboratory conditions for up to 12 years with 95 percent viability, but in the field there is little or no evidence
of persistence (Hulbert 1955; Hull 1973). In conjunction
with a study conducted near Snowville, UT (Pyke 1990),
I collected 20 random soil samples (10 by 10 by 3 em) per
sample session over a 2-year period. The soil was thinly
spread across the surface of sterile sand and kept moist
in a heated glasshouse (20/15 °C; day/night) to germinate
seeds. After 4 weeks, seedlings were identified and removed. The soil was then turned and allowed two additional weeks for germination. After the sixth week, the
soil was sieved and any ungerminated seeds were collected,
identified, and tested for viability using a 1 percent solution of 2,3,5-triphenyltetrazolium chloride (Bewley and
Black 1982; Woodstock 1973) for 48 h at 20 °C. The peak
seed population occurs immediately after dispersal and
declines to near zero by late spring (fig. 1). This decline is
consistent with declines in seed banks of other exotic annuals in other semiarid environments (Bartolome 1979; Rice
1985, 1989). The high amount of spatial and temporal variability in the cheatgrass seed bank is noteworthy (fig. 1).
This is consistent with the findings from other semiarid
communities (Coffin and Lauenroth 1989). Spatial variability demonstrates the importance of multiple sampling
times during the year and of adequate replication within a
sampling session (Gross 1990).
m
m
198
Seeds are dispersed into heterogenous and sometimes
unpredictable environments. Dormancy is an adaptation
that some species have evolved to prevent germination, of
at least some individuals, until conditions are suitable for
successful germination and establishment.
Two dormancy classifications have been proposed. Each
has its advantages and disadvantages. Harper (1977) proposed a phenological classification based on the time to germination and on the environmental conditions provided to
the seed. There are three categories under his classification: enforced, induced, and innate dormancy. Enforced
dormancy is viewed by many as a nondormant condition
where the requirements for germination, such as sufficient
moisture, have not been experienced by the seed Induced
dormancy occurs when a nondormant seed is exposed to
environmental conditions that cause the seed to become
physiologically dormant. Cheatgrass has the potential for
this dormancy type (Kelrick 1991; Young and Evans 1975).
Innate dormancy occurs when seeds are incapable of germinating immediately after they disperse. The seed is either morphologically or physiologically incapable of germinating or due to a physical barrier (a thick seed coat) is
125
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DATE (1985-1987)
Figure 1-Mean number of viable seeds of cheatgrass found in the litter and upper 3 em of soil over
2 years. Bars represent ±1 S.E. of the mean.
Table 2-The types, causes, and characteristics of seed dormancy from the dormancy classification system of Baskin and Baskin (1989) relative to the system of Harper (19n)
Baskin and Baskins' system
Type
causes
Characteristics
Harper's system
Type
Physiological
Embryo chemicals inhibit germination.
Fully developed, but dormant.
Innate or induced
Physical
Seed coat is impermeable to water
and inhibits imbibition.
Fully developed and capable
of germination.
Innate
Combinational
Impermeable seed coat and chemicals
In embryo inhibit germination.
Fully developed, but dormant.
Innate or induced
Morphological
Underdeveloped embryo.
Underdeveloped and nondormant.
Innate
Morphophysiological
Underdeveloped embryo with chemicals
in the embryo that inhibit germination.
Underdeveloped and dormant.
Innate or induced
Fully developed.
Enforced
None
seed predator normally acts as a predispersal agent. One
agent is the seed head gall fly (Urophora quadrifasciata)
that feeds on the knapweed complex (Centaurea sp.)
(Coombs 1992).
unable to germinate. Baskin and Baskin (1989) subdivide
innate dormancy into five categories of seed dormancy
(table 2).
Some species disperse seeds with two dormancy types
from the same plant. One type is capable of immediate
germination and the other type is physiologically dormant.
Jointed goatgrass (Aegilops cylindrica) reportedly contains
this combination of dormancy known as heteromorphic
seeds (Donald and Ogg 1991). Heteromorphic seeds allow
a species to spread germination over two or more seasons,
thereby reducing the risk of unpredictable environmentally
induced catastrophes that may reduce population sizes.
This type of strategy is most common in Gramineae, Compositae, and Chenopodiaceae (Fenner 1985). Venable and
Lawlor (1980) have proposed that reproduction is maximized
when species with dimorphic seeds have correlated dimorphic dispersal mechanisms. They proposed that seeds with
high dispersability should germinate quickly and those with
low dispersability should contain some form of dormancy.
Of the 27 species examined, 25 complied with their proposed model.
DISTURBANCE
Although prescribed fire is proposed as a technique for
reducing seed population sizes, it must be conducted at
the appropriate time of year. Slow, hot fires during the
seed maturation process ofmedusahead have been used
as an effective tool for reducing population sizes in the
following year (Furbish 1953; McKell and others 1962;
Murphy and Turner 1959). However, fires during the
wrong season (for example, after seed dispersal) are less
successful. Repeated fires may favor one exotic annual
over another, changing seedbed environments rather than
changing the seed bank populations (Young and Evans
1972).
A persistent seed bank is clearly a mechanism for some
plants to maintain themselves in environments with regular disturbances. Disturbances may initially reduce the
seed bank of a species, but those seeds that remain may
be capable of germinating and replenishing the seed bank
in a limited amount of time. Cheatgrass is able to respond
within the first growing season after a fire with a 50 percent increase in seeds on a burned site relative to an adjacent unburned site (fig. 2). This demonstrates the importance of revegetation following prescribed or wild fires in
controlling the spread of exotic annuals.
Revegetating degraded communities with monospecific
stands of exotic perennials like crested wheatgrass (Agropyron cristatum) may lead to reductions in the species
richness of the seed bank relative to native sites (fig. 3)
(Marlette and Anderson 1986). Revegetation of degraded
lands by sowing monospecific stands of highly competitive
species may be an effective deterrent to annual plant establishment, but it is likely to be a deterrent to the reestablishment of native perennials also (Marlette and Anderson
1986). Similar or possibly more dramatic reductions may
occur on sites dominated by exotic annuals, but I am unaware of studies demonstrating this.
GRANIVORY
Granivory can have an appreciable impact on the population sizes of some plants (see review by Louda 1989). In the
California annual grasslands, rodent seed predation can
significantly reduce populations of exotic annuals (Borchert
and Jain 1978; Marshall and Jain 1970). In the Intermountain West, the exotic annuals tend to be avoided by many
granivores if they are given alternative native species
(Goebel and Berry 1976; Kelrick and others 1986; McAdoo
and others 1983). Therefore, granivores may hasten a compositional change in a community from a dominance of natives to exotics.
Although granivory is a negative impact on plant populations, granivores that cache seed may act as beneficial
dispersal agents. Unrecovered seed caches of rodents can
act as an effective seed-dispersal mechanism (Price and
Jenkins 1987), while adding to the spatial heterogeneity
of the seed bank.
Seed predators can be used as biological control agents
to reduce the spread of undesirable species. An effective
199
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DATE (1981-82)
NATIVE
CRESTED
COMMUNITY
Figure 2-Mean number of seeds of cheatgrass on
a burned and unburned site of sagebrush steppe
during a single growing season (from Hassan and
West 1986). Bars represent ±1 S.E. of the mean.
Figure 3-Number of species, partitioned by life
form, in a native sagebrush steppe community and
on a sHe revegetated with crested wheatgrass in
southern Idaho (from Marlette and Anderson 1986).
Bars represent ±1 S.E. of the mean.
QUESTIONS CONCERNING ALIEN
ANNUALS
Bartolome, J. W. 1979. Germination and seedling establishment in California annual grassland. Journal of Ecology. 67: 273-281.
Baskin, J. M.; Baskin, C. C. 1989. Physiology of dormancy
and germination in relation to seed bank ecology. In:
Leek, M.A.; Parker, V. T.; Simpson, R. L., eds. Ecology of
soil seed banks. San Diego, CA: Academic Press: 53-66.
Bewley, J. D.; Black, M. 1982. Physiology and biochemistry
of seeds in relation to germination. vol. 2. Viability, dormancy, and environmental control. New York: SpringerVerlag. 380 p.
Borchert, M. 1.; Jain, S. K. 1978. The effect of rodent seed
predation on four species of California annual grasses.
Oecologia. 33: 101-113.
Coffin, D.P.; Lauenroth, W. K. 1989. Spatial and temporal
variation in the seed bank of a semiarid grassland.
American Journal of Botany. 76:53-58.
Coombs, E. M. 1992. Implementations of biological control
on federal lands in Oregon. In: Butler, T ., comp. Interagency noxious weed symposium: proceedings; 1991
December 3-4; Corvallis, OR: Oregon State University:
14-25.
Donald, W. W.; Ogg, A. G. 1991. Biology and control of
jointed goatgrass (Aegilops cylindrica), a review. Weed
Technology. 5: 3-17.
Duba, D. R. 1976. Plant demographic studies of a desert
annuals community in northern Utah dominated by nonnative weedy species. Logan, UT: Utah State University.
204 p. Thesis.
Fenner, M. 1985. Seed ecology. New York: Chapman and
Hall.151 p.
Furbish, P. 1953. Control of medusa-head on California
ranges. Journal of Forestry. 51: 118-121.
Of all the alien annual species that inhabit the Intermountain West, more is known about the seed and seed
bank characteristics of cheatgrass than any other exotic
annual. We cannot expect to manage and control the
spread of alien annuals without some basic information
on postdispersal seed behavior of other exotic species.
Studies need to be designed to specifically address this
information. We no longer can rely on tangential studies
or personal observations to provide the needed information. Studies need to be well designed and replicated in
space and in time to provide an adequate picture of the
dynamics of seed banks. The following questions should
be specifically addressed for each species:
1. Do its seeds persist in the soil or litter, and if they
persist, how long will they persist?
2. How quickly does the seed bank decline over a growing season?
3. Does dispersal occur immediately after maturation
of seeds or are seeds dispersed over an extended period?
With answers to these questions, managers can begin to
prescribe and test various control regimes and to restore
native plants across the Intermountain West.
REFERENCES
Archibold, 0. W. 1989. Seed banks and vegetation processes in coniferous forests. In: Leek, M. A.; Parker,
V. T.; Simpson, R. L., eds. Ecology of soil seed banks.
San Diego, CA: Academic Press: 107-122.
200
Goebel, C. J.; Berry, G. 1976. Selectivity of range grass
seeds by local birds. Journal of Range Management.
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Grime, J.P. 1981. The role of seed dormancy in vegetation
dynamics. Annals of Applied Biology. 98: 555-558.
Gross, K. L. 1990. A comparison of methods for estimating
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Hulbert, L. C. 1955. Ecological studies of Bromus tectorum
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Hull, A. C. 1973. Germination of range plant seeds after
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Kelrick, M. I. 1991. Factors affecting seeds in a sagebrushsteppe ecosystem and implications for the dispersion of
an annual plant species, cheatgrass (Bromus tectorum L.).
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Kelrick, M. I.; MacMahon, J. A.; Parmenter, R. R.; Sisson,
D. V. 1986. Native seed preferences of shrub-steppe rodents, birds and ants: the relationships of seed attributes
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Louda, S. M. 1989. Predation in the dynamics of seed reproduction. In: Leek, M. A; Parker, V. T.; Simpson, R. L.,
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McAdoo, K.; Evans, B. A.; Young, J. A; Evans, R. A.1983.
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201
Parker, V. T.; Kelly, V. R. 1989. Seed banks in California
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Price, M. V.; Jenkins, S. H. 1987. Rodents as seed consumers and dispersers. In: Murray, D. R., ed. Seed dispersal.
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introduced and native tussock grasses: persistence and
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Rice, K. J. 1989. Impacts of seed banks on grassland community structure and population dynamics. In: Leek,
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Roberts, H. A 1981. Seed banks in soil. Advances in Applied
Biology. 6: 1-55.
Roberts, H. A 1986. Seed persistence in soil and seasonal
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Young, J. A.; Evans, R. A. 1975. Germinability of seed reserves in a big sagebrush community. Weed Science. 23:
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