Trait-Based Community Assembly in a Changing Climate: Dispersal Dynamics and Ecological Filtering In a Grassland Metacommunity

Abstract

Research on plant community assembly often focuses on single life stages, or transitions between two life stages, and rarely integrates multiple life stage transitions into a more complete picture of the community assembly process. This is unfortunate because it limits our ability to assess the relative influence of each life stage transition on community assembly outcomes, and hence, predict community response to perturbations such as climate change. In this dissertation, I use observational and experimental evidence at different points in the plant life cycle to improve short- and long-term predictions of community response to climate change. I work in twelve grassland sites in southern Norway that fall along orthogonal gradients of temperature and precipitation, allowing me to disentangle the influence of these climate drivers.

I first combine seed, seedling, and adult plant survey data at the twelve sites to infer regional patterns of seed dispersal and immigration among climate zones. On average, 5 to 10 percent of seeds at a site putatively originated from different climates, suggesting significant connectivity among climate zones. However, immigrant seedlings were less likely to emerge and establish in experimental gaps than seedlings with locally-present conspecific adults, suggesting that a climate-based filters are in part responsible for maintaining regional vegetation patterns at the seedling stage. Despite the evidence for site connectivity, 66 of the 163 species in our system were not observed as immigrants at any point in the study, highlighting the potential for dispersal to limit species ability to track rapid changes in climate.

Second, I examine changes in species diversity and community-weighted mean trait values over plant life stages to characterize the strength and nature of ecological filtering at each life stage transition. Each surveyed life stage had fewer species than expected by chance, indicating that species sorting processes restricted community membership at multiple points of plant community regeneration. Furthermore, shifts in community weighted trait means suggest that different life stage transitions are influenced by qualitatively different mechanisms. The strength of filtering varied little with temperature and precipitation, suggesting that these stage-specific assembly processes are of broad relevance.

Third, I evaluate whether traits associated with regional temperature and precipitation patterns can predict community responses to rapid experimental climate change. To avoid the artifacts of in situ climate manipulation, 25 x 25 cm turfs of standing vegetation were transplanted to warmer and wetter sites. Changes in transplanted turf community composition were monitored over five years and compared to a field-parameterized null model. Three of the six traits with spatial associations to temperature predicted species success following transplantation. My results underscore the importance of using ecologically relevant traits when making predictions of community response, and suggest that in our grassland system, architectural traits may exert more influence on initial species response to rapid warming than the more commonly used growth-related traits.

This dissertation offers a much-needed empirical exploration of how regional dispersal dynamics, seed and seedling performance, and adult community response interactively shape patterns of plant community diversity. In addition, it demonstrates how species traits, when chosen for their potential mechanistic relevance to community assembly processes, can be valuable hypothesis generators. Future work on plant community assembly should consider plant life stages and relevant traits to refine predictions of community response to climate change.