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Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts
STUDENT POSTER PRESENTATION
Factors Affecting Alien Annual Plant Abundance at a Site in the Western Mojave Desert: Effects of Human Disturbance, Microhabitat, Topography, and Rainfall
Matthew L. Brooks
University of California, Riverside, Riverside, CA 92521
Mailing address: 41734 South Fork Dr., Three Rivers, CA 93271
Four alien annual weeds (the grasses Bromus madritensis rubens, Bromus trinii, and Schismus spp, and the forb Erodium cicutarium) have invaded the Mojave Desert. To identify conditions that may have promoted their spread, I evaluated the association of temporal and spatial variation in their biomasses with environmental factors during one high and one low rainfall year. Sampling was stratified to sample all combinations of the following factors: (1) topographic position (upland, washlet); (2) microhabitat (intershrub, north creosote bush canopy, and south canopy); and (3) human disturbance, in the combined form of sheep grazing and off highway vehicle use (protected, unprotected from disturbance). Biomasses were generally highest in washlets, under creosote bushes, and in the unprotected areas. The distribution of Schismus spp and Erodium cicutarium varied between years, with greatest amounts under the south canopy during the wet year and intershrub spaces during the dry year. These results suggest that human disturbance and comparatively mesic conditions present during wet years, under shrubs, and in washes, may facilitate annual plant invasions into the Mojave Desert. Relative abundance of aliens was highest during the dry year, suggesting that their dormancy requirements may be less stringent than that of natives, which may explain their population declines following droughts.
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