Species composition in California’s grasslands varies with the interactions between precipitation, nitrogen deposition, clipping and identity of plant competitors

Background/Question/Methods

It is well acknowledged that multiple interacting factors influence plant community composition, and that these interactions must be understood to increase our effectiveness in restoring native communities and controlling invasive species, particularly under changing environmental conditions. Particularly in annual-dominated systems, it is also critical to understand how the effects of these factors may vary due to inter-annual variability in weather patterns. We investigated how community composition in California grasslands is impacted by: the timing and amount of precipitation, nitrogen deposition, and the timing of clipping.  In the fall of 2007, we planted three community types prevalent in California grasslands (on their own and in mixtures):  (1) naturalized annuals (dominant for the last 200-300 years); (2) native species commonly used in restoration; and (3) more recent annual noxious weeds (Aegilops triuncialis, Elymus caput-medusae). These plant treatments were exposed to: precipitation manipulations (starting 2010- control; longer, wetter growing seasons;  shorter, drier growing seasons), nitrogen deposition (starting 2008- control, 45 kgN/ha/yr), and clipping (starting 2008- none, fall, spring). At the end of each growing season, we assessed the effects of these treatments on percent cover of species

Results/Conclusions

Drier seasons  generally increased the prevalence of natives , while decreasing noxious weeds, with the strength of this effect  varying by year. Wetter seasons consistently had strong impacts on species composition, but different suites of species responded each year. In mixes of the naturalized annuals, natives and invaders, precipitation largely influenced the prevalence of the natives and invaders. The responses of naturalized annuals were constrained by the other species groups, since the naturalized annuals showed strong responses to wetter conditions only when the invaders weren’t present, and strong responses to drier conditions only when the natives weren’t present. The effects of nitrogen deposition varied by year, significantly increasing the prevalence of noxious weeds in 2010 and 2013, while decreasing them in 2011 and 2012. Native bunchgrasses tended to increase in cover in response to N deposition. Clipping consistently increased the prevalence of weeds, while the effects on natives varied by year.  Significant interactions occurred among treatments. In particular, N deposition strongly mediated the impacts of clipping on community composition. Precipitation, clipping and N deposition had little effect on total plant cover, but caused strong annual variation in species composition, highlighting the importance of species richness in providing resilience to environmental changes.