Water availability in the southwestern US is projected decrease with climate change due to rising temperatures and more frequent and intense droughts. These impacts are projected to vary by season, with the largest decreases in precipitation occurring during the winter and spring. Such seasonal changes in water availability may have large ecological effects on ecosystems such as the Colorado Plateau, which rely heavily on cool season precipitation for soil moisture recharge. We investigated the impacts of extreme drought seasonality on a mixed grassland-shrubland in southeastern Utah. The experiment consisted of three precipitation treatments: a control (ambient precipitation), a warm season drought (-66 % precipitation reduction May-Oct.) and a cool season drought (-66% precipitation reduction Nov.-Apr.). We measured species-level responses (phenology and aboveground biomass) of the dominant C3 grass, Achnatherum hymenoidies, and the dominant C4 grass, Pleuraphis jamesii, in plots containing a mixed grass only, or mixed grass and Ephedra viridis (dominant shrub) community.
Results/Conclusions
After three years of the experiment, we found strong responses to the seasonal drought treatments, with some differential responses by plant functional type and treatments. For both grasses, cool-season drought delayed green up, while warm-season drought advanced senescence, with both drought treatments decreasing growing season length. We observed a significant interaction between drought treatments and plant community on green up for both grasses, suggesting that the effects of drought may be influenced by the presence of E. viridis. There was a significant effect of drought treatments on biomass of both C3 and C4 grasses, but this effect varied by year, season, and community. There were divergent impacts on biomass depending on the seasonal timing of drought on the two grasses; cool-season drought had greater impacts on the C3 grass and warm season drought had greater impacts on C4 grass. These results suggest that community composition and the seasonal timing of drought in this semi-arid grassland may have negative impacts on the phenology and biomass of these important herbaceous species with the potential for significant ecological impacts.