Exotic species can have large environmental impacts. In the Sonoran and Mojave deserts, Schismus arabicusNees and Bromus rubensL., are able to invade open, intershrub and shaded, undercanopy habitats of the creosote bush, Larrea tridentata(DC.) Coville, creating pathways for fire to spread. Increasing fire risk, an uncommon disturbance in these deserts, is strongly linked to changes in climate, particularly precipitation patterns. Years with aboveaverage precipitation, such as those observed in 2004/2005, have been shown to cause increased fuel load through an increase in invasive grass biomass production. L. tridentata, is known to produce “islands of fertility” where pockets of soil with increased nitrogen availability are observed. Understanding how these individual species respond to a varied environment can help inform us of their potential to increase fire risk in the community. The shrub microhabitat effect on B. rubens was studied in a greenhouse setting. Individuals were exposed to varying shade (50%, 30%, 0%), water (10 mL, 5 mL), and nitrogen (40 ppm added, 12.3 ppm added, 0 ppm added) treatments to simulate the microclimate environments offered by the shrubs. Individuals were harvested after maturity and measurements of final height, longest leaf, and dried above-ground ground biomass were recorded.
Results/Conclusions
A generalized, linear, mixedeffects model indicates that final height was most influenced by water treatments (p < 2.2e16), waternitrogen treatment interactions (p = 0.028), and shadewaternitrogen treatment interactions (p = 0.008). Longest leaf measurements showed that leaf growth was influenced by water treatments (p = 1.791e10). Biomass production was strongly impacted by water (p < 2.2e-16) and nitrogen (p = 1.463e-08) while also showing a significant water-nitrogen interaction (p = 0.01). Under higher water treatments, B. rubensgrew taller (average = 22 cm), produced longer leaves (average = 18 cm), and produced more biomass (average = 268 mg). These responses indicate that B. rubenshas the potential to produce greater amounts of biomass in wetter years. Current climate models predict drier conditions, however precipitation patterns are expected to become more variable and make certain areas wetter. This study suggests that in wetter environments, invasive grasses could produce a greater fuel load, increasing potential fire risk. However, previous work suggests that for fuel loads in arid regions to build up, two consecutive years of heavy rainfall are necessary to provide proper fuel sources for fire spread. More work is needed to understand how species may respond to longterm changes in climate.