While altered precipitation regimes can greatly impact biodiversity and ecosystem functioning, we lack a comprehensive view of how these impacts are mediated by changes to the seasonality of precipitation (i.e., whether it rains more/less in one season relative to another). Over 2 years, we examined how altered seasonal precipitation influenced annual plant biomass and species richness, Simpson’s diversity, and community composition of annual plant communities in a dryland ecosystem that receives both winter and summer rainfall and has distinct annual plant communities in each season. Using a rainfall exclusion, collection, and distribution system, we excluded precipitation and added water during each season individually and compared responses to control plots which received ambient summer and winter precipitation. In control plots, we found five times greater annual plant biomass, twice as many species, and higher diversity in winter relative to summer. Adding water increased annual plant biomass in summer only, did not change richness or diversity in either summer or winter, and modestly shifted community composition. Excluding precipitation in either season reduced annual plant biomass, richness, and Simpson’s diversity. However, in the second winter season, biomass was higher in the plots where precipitation was excluded in the previous summer seasons suggesting that reduced productivity in the summer may facilitate biomass in the winter. Our results suggest that increased precipitation in summer may have stronger short-term impacts on annual plant biodiversity and ecosystem function relative to increased winter precipitation. In contrast, decreasing precipitation may have ubiquitous negative effects on annual plants across both summer and winter but may lead to increased biomass in the following off-seasons. These patterns suggest that annual plant communities exhibit asymmetries in their community and ecosystem responses to altered seasonal precipitation and that considering the seasonality of precipitation is important for predicting the effects of altered precipitation regimes.

1. Climate change is causing the rapid redistribution of vegetation as plant species move to track their climatic optima. Despite a global trend of upward movement in latitude and elevation, there is extensive heterogeneity among species and locations, with few emerging generalizations. Greater generalization may be

Many studies of community assembly focus on a single ontogenetic stage (typically adults) when trying to infer assembly processes from patterns of biodiversity. This focus ignores the finding that assembly mechanisms may strongly differ between life‐stages, and the role of ontogenetic dependency: the mechanisms by which one life stage directly affects the composition of another life stage. Within a 4‐ha forest dynamics plot in California USA, we explored how the relative importance of multiple assembly processes shifts across life stages and assessed ontogenetic dependency of seedlings on adults in woody plant communities. To assess variation in assembly processes across life stages, we examined how β‐diversity of adult and seedling communities were each influenced by space and 13 environmental variables (soils, topography) using distance‐based redundancy analysis and variation partitioning. We then assessed the ontogenetic dependency of seedlings on adults by including adult composition as a predictor in the seedling community variation partitioning. We found differences between adult and seedling composition. For the adults, we found 18 species including pines, oaks and manzanitas characteristic of this mid‐elevation forest. For seedlings, we found 11 species, and that oaks made up 75% of all seedlings while only making up 45% of all adults. Adult β‐diversity was primarily explained by space (44.0%) with environment only explaining 18.6% and 37.4% unexplained. In contrast, most of the explained variation in seedling β‐diversity was due to ontogenetic dependency alone (13.6% explained by adult composition) with 1.6% explained by space and the environment jointly, and 62.8% unexplained. Synthesis: Here, we describe a conceptual framework for integrating ontogeny more explicitly into community assembly research and demonstrate how different assembly processes structured adult and seedling β‐diversity in a temperate dry forest. While adult β‐diversity was largely driven by spatial processes, seedling β‐diversity was largely unexplained, with ontogenetic dependency comprising most of the explained variation. These patterns suggest that future assembly research should consider how assembly processes and their underlying mechanisms may shift with ontogeny, and that interactions between ontogenetic stages (ontogenetic dependency) are critical to consider when assessing variation in assembly processes.