Description
Nitrogen deposition and precipitation variability can have large effects on the structure and composition of Coastal Sage Scrub communities. These can occur as direct responses to changes in resource availability or indirectly through shifts in the competitive relationships among the many native and non-native plant species. These interactions can influence phenology, limit species distributions, and may play a role in the extinction risk of rare plants. There have been studies on how resources impact invasive species in Coastal Sage Scrub but few have focused on the impacts of invasive grasses on rare, endemic native forbs. I conducted a greenhouse study of the effects of nitrogen and moisture on the relationship between Acanthomintha ilicifolia (a rare native forb) and an invasive grass Brachypodium distachyon. I employed a 3-way factorial design using three moisture levels, three nitrogen treatments, and the application of a selective herbicide used to manage Brachypodium distachyon. Multiple metrics were used to assess the impact of the treatments including aboveground biomass, flower production and timing, leaf size, and chemical composition. For both species, inter and intraspecific competition limited vegetative growth and competitor densities was often more important than the identity of the competitor. We observed weak nitrogen addition effects, likely due to unexpectedly high levels of nitrogen in the natural soil used in the experiment. Total amount of water was not an important factor in this experiment. This could be the result of the relatively mild conditions in the greenhouse or because our experiment could not mimic other stressors like extreme variability in soil moisture and large changes in temperature and insolation. Finally, herbicide treatment was effective at controlling Brachypodium distachyon however significant delays in growth and flowering of Acanthomintha ilicifolia were observed. Herbicide treatment also appeared to alter soil nitrogen relationships. These observed changes in phenology of the endangered species and belowground soil processes need to be considered prior to its continued and wide-spread use in conservation management. Although it can be difficult to extrapolate the results of greenhouse experiments to the field, our results have significant implications for current management practices and warrant further research.
