A field experiment testing the effects of climate change, nitrogen deposition, and invasion on a Chihuahuan Desert ecosystem

Background/Question/Methods The Chihuahuan Desert has undergone dramatic shifts in vegetation structure and composition due to invasions of non-native plant species.  Our long-term dataset has revealed a similar invasion of a winter annual plant community in the San Simon Valley of southeastern Arizona, where diversity and abundance of native annuals has dramatically declined due to a sustained irruption of an exotic Eurasian species, Erodium cicutarium.  In conjunction with vegetation change, our study system is experiencing shifts in climate and nitrogen deposition.  The objective of our study is to determine the role of temporal patterns of water availability and nutrient levels in the sustained irruption of E. cicutarium and to determine the impact of E. cicutarium on the native plant community.  To fully understand the impacts of these physical and biological factors, we implemented a factorial experiment manipulating precipitation (early versus late season addition), nutrients (carbon and nitrogen additions), and E. cicutarium (removal versus control).  We monitored soil moisture and temperature, and germination and survival of three cohorts of seedlings throughout the season.  End of season plant harvest measurements will determine abundance, plant cover, seed production, and plant physiological characteristics (C and N content, and water-use efficiency).  
Results/Conclusions Timing of precipitation addition strongly affects relative abundance of the annual plants, with E. cicutarium responding stronger to early than late season addition.  Early season precipitation addition initiated and maintained higher species diversity than a late season addition across all nutrient treatments.  Late season precipitation addition resulted in less germination, but germinants survived better.  Results to date show a strong effect of E. cicutarium removal and precipitation addition on the density and number of species.  E. cicutarium abundance was highest in early season precipitation addition and nitrogen treatments.  Plots that received the carbon treatment and precipitation addition early in the season experienced reduced abundance of E. cicutarium.  Whereas plots that received nutrients and precipitation addition late in the season showed little contrast in E. cicutarium abundance.  Timing of precipitation treatments tests whether early or late season favors natives or E. cicutarium for germination and growth.  This factor combined with the differential effects of nutrient additions elucidates the potential role of human drivers of community change.  Ultimately, our results may provide a better understanding of community shifts in diversity and composition as a result of changing climate and nitrogen deposition in this arid ecosystem.