Seedling recruitment is hypothesized to be a key bottleneck in forest responses to environmental change due to the sensitivity of early life stages to environmental conditions and the critical role of recruitment in population persistence and migration. However, the effects of environmental change on recruitment may be modulated by the buffering effects of the forest canopy, particularly in productive, dense forests. Canopy density, light penetration through the canopy, and understory vegetation likely interact in complex ways to determine the environmental conditions experienced by early life stages, and the effects of these conditions on recruitment are likely to vary with a species’ degree of shade tolerance. The aim of this study was to quantify the hierarchical and interactive effects of forest structure, canopy cover, and understory vegetation on seedling germination and survival in naturally regenerated conifer forests in the Pacific Northwest. To address this aim, we monitored seedling germination and survival of two conifer species with differing shade tolerance over three years and across five long-term monitoring plots spanning the elevational distribution of these species. We fit hierarchical Bayesian regression models to germination and survival, forest structure, and light data to investigate interactions between these variables and their effects on recruitment.
Results/Conclusions
Model results support the hierarchical nature of light availability and its effects on seedling germination and survival. For both species—Pseudotsuga menziesii and Tsuga heterophylla—the amount of light available at a given seedling quadrat, modeled as a latent variable, was positively influenced by the basal area of neighboring trees and light penetration through the canopy, represented as global site factor. The percent cover of understory vegetation was not significantly related to light availability for either species, which may be explained by variation in the height of understory vegetation relative to the height of seedlings. Light availability had a significant and positive effect on Tsuga seedling counts, although its effect on Pseudotsuga was nonsignificant. The amount of light available in these closed-canopy mature and old-growth forests may be insufficient to exert substantial influence on germination and survival of Pseudotsuga – a shade-intolerant species – while relatively shade-tolerant Tsuga heterophylla seedlings may benefit from the moderate light levels that characterized the most well-lit locations in these plots. These findings indicate that light availability to seedlings in closed-canopy forests is influenced by forest structure in a hierarchical and nonlinear fashion.