Semi-arid grassland plant community responses to 7 years of experimental warming, elevated CO2, and irrigation

Background/Question/Methods

Grasslands cover 30-40% of the Earth’s terrestrial surface and constitute the largest intact biome in the conterminous US. Semi-arid grasslands are expected to be particularly sensitive to climate change because of the overriding control of soil moisture on virtually all ecological processes in these ecosystems. Over 7 years in the Prairie Heating and CO₂ Enrichment (PHACE) experiment in the mixed-grass prairie ecosystem of southeastern Wyoming USA, we evaluated how manipulations of soil moisture and elevated atmospheric CO₂ and temperature influenced plant community composition and species dominance patterns. We predicted that in dry years, increasing soil moisture and elevated CO₂ would have similar effects on plant community composition, but we expected to see those effects diminished in wet years. In contrast, we expected warming to counteract the effects of elevated CO₂. In addition to the effects on plant community composition, global change may have a destabilizing effect on community composition. We assessed how the global change factors affect community stability and in particular, determined whether resistance to change depended on the initial plant community.  

Results/Conclusions

Overall plant community composition shifted over the 7 years of global change manipulations, and a divergence in plant community composition among the PHACE treatments was apparent. The direction and magnitude of the global change effects differed among years and also between the dominant and subdominant community members. Differences in plant community composition among treatments were more pronounced in drier years. In addition, the relative biomass of dominant species decreased in the elevated CO₂ plots over time and the relative biomass of subdominant species proportionally increased. Compositional dissimilarity decreased over time in plots that received both elevated CO₂ and warming treatments, suggesting that these plots became more similar to each other over the course of the experiment. We did not find significant direct effects of altered soil moisture on most plant community measures. Our results suggest elevated CO₂ and warming have very different effects on plant community composition, largely through their influence on patterns of community dominance. We also found that interannual climate variation mediated the effects of climate change treatments on plant community composition. Long-term field experiments, such as the PHACE experiment, help reveal complex, and in our case, nonlinear responses to climate change that propagate through plant communities. These responses include both compositional shifts and changes in community dominance structure, with important consequences for the structure and function of these semi-arid grasslands.