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Plant Species Diversity in a Grassland Plant Community:
Evidence for Forbs as a Critical Management Consideration
Monica L. Pokorny, Roger L. Sheley, and Tony J. Svejcar
Introduction
Grasslands are the earth’s largest
biome, comprising 24 percent of the
world’s vegetation and about 309
million acres in the United States.
Grassland habitat types in the northwestern United States are based on
grassland vegetation types, serial
stages of each type, and response
to grazing management practices.
While forb species are listed as
diverse components of grassland
communities, grasses have been the
primary focus in classification and
land management practices. Vegetative classifications historically have
assessed species composition once
during the growing season, which
does not account for the diversity of
spring or fall forbs. Although ecologists and land managers have recognized the importance of diverse
plant communities for the maintenance of healthy ecosystems, only
limited attention has been given
to the role of forbs in grasslands.
The purpose of this research was
to quantify species and functional
group diversity in a grassland plant
community in southwestern Montana. Specific objectives included
identifying plant species richness,
density, and biomass within an
Idaho fescue/bluebunch wheatgrass
grassland habitat type, using a
multiple-season sampling method,
identifying various functional
groups based on their morphology,
and comparing the richness to previously described diversities of this
habitat type.
Experimental Protocol
The study was conducted on two
sites within the Idaho fescue/bluebunch wheatgrass habitat type,
which lies at the cool-wet end
of grassland habitats. Sites were
located approximately 43.5 miles
west of Bozeman, Montana, on an
east-northeast aspect of a 20-degree
slope at 5,327 ft elevation. Species
richness and density were measured
during the spring, summer, and fall
of 2000 and biomass data were collected during the spring, summer,
and fall of 2001. Species richness
was measured by counting all species present on 4.8-yard2 plots. Forb
density was measured per 4.8-yard2
plot, while grass density was determined by counting tillers per species within a 0.7- by 1.6-ft frame.
Diversity indices were calculated
for each functional group. Biomass
by functional group was clipped
from three 0.7- by 1.6-ft frames per
4.8-yard2 plot.
Results and Discussion
Species diversity was high in both
sites. Sampling over time allowed
documentation of greater species
richness than previously was suggested for this habitat type. Forb
functional groups represented the
majority of the richness and biomass of the grassland community
studied. Forbs accounted for 83
percent of the vascular species richness in our research. In addition,
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forbs represented a greater proportion of plant biomass than grasses
on the study sites. Although three
to four grass species may comprise
a large portion of the biomass in
grasslands, forbs contribute more
to community diversity. The data
indicate that greater species richness coincided with greater overall
biomass or productivity. This finding is consistent with other research
suggesting increased diversity is
positively correlated with increased
community productivity and stabilization due to more complete use
of resources. Because maintenance
of functional group diversity is
suggested for maintaining optimum
plant community function, more
emphasis should be placed on managing grasslands for forb functional
group diversity.
Management Implications
Land managers should recognize
forb species and forb functional
group diversity in grassland classifications and should quantify species
at least twice during the growing
season for these community types.
By sampling once in the spring and
once in the summer, land managers
should be able to measure approximately 95 percent of the diversity in
Idaho fescue/bluebunch wheatgrass
habitat types. In comparison, a
maximum of 76 percent of community diversity was recorded with
only one summer field sampling.
Land managers should establish
and maintain forb species and forb
functional group diversity in land
Native community of forbs and grasses representing many functional groups.
management decisions. Intermediate levels of disturbance through
regulated grazing timing and intensity, planned herbicide application,
and periodic prescribed burning
have been proposed to maintain the
highest level of diversity.
Maintaining functional group
diversity should be a primary
objective of land managers because
increased functional group diversity
correlates with increased stability and productivity of the land.
Increasing functional diversity also
decreases the risk of invasion by
undesired species. Indigenous forb
functional groups should be recognized as an essential component for
proper land management because
they may increase community resistance to noxious weed invasion.
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