Seasonality of precipitation interacts with exotic species to alter composition and phenology of a semi‐arid grassland

Summary While modelling efforts suggest that invasive species will track climate changes, empirical studies are few. A relevant and largely unaddressed research question is ‘How will the presence of exotic species interact with precipitation change to alter ecosystem structure and function?’ We studied the effects of changes in seasonal timing of precipitation on species composition and resource availability in a grassland community in Colorado, USA . We examined how seasonal precipitation patterns affect the abundance of historically present (native) and recently arrived (exotic) plant species, as well as soil moisture, nitrogen and above‐ground biomass. Over 4 years, we applied four precipitation treatments based on climate model predictions for the study area: winter‐wet/summer‐ambient, winter‐wet/summer‐dry, winter‐wet/summer‐wet and winter‐dry/summer‐wet. Cover of exotic winter‐active grasses was greater in winter‐wet treatments than in control or winter‐dry treatments. Cover of native warm‐season grasses and forbs was greatest in the winter‐dry/summer‐wet treatment, and lowest in the winter‐wet/summer‐dry treatment. These results support the expectation that increased winter precipitation benefits new arrivals, whereas increased summer precipitation benefits later‐growing native plants. Structural equation models showed that interactive effects of increased winter precipitation and increased cover of winter‐active grasses reduced growing season soil water content and species diversity. In addition, the dominant winter‐active species, Bromus tectorum, flowered and senesced earlier in plots receiving increased winter precipitation and reduced summer precipitation, suggesting that earlier growth of winter‐active grasses decreases available soil resources and impacts later‐growing native plants. Peak above‐ground biomass was lowest in the treatment receiving reduced summer precipitation, but only in years with dry springs. Plant‐available nitrogen in spring was lower in plots receiving supplemental winter precipitation, and highest in plots with reduced winter precipitation. Synthesis . Our results indicate that altering the seasonality of precipitation can have large direct effects on plant community composition and phenology, as well as significant indirect effects, mediated through exotic species, on plant‐available resources and plant interactions.