Climate change is predicted to have significant impacts on precipitation patterns, including changes in seasonality along with the occurrence of more severe and prolonged droughts. The effects of prolonged drought have been widely investigated, but studies on changes in precipitation seasonality are much less common. In the southwestern United States, most annual rainfall occurs during the summer monsoon from July through early September. The monsoon season typically starts after a two month dry period following spring rains. Downscaled climate models indicate a delayed start to the summer monsoon, creating a more extended hot, dry period between rainy seasons. We experimentally investigated the impacts of chronic drought and delayed monsoon rainfall on plant community composition and structure in blue grama and black grama dominated grassland in central New Mexico, USA. To assess the effects of drought and delayed monsoon, we conducted two concurrent rainfall manipulation experiments. For the delayed monsoon, we used complete rainout shelters to capture all rainfall during July and August and reapplied this rainfall in September and October each year from 2013 to 2017. For the chronic drought we erected partial rainout shelters that removed 66% of ambient rainfall from April through September each year.
Results/Conclusions
We found no differences in community composition and structure in pretreatment data. Based on PERMANOVA, differences (P<0.0001) in community composition in black grama grassland occurred between control and drought treatments in year 1, and between all treatments (P<0.0001) during years 2 through 5. For blue grama grassland, differences in community composition among all treatments started in year 1, but differences between control and delayed monsoon treatments were weaker than control vs. drought (0.01>P<0.05). We found significant year by treatment effects on species diversity, richness, and evenness in both blue and black grama grassland. In black grama grassland, diversity was highest in year 5 in the drought treatments, whereas diversity was highest in drought treatments in blue grama grassland during years 3 through 5. Across all years, the drought treatment increased community heterogeneity in blue grama grassland, whereas the delayed monsoon increased community heterogeneity in black grama grassland. Differences among treatments were primarily driven by large declines (75-90%) in dominant grasses under drought, as well as increases in abundance and richness of several subdominants in both drought and delayed monsoon treatments. Overall, our results suggest that these semi-arid grasslands are sensitive to changes in both the amount and seasonality of precipitation.