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PERENNIAL FORB LIFE-HISTORY
STRATEGmS ON SEMIARID
RANGELANDS: IMPLICATIONS
FOR REVEGETATION
Stanley G. Kitchen
ABSTRACT
aboveground structures. As a general rule, these plants
have relatively rapid potential growth rates; there is no
investment in aboveground woody tissues. Because they
run the risk of being shaded by taller woody species, some
benefit from frequent aboveground disturbance such as
herbivory and fire. Herbaceous perennials are divided
into two fairly distinct groups: grasses and grasslike species and forbs (dicots and some monocots). Though extremely diverse, perennial forbs are competitively distinct
from grasses, differing significantly in anatomical, physiological, and reproductive features.
The seasonal nature of resource pulses in the Intermountain West favors species with perennial life histories. The
often short and unpredictable growing season (as dictated
by soil moisture and soil and air temperature) typical of
valley and foothill landscapes results in numerous combinations of perennial shrub, grass, and forb species (Ream
1963). Perennial forb species are responsible for much
of the diversity in these communities even though coolseason grasses often dominate the herbaceous component.
Frequent spring cold spells and a continental climate also
favor wind-pollinated grasses and shrubs over the predominantly insect-pollinated forbs. Forb diversity is generally greater in communities associated with more mesic
sites. On these sites, the canopy cover of tall shrubs and
trees may increase, resulting in a shift to conditions that
favor shade-tolerant forbs.
Establishment and persistence ofperennial (orbs are
priority objectives in revegetation. Species adapted to
semiarid regions of the Western United States employ lifehistory strategies suited to unpredictable and often extreme environmental conditions. Long-lived (orbs grow
rapidly in spring and survive drought through vegetative
dormancy. Short-lived species are opportunistic and utilize soil seed reserves to persist through long periods of
unfavorable conditions. Seeded (orbs must be adapted to
site conditions for successful establishment and long-term
survival.
INTRODUCTION
The evolution of terrestrial plant morphologies reflects
tradeoffs between maximization of growth rates by allocation of photosynthate to production and display of photosynthesizing tissues (principally leaves) and the need to
invest in other structures (roots, stems) needed to survive
in resource-limited environments (Tilman 1988). The development of roots increases in importance as soil resources
(such as water and mineral nutrients) become less available. Conversely, plant height becomes increasingly important as soil-surface light intensity decreases due to
increases in plant canopy shade, a change generally associated with increases in soil resource levels. Plant height is
greatly increased through the development of perennial
(woody) stems. The "cost" of root and stem investment is
a decrease in growth rate. Competitive strength of a species is dictated by its ability to adjust photosynthate allocation between plant parts, balancing the tradeoff between
growth rate and stress tolerance for the often-changing conditions of its environment.
Perenniality is generally favored in environments experiencing seasonal pulses in resource availability (Tilman
1988). While increases in both frequency and severity of
disturbance favor annual growth, herbaceous perennials
may be favored when disturbance-caused losses are primarily to aboveground structures.
Herbaceous perennials are characterized by perennating belowground structures and herbaceous, nonwoody,
PERENNIAL FORB STRATEGIES
Grime (1977) proposed three primary plant strategies
in which traits have evolved in response to different levels
of competition, stress, and disturbance. He described
competitive, stress-tolerant, and ruderal strategies, each
adapted to conditions of high levels of one of these environmental forces and lesser levels of the remaining two.
He also described conditions that select for secondary or
intermediate strategies and suggested that perennial
forbs have a range of strategies wider than other plant
groups.
Successful forb strategies have evolved to address both
norms and extremes of the environment. Physical factors
such as dependability of growing season, severity and
length of drought, and nature and frequency of disturbance are important in determining the "shape" of each
strategy. Abrahamson (1979) documented differences in
resource allocation between forb species and ecotypes associated with forest understories and those found in open
fields. Field forbs allocated a greater share of resources to
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Stanley G. Kitchen is Botanist, Shrub Sciences Laboratory, Intermountain Research Station, Forest Service, U.S. Department of Agriculture,
Provo, UT 84606.
342
reproductive organs and a lesser share to leaves and
belowground organs than did forest understory species.
Specialized animal pollination strategies are favored in
wooded understory habitats because of improved efficiency
in long-distance pollination among dispersed plants bearing relatively few flowers (Regal 1977).
This paper describes life-history strategies suited to
non- or open-forested habitats and gives examples native
to the Intermountain West.
most plants do not flower for several years after seedling
establishment (Shaw and Monsen 1983).
Other long-lived forb species used in revegetation with
high levels of belowground resource storage include
Pacific aster (Aster chilensis), western yarrow (Achillea
millefolium), Louisiana sagewort (Artemisia ludoviciana),
and scarlet globemallow (Sphaeralcea coccinea) (Menke
and Trlica 1981; Shaw and Monsen 1983). As with these
species, belowground reserves are often related to vegetative reproduction. Allium, Calochortus, Erigeron, Eriogonum, Lomatium, Senecio, Solidago, and Wyethia are examples of other forb genera represented by Strategy A
species.
Strategy A: Long-Lived/Summer
Dormant
Strategy A species are typically slow to reach reproductive maturity and are long lived. They inhabit communities where the favorability of microsites is relatively
constant (Gadgil and Solbrig 1972). Resource pulses are
generally predictable (seasonal) and at least adequate for
maintenance growth. This strategy is intermediate between the competitive and stress-tolerant strategies proposed by Grime (1977), though species are also tolerant
of some forms of disturbance.
A distinguishing trait of Strategy A species is the storage of large quantities of mineral resources and photosynthate in belowground perennating structures. These reserves are mobilized rapidly to produce photosynthesizing
leaves and stems early in the growing season, making mature plants excellent competitors. Reproductive output
intensity is tied to these stored reserves; mast seed production (cycles ofbumper crops spaced by years of minimal or no seed production) is common. Summer drought
is avoided through vegetative dormancy. The successful
establishment of new individuals or cohorts may be either
slow and gradual or rare, as these species are typically associated with somewhat closed, late seral communities
(Gadgil and Solbrig 1972).
Arrowleafbalsam.root (Balsamorhiza sagittata) is a
typical Strategy A species. This long-lived species produces a large taproot with deep soil penetration (Shaw
and Monsen 1983). Large basal leaves emerge quickly
from the taproot as spring conditions permit. Mast seed
production is synchronized, at least in part, by the effects
of weather (such as precipitation and spring frost) on
resource allocation. Plants enter dormancy in early to
midsummer.
Seeds are relatively large and are sought out and
cached by rodents (Everett and others 1978). Seed dormancy prevents precocious summer or fall germination.
Young and Evans (1979) speculated that excessive seed
stratification requirements reduce spring germination.
In laboratory experiments, I found that seed dormancy is
broken with moderate periods of moist chilling (stratification) (Kitchen 1993). Mean germination percentages for
10 collections after 8 and 10 weeks of moist chilling were
59 and 91 percent, respectively. In a parallel field retrieval experiment conducted on an arid Wyoming sagebrush (Artemisia tridentata spp. wyomingensis) site in
southern Idaho, less than 1 percent of seeds planted in
midsummer and fall remained dormant in the following
spring. Seedling growth and maturation rates are slow;
343
Strategy B: Short-Lived, Opportunistic
Strategy B is associated with species that experience
episodic recruitment and die-off events, often correlated
with extreme weather events or harsh disturbance.
Though these species usually benefit from seasonal pulses
in soil moisture, vegetative and reproductive growth responses can occur any time effective moisture becomes
available and temperatures permit. High mortality rates
often follow explosive seed production events apparently
due to depletion of stored reserves (Fenner 1985). Species
are generally classified as short lived. Strategy B is best
described as a stress-tolerant ruderal strategy when
Grime's (1977) classification is applied.
Strategy B species rapidly reach reproductive maturity
and have a proportionately high investment of resources
in seed production (Fenner 1985; Gadgil and Solbrig
1972). Individual seeds require relatively small investments in resources and are mobile, allowing the species
to explore numerous new sites on both a spatial and temporal scale. Seed dormancy mechanisms, such as hardseededness, that function to ensure that a portion of the
seeds produced are committed to a soil seed reserve (remain ungerminated through periods favorable for seedling growth) are essential for species or ecotypes employing this strategy. The successful establishment of new
cohorts, though more common than with Strategy A species, is probably tied to disturbance events or sequences of
one or more seasonal pulses with higher than normal amplitude or longer than normal duration.
A good example of a Strategy B species is Palmer penstemon (Penstemon palmeri). This short-lived perennial
forb flourishes on disturbed sites such as washes and canyon bottoms (Cronquist and others 1984). Populations
have been successfully established and have persisted on
human-caused disturbances such as roadcuts. When soil
moisture is plentiful during the growing season, mature
plants produce copious quantities of seeds. Typically,
these plants become weakened and many die by the following spring (personal observation). Mortality is likely
due, at least in part, to stresses caused by high reproductive output. Cold-induced seed dormancy, a light requirement, and a long imbibition requirement before radical
emergence ensure maintenance of a soil seed reserve
(Kitchen and Meyer 1992; Meyer and Kitchen 1992).
Numerous other penstemon species exhibit strategies for
preserving a soil seed reserve (Kitchen and Meyer 1991;
Meyer 1992; Meyer and Kitchen 1993).
Gooseberry-leaf globemallow (Sphaeralcea grossulariifolia) has a strategy similar to that of Palmer
penstemon. This short-lived species (West 1979) is well
adapted to drought and is widely distributed in desert
shrub, pinyon-juniper, and low-elevation mountain brush
communities (Welsh and others 1987}. Persistent soil
seed reserves allow populations to cycle in response to
variable weather patterns (Sharp and others 1990). Typically, extended periods of favorable conditions result in
populations spending themselves in explosive reproductive events often followed by episodic die-off (personal observation). The strategy of the closely related munro
globemallow (S. munroana) is similar.
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Figure 1-Survival of Lewis flax collections in a
common garden near Nephi, UT. Greenhouse~
reared seedlings were transplanted in April 1989.
Mortality from 1990 to 1991 was significantly correlated with an index of severity of rust infection
(p < 0.05, r 2 = 0.75). All plants were treated for
rust in 1991, minimizing infection. Mortality from
1991 to 1992 is presumed unrelated to rust infection. The cultivar 'Appar' belonging to the European species perennial blue flax (Linum
perenne), remained free of infection though it
experienced 26 percent mortality, similar to that
experienced by some of the North American
Lewis flax collections.
Strategies A and B are not mutually exclusive. Species
with intermediate strategies combine elements of both.
In addition, the relative importance of each strategy, as
expressed by key traits, may vary considerably among
ecotypes of a single species occupying a wide range of
habitats. This variability is not surprising when the differences in moisture regime, disturbance factors, and
competition patterns among these habitats are considered. Solbrig and Simpson (1974) observed phenotypic
variability in seed production and competitive ability
among biotypes of common dandelion (Taraxacum officinale). Biotypes with relatively high reproductive output
dominate on sites with frequent disturbance (mowing),
while more competitive biotypes dominate less frequently
disturbed sites.
Ecotypes of Lewis flax (Linum lewisii) are adapted to
shadscale, sagebrush, pinyon-juniper, grassland, mountain brush, ponderosa pine, aspen, and spruce-fir communities (Welsh and others 1987). In a common garden study
with populations selected from diverse habitats, reproductive output varied by more than two times as measured by
flower number and total seed weight per plant (Kitchen
and others 1993). After 3 years, mortality among ecotypes ranged from 3 to 100 percent (fig. 1). Differential
resistance of ecotypes to rust infection significantly contributed to differences in second-year losses. Short-lived
ecotypes subject to frequent density-independent mortality events may gain little or no advantage by investing in
rust resistance. In addition, much of the mortality over a
3-year period occurred in plants that were rust-free and
prolific during the year before death, suggesting that patterns of reserve depletion during seed production may be
important to longevity among ecotypes of this and possibly other species (Gadgil and Solbrig 1972).
In laboratory germination studies with Lewis flax seeds,
a significant fraction of seeds developed cold-induced secondary dormancy for five of21 collections tested (Meyer
and Kitchen, in press). Nine of the remaining collections
had a portion (13 to 68 percent) of viable seeds that did
not become germinable after 24 weeks of stratification.
Field retrieval experiments verified the ability of this species to seedbank for multiple years. These results suggest
that Lewis flax tactics include provisions for maintaining
a soil seed bank, and that the relative importance of a
seed bank varies among ecotypes.
Another forb with an intermediate strategy is northern
sweetvetch (Hedysarum boreale). This moderate to
long-lived legume is most common in midelevation
communities and may have been reduced in distribution
and density by past overgrazing (Plummer and others
1968). Its roots penetrate deeply into the soil. Vegetative
dormancy occurs as soil moisture is reduced in mid to late
summer. First bloom occurs in spring to midsummer. A
second minor flowering sometimes results when initial
fruit set fails (personal observation). Both weather and
carbohydrate reserves probably play an important role in
determining the intensity of single-year reproductive allocation. Seeds are generally hard (do not imbibe water)
(Kitchen and others 1987; Redente 1982) and are sought
out and possibly cached by rodents. The hard-seeded trait
allows for a significant portion of uneaten seeds to carry
over in the soil. The species responds favorably to removal of competitive shrub species such as big sagebrush
(Artemisia tridentata) and Gambel oak (Quercus
gambelii). Large increases in vegetative and reprodu?tive
output are common following fires (personal observation).
EMPUCATIONSFORREVEGETATION
Familiarity with perennial forb strategies is essential to
avoid costly mistakes in revegetation projects and in formulating appropriate expectations for seeded species in
modified environments. Species and ecotype life-history
strategy must be matched to planting site environmental
conditions. The following questions and discussions
344
cons~ deration
for any seeding project. However, if seeded
species are short lived, requiring relatively frequent
recruitment episodes, long-term persistence may be prevented by weeds.
5. Will a fraction of viable seeds remain ungerminated
after the first season of growth? The seeds of most rangeland species are best planted in the fall. This permits
moist chilling of dormant seeds and accelerates spring
germination and growth of nondormant seeds. A viable
portion of the seeds of many forbs, including penstemons,
globemallows, Lewis flax, and northern sweetvetch, will
remain ungerminated the first spring following planting
(Strategy B). A knowledge of the seed-banking tendencies
of each species is useful in planning seeding rate and in
judging first- and subsequent-year establishment success.
Germination percentage can be increased for some species
using appropriate seed pretreatments. For example, hardseeded species such as sweetvetch and globemallows can be
scarified prior to planting (Redente 1982; Roth and others
1987) while species such as Lewis flax need only time in
dry storage to increase first-year germination (Meyer and
Kitchen, in press). Such tactics conserve the genetic capacity for seedbanking, a trait important for the longterm persistence of some species and ecotypes.
6. How important is seed size in planning seeding rate
and method? The seeds of some forb species are very
small (table 1) and are not able to emerge from typical
drilling depths. These seeds must be sown at or near the
soil surface. Seeding rates of these species can often be
quite low, a real plus considering the cost of obtaining the
seeds.
should be addressed in determining appropriate species or
ecotypes for each revegetation project.
1. Is the species adapted to the climate of the site? For
example, when considering precipitation, seasonality and
extremes may be as important as annual means in affecting success.
2. Is the expected disturbance regime compatible with
the long-term persistence of each planted species? Disturbance effects can be positive or negative depending on the
nature and timing of their occurrence. Intensity and frequency of disturbance are also important. For example,
occasional fire is beneficial for northern sweetvetch and
arrowleafbalsamroot, because of fire's negative effect on
competing species, while herbivory is not. Other species
such as Palmer penstemon benefit from frequent soil erosion such as is found on steep slopes and in wash bottoms.
The severity of damage, if any, to perennial forbs caused
by selective herbivory and trampling associated with
large ungulates (whether livestock or wildlife) is largely
dependent on season of use. There may be times when it
is appropriate to plant Strategy B species on sites where
they may not persist; their value is as nurse plants for
later seral species.
3. Are the forb species to be seeded compatible with
other seeded species and remnant perennials on this site?
An understanding of the competitive relationships among
species in seeded communities is largely lacking, so evalu":tion of this question is difficult. At the very least, caution should be used to not overplant aggressive long-lived
grasses on sites where a good representation of forbs is
desired.
4. How weed prone is the site? Competition from annual
grasses such as cheatgrass (Bromus tectorum), medusahead (Taeniatherum asperum), and jointed goatgrass
(Aegilops cylindrica) inhibits successful establishment
of perennial seedlings. Weed control should be a standard
Table
Efficacy in the use of a great variety of perennial forb
species will improve as research provides a more complete
understanding of perennial forb life-history strategies and
their interactions within different communities. Many species with potential value are presently poorly understood.
1-s~ed
size (seeds/lb) for 10 native perennial forb species. Quantities reported are adapted from
K1tchen and other~ (198~) and Shaw an~ Monsen (1983). Recommended seeding rates (lblacre) are
based ~n the spectes betng a part of a dtverse seed mix. Alfalfa and small burnet are included for
companson
Recommended
seeding rate
Scientific name
Seed
weight
Western yarrow
Louisiana sagewort
Pacific aster
Arrowleaf balsamroot
Lewis flax
Palmer penstemon
Firecracker penstemon
Rocky Mountain
penstemon
Gooseberry-leaf
globemallow
Northern sweetvetch
Achillea millieolium
Memisia ludoviciana
Aster chilensis
Ba/samorhiza sagittata
Linum lewisii
Penstemon palmeri
Penstemon eatonii
4,000,000
3,800,000
2,700,000
55,000
280,000
600,000
350,000
0.1
0.2
0.5
1.0
0.25
0.25
0.5
Penstemon strictus
Sphaeralcea
grossulariifolia
Hedysarum boreale
760,000
0.25
500,000
59,000
0.75
1.5
Alfalfa
Small burnet
Medicago sativa
Sanguisorba minor
225,000
45,000
0.5
2.0
Common name
No.llb
345
Lblacre
-
-
-
-
-
1.0
1.25
1.5
4.0
2.0
2.0
2.5
2.0
3.0
4.0
3.0
4.0
Our success in managing diverse communities in the
semiarid West will depend on carefully planned research
and its application.
Meyer, S. E. 1992. Habitat-correlated variation in
firecracker penstemon (Penstemon eatonii Gray:
Scrophulariaceae) seed germination response. Bulletin
of the Torrey Botanical Club. 119:268-279.
Meyer, S. E.; Kitchen, S. G. 1992. Cyclic seed dormancy in
the short-lived perennial Penstemon palmeri. Journal of
Ecology. 80: 115-122.
Meyer, S. E.; Kitchen, S.- G. [In press]. Life history variation in blue flax (Linum perenne: Linaceae): seed germination phenology. American Journal of Botany.
Meyer, S. E.; Kitchen, S. G. 1993. Habitat-correlated
variation in seed germination response to chilling in
Penstemon Section Glabri (Scrophulariaceae). Provo,
UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station. 37 p. Review draft.
Plummer, A. P.; Christensen, D. A.; Monsen, S. B. 1968.
Restoring big-game range in Utah. Publ. 68-3. Salt
Lake City, UT: Utah Division ofFish and Game. 183 p.
Ream, R. R.1963. The vegetation of the Wasatch Mountains, Utah and Idaho. Madison, WI: University of
Wisconsin. 178 p. Dissertation.
Redente, E. F. 1982. Sweetvetch (Hedysarum boreale var.
boreale) seed germination. Journal of Range Management. 35: 469-4 72.
Regal, P. R. 1977. Ecology and evolution of flowering
plant dominance. Science. 196: 622-629.
Roth, T. E.; Holecheck, J. L.; Hussain, M. Y. 1987. Germination response of three globemallow species to chemical treatment. Journal of Range Management. 40:
173-175.
Sharp, L.A.; Sanders, K.; Rimbey, N. 1990. Forty years of
change in a shadscale stand in Idaho. Rangelands. 12:
313-328.
Shaw, N.; Monsen, S. B. 1983. Nonleguminous forbs for
rangeland sites. In: Monsen, S. B.; Shaw, N., comps.
Managing Intermountain rangelands-improvement of
range and wildlife habitats. Gen. Tech. Rep. INT-157.
Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 123-131.
Solbrig, 0. T.; Simpson, B. B. 1974. Components of regulation of a population of dandelions in Michigan. Journal
ofEcology. 62: 473-486.
Tilman, D. 1988. Plant strategies and the dynamics and
structure of plant communities. Princeton, NJ:
Princeton University Press. 360 p.
Welsh, S. L.; Atwood, N. D.; Higgins, L. C.; Goodrich, S.
1987. A Utah flora. Great Basin Naturalist Memoirs 9.
Provo, UT: Brigham Young University. 894 p.
West, N. E. 1979. Survival patterns of major perennials in
salt desert shrub communities of Southwestern Utah.
Journal of Range Management. 32:442-444.
Young, J. A.; Evans, R. A. 1979. Arrowleafbalsamroot
and mules ear seed germination. Journal of Range
Management. 32:71-74.
REFERENCES
Abrahamson, W. G. 1979. Patterns of resource allocation
in wildflower populations of fields and woods. American
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J. L.; Holmgren, P. K. 1984. Intermountain flora; vascular plants of the Intermountain West, U.S.A. (vol. 4).
Bronx, NY: New York Botanical Garden. 573 p.
Everett, R. L.; Meeuwig, R. 0.; Stevens, R. 1978. Deer
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Fenner, M.1985. Seed ecology. New York: Chapman and
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Gadgil, M.; Solbrig, 0. T. 1972. The concept ofr- and Kselection: evidence from wild flowers and some theoretical considerations. The American Naturalist. 106:
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1169-1194.
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Kitchen, S. G.; Meyer, S. E. 1991. Seed germination of
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Kitchen, S. G.; Meyer, S. E. 1992. Temperature-mediated
changes in seed dormancy and light requirement for
Penstemon palmeri (Scrophulariaceae). Great Basin
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Kitchen, S. G.; Meyer, S. E.; Monsen, S. B. 1993. Lifehistory variation in Lewis flax (Linum lewisii: Linaceae): fecundity and plant longevity. Data on file at:
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Stevens, R. 1987. Addition of Hedysarum borealenorthern sweetvetch to the Rules. Association of Official
Seed Analysts Newsletter. 61: 65-66.
Menke, J. W.; Trliea, M. J. 1981. Carbohydrate reserve,
phenology, and growth cycles of nine Colorado range
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346