Integrating LiDAR measurements of canopy structure, forest inventory data, and a simple forest dynamics model to understand fundamental drivers of forest canopy structure

Abstract:

Forest structure is a critical emergent property of forest ecosystems. It affects ecological interactions within forests and is the foundation of forest carbon stock estimates. Despite diversity in climate, substrate, and demography, forests exhibit striking consistency in patterns of certain structural attributes - specifically tree diameter distributions and gap size frequency distributions - hinting at the existence of fundamental mechanisms shaping forest structure. Recently, a model including only patch-scale disturbances, phototropism (i.e. the Perfect Plasticity Approximation or PPA), crown area allometry, and the dependence of tree growth and mortality on light availability could explain tree diameter distributions across tropical and temperate forests. However, evaluating the generality of these mechanisms to a comprehensive definition of forest structure requires testing with additional structural dimensions of forests. In this study, we are integrating new measurements of forest structure derived from LiDAR data with tree diameter data from a forest inventory and a simple model of forest dynamics to investigate fundamental drivers of forest canopy structure. Our study site is the Wind River Forest Dynamics Plot (WFDP), a 25.6 hectare permanent plot in southwestern Washington, USA. The distribution of forest canopy height from LiDAR data was bimodal, with one peak centered at 34.55 meters and another at 6.1 meters. Comparisons between the distribution of canopy height derived from LiDAR and predictions of canopy height from diameter data and alternative allometric models suggested that small- to medium- scale gaps, variation in crown allometry among gap-specialist and canopy-specialist species, and horizontal foraging for light (the PPA) could explain the bimodal distribution of forest canopy height in this ecosystem. Now, we are working on simulating forest canopy structure from a simple forest dynamics model to investigate the importance of patch-level disturbances and diversity in tree form and strategy in emergent patterns of forest canopy structure.