Invasive annual grass interacts with drought to influence plant communities and soil moisture in dryland restoration

Abstract Understanding the combined effects of drought and invasive species on plant community development and soil moisture could provide valuable insight into the mechanisms hindering successful native plant establishment in dryland restoration projects. We implemented a re‐vegetation experiment at two sites in Colorado, USA (one each in the Western Great Plains and Cold Desert ecoregions) to investigate the effects of drought (66% reduction of ambient growing season rainfall), non‐native Bromus tectorum seed addition (465 seeds/m2), and superabsorbent polymer soil amendment (25 g/m2) on plant community development and soil volumetric water content at 5 and 30 cm depth. Drought resulted in higher B. tectorum cover at the Western Great Plains site but lower B. tectorum cover at the Cold Desert site. These contrasting results suggest drought may interact with site‐specific precipitation patterns to influence B. tectorum establishment. At the Western Great Plains site, drought reduced seeded forb cover and B. tectorum seed addition reduced seeded grass cover, highlighting how the effects of drought and invasive species may vary depending on which functional group is assessed. At the Cold Desert site, drought and B. tectorum seed addition each decreased seeded species cover from approximately 8% to 3%. Superabsorbent polymer effects were limited to slight increases in overall seeded grass cover at the Western Great Plains site from 2.2% to 4.9%. Both drought and B. tectorum seed addition reduced soil volumetric water content but in some cases effects depended on interactions between the two treatments, site, or soil depth. Notably, at the Cold Desert site the reduction in soil volumetric water content resulting from B. tectorum seed addition with ambient precipitation exceeded that of the drought treatment alone at 5 and 30 cm depth. Our results demonstrate that drought and invasive species both negatively influence native plant establishment and soil moisture, and in some cases may interact. Considering both abiotic and biotic stressors as well as their interactions in restoration planning could improve our understanding of native plant establishment and improve re‐vegetation outcomes.