Global climate change is driven by a complicated set of interacting environmental pressures, such as rising temperatures and nitrogen deposition. While substantial progress has been made in examining the impacts of one or two concurrent global change pressures, few studies manipulate 3 or 4 variables simultaneously over the long-term. Because of this experimental knowledge gap, we are unsure how combinations of global change drivers will interact to affect the environment. In fact, global change drivers may be additive, synergistic, or counter-active. Nevertheless, it is important to understand how species richness is affected by changing climatic variables as biodiversity is linked to important processes including community productivity, stability, and invasive species resistance. Here, we seek to understand how species richness is affected by multiple global change pressures by using an experiment at Cedar Creek Ecosystem Science Reserve (East Bethel, MN) that manipulates temperature, rainfall, CO2, and nitrogen (TeRaCON) in a full factorial design (n = 3) on a tallgrass prairie community. The treatments were applied for 5 to 18 years and are as follows: temperature increased by 3oC, growing season rainfall reduced by 45%, nitrogen increased by 4gN m-2 yr-1, and CO2 elevated by 180 ppm.
Results/Conclusions
We tested the effects of the four global change variables on species richness using mixed effects models based on species richness data for the year 2016. We found no significant effects on species richness in the 3 or 4 way interactions. The water and temperature treatments individually resulted in significant decreases to species richness (p = 0.04, p = 0.01 respectively), however the combination of these two treatments increased species richness but still resulted in species richness below ambient levels (p = 0.01). A similar pattern was found in the interaction between the water and nitrogen treatments as the water and nitrogen effects decreased species richness (p = 0.04, p = 0.09 respectively) while this species loss was mitigated by the combination of both nitrogen addition and water exclusion (p = 0.03). The overall tempering effects of interacting variables are contrary to predictions that stacking changes would result in similar or exaggerated species loss, emphasizing the importance of studying how multiple global change factors interact to impact communities.