Exotic species can have large environmental impacts. In the Sonoran and Mojave deserts, Schismus arabicus Nees and Bromus rubens L., are able to invade open, inter-shrub and shaded, under-canopy 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 above-average 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 in the open and shaded environments offered by the shrubs. Individuals were harvested after maturity and measurements of final height and longest leaf were recorded.
Results/Conclusions
A generalized, linear, mixed-effects model indicates that final height was most influenced by water treatments (p < 2.2e-16), water-nitrogen treatment interactions (p = 0.028), and shade-water-nitrogen treatment interactions (p = 0.008). A similar model analyzing longest leaf measurements showed that leaf growth was only strongly influenced by water treatments (p = 1.791e-10) and showed that no significant interaction was found between shade-water-nitrogen treatments (p = 0.081). Under higher water treatments, B. rubens grew taller (average = 22 cm) and produced longer leaves (average = 18 cm). This response indicates that B. rubens has the potential to produce greater amounts of biomass in wetter years. Current climate models predict drier overall conditions, however precipitation patterns are expected to become more severe and ultimately make certain areas wetter. This study suggests that in wetter environments, invasive grasses like Bromus rubens could produce a greater fuel load for greater 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 ignition and fuel sources for fire spread. More work is needed to fully understand how species may respond to long-term changes in climate.