The role of fire and a nonnative grass as disturbances in semi-desert grasslands of southeastern Arizona.

摘要

Disturbances are key processes that alter environmental conditions which have consequences for species interactions; therefore, disturbances are important to maintenance of biological diversity. Climate, fire, livestock grazing, introduction of nonnative species, and humans all influence semi-desert grasslands of the southwestern United States. In southeast Arizona, a nonnative perennial grass, Lehmann lovegrass (Eragrostis lehmanniana), was introduced in the 1930s in an effort to restore degraded rangelands. I quantified annual fluctuations of Lehmann lovegrass and the response of native plants along a gradient of dominance by this nonnative grass. Biomass and richness of native plants declined with increasing dominance by Lehmann lovegrass. Substantial increases in biomass by Lehmann lovegrass in response to above-average rainfall did not result in decreases in native plants. Fire is an important disturbance in semi-desert grasslands. Herbaceous plants are favored over trees and shrubs in areas with frequent fires, therefore fire maintains grassland physiognomy. Humans have manipulated natural fire regimes however, by altering their frequency, season, and spatial extent. I experimentally investigated the possibility of a positive feedback cycle in grasslands invaded by Lehmann lovegrass by assessing whether the presence of Lehmann lovegrass was enhanced by fire to the detriment of native plants. In the two years post-fire, the proportion of Lehmann lovegrass on burned sites did not increased compared to unburned sites or to pre-fire conditions. I also detected no changes in species richness, diversity, or biomass of native plants in response to fire for 2-3 years after fires. Changes in biomass of all plants varied among years, especially due to exceptionally wet conditions during spring 2001, and varied if fires were set in spring versus summer. Lastly, I investigated the response of Agave palmeri to fire, a species that provides forage for a migratory bat that is federally endangered. Survival of agave varied with soil type, fire season, and size of the plant. In general, agave can tolerate up to 85% damage before mortality. Germination of agave seed was decreased by fire. Based on my research, I recommend setting fire at natural frequency and season for the maintenance of semi-desert grasslands.