US Geological Survey - Southwest Biological Science Center Moab, United States
Climate change is projected to reduce water availability in many dryland ecosystems due to rising temperatures, more frequent and intense droughts, and seasonal changes in precipitation. Experiments and observations suggest that grass-dominated drylands have low resistance and resilience to drought. We investigated the impacts of extreme drought on a mixed grassland-shrubland in the semi-arid Colorado Plateau of the southwestern US. 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.) imposed over a four-year period, followed by two years of recovery (ambient precipitation). We measured plant community and ecosystem dynamics in plots containing a mixed grass only, or mixed grass and Ephedra viridis (the dominant shrub) community to evaluate drought resistance and resilience over the six-year study.
Four years of drought had large impacts on the herbaceous plant community, particularly the cool-season grasses, which suffered high mortality. In contrast, the dominant shrub, E. virdis had few effects of drought and thus demonstrated high drought resistance. Both seasonal drought treatments lead to a decline in total biomass but had differential impacts on the plant community. The multi-year drought also had several belowground impacts, including shifts in the vertical distribution of roots and increases in soil nitrate with drought. Recovery was limited in the two years following the drought treatment, which is consistent with previous studies. However, a naturally occurring drought in the region may exaggerated low drought resilience. Overall, these results suggest that extreme multiyear drought can have large and prolonged ecological impacts in this semi-arid ecosystem.