Changes in species cover due to interannual variation in temperature and precipitation in a tallgrass prairie experiment

As part of an experiment examining the effects of a single anomalously warm year and increased precipitation on ecosystem processes, we previously found that above-ground biomass of C₄ plants was increased compared to C₃ plants in warmed plots and that this difference carried over into the post treatment year. Here we examine changes in species composition as measured by relative cover. The experiment consisted of 20 plots in a randomized block design with 2 levels of temperature (ambient and +4^oC) and 2 levels of precipitation (ambient and 2x ambient). Measured in mid-summer, non-leguminous annual forb species decreased in relative cover 40.5% overall between the pretreatment and treatment year (which was especially dry) while relative cover of legumes and perennial forbs increased 42.6%. In warmed treatments, increased relative cover of C₄ grasses was largely due to decreases in several low abundance forbs. Non-graminoid C₃ cover increased 5% in 2x precipitation plots. Significant differences in the magnitude of change between treatments were found in Achillea millefolia, Ambrosia trifida, and Sorghastrum nutans. Ambrosia psilostachya showed a greater increase in cover in warmed plots than in control and 2x precipitation plots, and little bluestem decreased in control and 2x precipitation plots while increasing in warmed plots. Overall, the relative increase in C₄ plants in warmed plots can be contributed to a dramatic drop in minor forb species in those plots, and decreased relative cover of little bluestem in control and 2x precipitation plots.