Most plant communities will face multiple environmental changes in the future, and predictions on the composition of future communities could be improved by experiments that explore the potential for non-additive interactions. The Warming / El Niño Nitrogen Deposition Experiment (WENNDEx) is a multi-factorial experiment investigating interactions among three key global change factors: increased nighttime temperatures, nitrogen addition, and increased winter precipitation. These treatments represent the higher temperatures, nitrogen deposition and more frequent El Niño events predicted to occur in the future because of climate change and nitrogen pollution. The experiment is located on the Sevilleta National Wildlife refuge in New Mexico, at a transition zone between Chihuahuan Desert grassland (dominated by black grama, Bouteloua eriopoda), and shortgrass prairie (dominated by blue grama, Bouteloua gracilis). The WENNDEx experiment was established in 2006, and in 2009 a wildfire burned the experiment. Here, we present analyses on data from 2010 onward, as plant communities recovered from fire. We examined how the three treatments singly and interactively altered plant biomass of the two dominant grasses to test for effects in re-ordering the foundation species at their ecotone.
Results/Conclusions
Overall, black grama declined under nitrogen addition and increased with winter precipitation additions, while blue grama increased under nitrogen addition and decreased under warming. However, only some treatment combinations differed significantly from the unmanipulated control. Black grama biomass was significantly reduced by precipitation + nitrogen (PN; P<0.01), and by nitrogen + warming (TN; P=0.03). PN plots had an average of 50% less black grama biomass than control plots, and TN plots had 60% less black grama than control plots. In contrast, blue grama biomass was significantly lower than control in precipitation + warming plots (TP; P=0.03), and in plots subject to all three treatments (TPN; P<0.01). TP plots had 47% less blue grama biomass than control, and TPN plots had 36% less.
Our results indicate that not all environmental changes favor the same species, so the future plant communities of central New Mexico may depend on the strength of different change drivers. Since all three of the factors simulated by this experiment are likely to occur in the future, the TPN treatment may represent the most realistic scenario. This treatment favored black grama over blue grama, with the ratio of black to blue grama 3.3x larger in TPN than control, suggesting future expansion of desert plant communities at the expense of prairie in this region.