SYMP 19-2 - Will climate change lead to ecological transition in a grassland-shrubland ecotone in northeast Wyoming?

Ecotones, or boundary zones between ecoregions, can be especially sensitive to shifts in abiotic conditions because species in these regions are coexisting near the edges of their respective ranges. Global change drivers are more likely to cause ecosystem transitions in ecotones, but it is unclear which drivers or combinations of drivers are most likely to cause change. Climate change is predicted to increase interannual precipitation variability in many regions, including the semi-arid western edge of the North American Great Plains. In this region, heightened variability is predicted to be coupled with increased winter precipitation and more extreme droughts. The consequences of these shifts in precipitation variability and seasonality for plant communities and ecosystem function remain uncertain. We investigated the separate and combined effects of shifting precipitation variability and seasonality on plants in a shrubland-grassland ecotone. We experimentally enhanced interannual precipitation variability by implementing precipitation reductions (-50% of ambient) or precipitation additions (+50%), and then reversing these treatments every other year. Precipitation variability treatments were crossed with added winter moisture treatments, in which we increased winter moisture by 50% at the end of the dormant season. We predicted that increases in interannual variability and winter precipitation should favor deep-rooted shrubs at the expense of perennial grasses in this ecosystem due to differences in rooting profiles and soil moisture availability.
During the first five years of the experiment, shrub density increased in plots receiving increased interannual variability as well as added spring moisture, while densities remained stable in all other treatment combinations. Precipitation manipulation treatments also had strong and lagged impacts on invasive annual grasses. When compared to plots receiving ambient precipitation, these annuals were more abundant in plots that had been watered in the previous year, despite the fact that these same plots were receiving reduced precipitation during the current growing season. Perennial grasses exposed to increased variability treatments experienced more water stress but generally maintained their aboveground production and composition relative to ambient plots. These results indicate that predicted climate changes may benefit annual grasses and shrubs, but not perennial grasses. Ecosystem transition may not be driven by increasing precipitation variability and shifting seasonality alone, but transitions may become more likely when these changes are combined with other factors that reduce perennial grass resilience, such as multi-year droughts or heavy grazing.