COS 257-3 - The Importance of Regeneration on Forest Biodiversity

Abstract: Recruitment and regeneration of trees are critical processes that can determine species composition in forests. Both seed production and seedling survival are sensitive to environmental conditions and vary amongst species. Especially under climate change, it is critical to consider how those processes may change and the impact this can have on species diversity and composition in forests. The overall research question of our study, is how does variation in seed production between tree species influence forest species composition? How important are seed production and seedling survival for diversity? Models are one approach for projecting the impact of climate change on forests. However, forest gap models typically use a “sapling rain” method to represent the recruitment process, which assumes seed production is unlimited for all tree species and saplings establish if species-specific environmental conditions are met. Thus, to address our specific question, we improved the recruitment process in a forest gap model by simulating seed production and seedling survival, as species-specific functions of weather and size. Seed production equations were made using long-term seed data provided by the Mast Inference and Prediction (MASTIF) network with climate data from the Parameter-elevation Regression on Independent Slope Model (PRISM). Seedling survival equations were made using basal area data from Forest Inventory Analysis (FIA) data and climate data from PRISM. The ForClim model was used and applied to forest stands in the Pacific Northwest where simulations with and without seed production were done. Model results were then compared with observed tree species composition from the FIA data. The results show that the regeneration can have a significant reduction on tree diversity and that the addition of seed production and seedling dynamics into the forest model made the simulated forests match closer to the observed FIA forests. The average observed species richness of observed forests was 2.5. The simulated forests with the new regeneration processes had an average species richness of 2, compared to the average species richness of 8 in the original model. The average Simpson diversity index was 0.3 in the observed FIA data, and this was also more similar to simulated forests in the updated model version (0.7) compared to the original model (0.12). In conclusion, regeneration processes have a significant impact on forest demography and can act as a bottleneck in determining the species composition of a forest.