Assistant Professor University of California, Riverside Riverside, CA, United States
Understanding the factors that structure plant communities is critical for predicting biodiversity change. Importantly, changes in the relative importance of assembly processes across life stages may determine which species will be able to persist under changing environmental conditions. For example, in forest ecosystems seedlings often have narrower environmental tolerances than adult conspecifics. Changing environmental conditions could therefore limit the ability of certain species to regenerate which can foreshadow future changes in forest composition. Within the 4ha San Jacinto Forest Dynamics Plot (SJFDP) in Southern California, we explored how the relative importance of multiple assembly mechanisms shifts across life stages in trees and shrubs. To assess drivers of variation in adult composition, we examined how spatial variation in the biodiversity of woody stems larger than 1cm DBH was influenced by space (using Moran’s Eigenvector Mapping) and 13 environmental variables (soils and topography) using dbRDA and variation partitioning. We then assessed drivers of variation in seedling composition (individuals under 1m in height) across 256 1m2 seedling plots within the SJFDP using the same analytic approach, but also included adult composition as a predictor in the variation partitioning analysis.
We found a significant mismatch between adult and seedling composition in the SJFDP. For the adult community, we found 18 species across the 4ha SJFDP including a mix of pines, oaks, and manzanitas common to this mid-elevation forest. For the seedling community, we found 11 species and that oaks made up 75% of all seedlings while only making up 45% of the adult community. We found that variation in adult composition was primarily explained by space (41.7%) with environmental processes only explaining 18.2%. Seedling composition was not strongly predicted by any of our measured variables with 25.8% explained by adult composition, 11.8% explained by a combination of our other variables, and 62.4% of the variation in seedling composition unexplained. Our results suggest that contemporary adult composition is largely driven by spatial processes associated with the long history of fire suppression and drought in this forest. For seedlings, our results suggest that this forest is primed for change with pines likely to experience strong demographic stochasticity from the low numbers of individuals and weak species sorting, and oaks becoming the dominant species in this forest, resulting in substantial changes to biodiversity and ecosystem functioning.