Abstract:
Vegetation diversity in the Wyoming big sagebrush/Thurber needlegrass
(Artemisia tridentata Nutt. ssp. wyomingensis Beetle & Young/Achnatherum
thurberianum (Piper) Barkworth. syn. Stipa thurberiana Piper) plant association
was studied across a condition class gradient in southeastern Oregon.
All observable vascular plant species were counted during 6 seasons and
2 years and diversity indices were calculated using both density and percent
cover. Soil surface characteristics were evaluated once per year for 3 years.
The measures of vegetation diversity evaluated in this thesis are: Species
Richness, Simpson's Index, Shannon's Index, and Hill's diversity numbers, NO,
NI, N2, and evenness ratio E5. Functional categories were addressed using
Raunkiaer's Biological Classification system (1934) and life form classes used
by the USDA. Soil surface patterns were evaluated using types described by
Eckert et al. (1986a).
High seasonal and yearly variation exhibited by all classification
schemes studied (species richness counts, diversity indices, functional groups, and soil surface microtopography) confounded their ability to consistently
distinguish among condition classes. Highest diversity was obtained in the
spring and least in the fall.
A significant correlation was obtained between diversity and a modified
condition class index calculated using actual field data by season, and showed
that based on diversity, sites might be classified as poor in one year and good in
the next.
Five distinct soil surface types showed apparent trends across the
gradient, but no statistically significant results were obtained between the
amount of cover of any of the soil surface types and range site condition.
Significant natural changes in the amount of cover by soil surface
microtopography category did occur in the year following grazing exclusion
regardless of range condition. This may indicate system resilience is initially
high and that recovery thresholds may be approached quickly regardless of
initial range condition.
Our results indicate that in ecosystems where distinct seasonal changes
affect plant community composition, species richness numbers and diversity
indices should be used with caution as a criterion for ecological assessment. An
a priori delineation of the temporal scale for measurement is an essential
condition of using any vegetation or soil indicator as an evaluation tool.
