Invasive plants can alter ecosystem function by affecting the controls that regulate ecosystem processes or by responding differently than native species to environmental change. In particular, invasion can alter nitrogen (N) cycling, which is a primary driver of ecosystem function due to its effects on plant productivity. However, much less is known about how invasions and environmental change may interact to alter N availability. Here we evaluated how the problematic invasive cogongrass (Imperata cylindrica) and reduced water availability (hereafter “drought”) independently and interactively influenced plant productivity and soil N availability in two experiments. The first was a common garden experiment with a factorial combination of invasion and drought treatments: uninvaded (native-only vegetation) with ambient precipitation, invaded (native vegetation plus cogongrass) with ambient precipitation, uninvaded exposed to drought, and invaded-drought. The second was a complementary field experiment with nine sites naturally invaded by cogongrass and spanning a soil moisture gradient with three replicated treatments established at each site: invaded, nearby uninvaded, and invader removal.
In the common garden experiment invasion and drought independently increased nitrate concentrations relative to the ambient, uninvaded treatments by 943 and 708%, respectively. The invaded plus drought treatment only increased nitrate concentrations by 562% with a corresponding increase in invader biomass. Thus under drought conditions cogongrass appeared to maintain productivity by assimilating soil nitrate, although further work is needed to uncover mechanisms.
Across the soil moisture gradient, ammonium concentrations were highest at sites with greatest soil moisture in uninvaded and invader removal plots (linear regression; r2 = 0.68 and r2 = 0.70, respectively) but this relationship was weaker in invaded plots (r2 = 0.26). Invasion seems to decouple the relationship between moisture and nutrient availability, thereby minimizing the effect of environmental change on biogeochemical cycling in invaded systems. In the invaded plots, aboveground biomass was greatest at sites with the highest soil moisture (r2 = 0.58), driven primarily by cogongrass biomass (r2 = 0.47). These data suggest that nutrient availability and plant biomass responses to soil moisture availability differ between invaded and native-dominated communities. Thus, invasion seems to buffer the effects of drought and lower soil moisture on biogeochemical processes and may provide a mechanism for cogongrass to outperform native species in a changing climate.