Interactive Effects of Drought and Grazing on Vegetation Dynamics in the Shortgrass Steppe

Grasslands comprise a large portion of Earth’s terrestrial surface and they occur in regions with low precipitation and have coevolved with grazers. The shortgrass steppe (SGS) of eastern Colorado is dominated by C 4 perennial grasses ( Bouteloua gracilis and Bouteloua dactyloides account for >70% of vegetation cover) and is highly resistant to grazing and droughts. The dominance of these two Bouteloua species is considered stable in this ecosystem, taking one or more decades of changes in grazing or precipitation to result in changes in species composition. However, the interaction of grazing and drought are rarely studied together and, when done, have only included droughts with a 1-yr duration. Therefore, we focused our research on the interaction of droughts and grazing across varying grazing intensities (nongrazed, heavy cattle grazed, and moderate cattle grazed with prairie dogs) and two lengths of drought (single and multiyear). During a year of normal rainfall (i.e., a recovery year) following a single-year drought, vegetation cover returned to >60% of predrought cover across all grazing intensities. In contrast, following a multiyear drought (MYD) cover returned to 50%, 40%, and 38% of predrought levels for ungrazed, cattle grazed, and cattle + prairie-dog-grazed vegetation, respectively. A particularly strong drought × grazing interaction was observed for aboveground net primary production (ANPP), where ungrazed and cattle grazed sites returned to >80% of their predrought ANPP levels regardless of drought duration, but with intense grazing by both cattle and prairie dogs the vegetation only returned to 70% of predrought ANPP following a single-year drought and returned to <50% of predrought ANPP levels following the MYD. Overall, our results suggest that the combination of high intensity grazing of cattle and prairie dogs combined with MYD could lead to a tipping point from which the SGS cannot quickly recover from.