Quantification of vegetation diversity and variation on intact and degraded rangelands (Artemisia tridentata, Achnatherum thurberianum, Stipa thurberiana).

摘要

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, N0, N1, 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.