Competition reverses the response of shrub seedling mortality and growth along a soil moisture gradient

Data associated with the following manuscript abstract: 1. Mediterranean climate regions are being increasingly invaded by exotic annual plants, which often increase in abundance in years of high rainfall. For native shrubs in these regions, seedling establishment is a critical demographic stage and often depends on precipitation, as seedlings are sensitive to drought stress. Thus, high rainfall years are well known to promote shrub establishment. However, greater competition from exotic annual species in high rainfall years could prevent shrub seedling establishment, resulting in higher rates of establishment in dry years. Models predict non-linear relationships between competitive suppression and soil moisture, but there are few experimental evaluations of competitive interactions across gradients of soil moisture availability. 2. Here we examined how competition from an exotic annual influenced native shrub establishment, across an experimental soil moisture gradient. Seedlings of two native shrub species (Encelia californica and Eriogonum fasciculatum) were grown with and without an exotic grass competitor (Avena fatua) across eight water availability levels, and monitored for growth and survival. These focal species are common and abundant in the Mediterranean climate region of coastal Southern California, where climate change projections for the coming decades include long-term drought, but also high interannual variation in rainfall. 3. Without competition, shrub seedlings achieved higher growth and survival at high water availability levels. However, when grown in competition with the exotic grass, shrub seedlings had higher growth and/or survival under dry conditions, suggesting competition can modify and even reverse species responses to changing rainfall patterns compared to predictions made when they are grown in the absence of competitors. The exotic grass strongly reduced soil nitrate and water availability when it was planted with the native shrub seedlings, and the exotic grass responded positively and non-linearly to increasing levels of water addition. Synthesis. These non-linear responses demonstrate how community context and competition can alter native species response to climate change, with important ramifications for community and ecosystem processes in areas experiencing both climate change and invasion by exotic species.