28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

POSTER

Can Removal of Alien Annual Grasses Significantly Benefit Native Annual Plants?

M. Brooks and S. Santos
U.S. Geological Survey, Western Ecological Research Center, Las Vegas Field Station, 160 N. Stephanie St., Henderson, NV 89014

Alien annual grasses in the genera Bromus (B. rubens, B. tectorum, and B. trinii) and Schismus (S. arabicus and S. barbatus) are widespread and abundant in the Mojave Desert, and negative correlations between the abundance and diversity of these aliens and native annual plants suggest that competition may occur between them. It is assumed that this competitive effect reduces density and biomass, and therefore reproductive success and diversity, of native plant species. General land management plans for large administrative units, and specific restoration plans for individual sites, typically include stipulations for reducing dominance of alien plants for the purpose of benefiting native plants. However, the hypothesis that thinning of alien annual plants will significantly increase density, biomass, and diversity of native annual plants in the Mojave Desert has not been substantiated experimentally, and is only supported by correlative data. This poster presents an experimental test of this hypothesis involving the thinning of the two dominant alien annual grass taxa in the Mojave Desert, Bromus and Schismus.

Alien annual grass seedlings were thinned from twenty-five replicate 20 x 25cm treatment plots, in beneath-shrub (north side of Larrea tridentate canopy) and interspace (>1m from the nearest shrub canopy) microhabitats, at three sites in the central, southcentral and southwestern Mojave Desert, during two years of contrasting plant productivity (1996 and 1997). All alien annual grass seedlings were removed first at the end of January, then a second time approximately 2 weeks later at the beginning of February, during each year. Effects of Bromus and Schismus thinning were evaluated separately in the microhabitat where each was most abundant, in the beneath-shrub microhabitat for Bromus and in the interspace for Schismus.

Thinning of Bromus and Schismus significantly increased density, biomass, and species richness of native annuals at all three sites, but only during a year of high annual plant productivity and species richness. Effects of thinning were strongest for the native Amsinckia tessellata and for a group of relatively uncommon native annuals. Thinning also significantly increased the density and biomass of the alien forb, Erodium cicutarium, highlighting the need to evaluate the entire alien plant flora before instituting control efforts for individual alien species. If the target alien annual plant may be replaced by an even less desirable species, then control efforts should be avoided.

These results indicate that removal of alien annual grasses can increased density, biomass, and species richness of native annual plants, but that this type of control method may only be effective during years of high annual plant productivity. This study is limited at temporal and spatial scales to 2 years of native plant response in small experimental plots. The longer-term effects of thinning at larger spatial scales, and the effects of thinning using more cost-effective methods such as grass-specific herbicides, remain unknown. Native plant responses to control methods for invasive alien forbs and perennial grasses are also needed. These types of studies are the next step in providing land managers with the information they need to reliably predict the effects of efforts to manage alien annual plants in the Mojave Desert.