Simulating the impact of small-scale extrinsic disturbances over forest species volumetric light environment

Abstract

Small-scale disturbances (SSD) creating canopy gaps are fundamental to successional dynamics in temperate forests. As gap-oriented management becomes very popular, spatial aspects of gap dynamics, especially the detailed impact of disturbances on the light environment for different species, remain understudied. The aim of this study is to evaluate this effect using the individual-based model SORTIE. Using different initial conditions, 10 simulated data sets, each representing temperate forests, were artificially disturbed using four disturbance sizes. For each 3D location of the simulation space, light availability was computed using the gap light index to create volumetric light data sets. The growth functions of the nine tree species incorporated in the simulation were mapped to each light data set, generating species-dependent 3D cubes illustrating the effect of small-scale disturbances over the different species according to their autecologic relationship to light. The general impact of the simulated SSD was assessed (1) by extracting the 3D boundaries associated to the absolute spatial influence of each replicated SSD and (2) by analyzing the variation of light inside and outside these boundaries, at different height levels. Results were compared for each disturbance size. The species response to different disturbance sizes was evaluated globally and also as a function of height levels under the canopy. This study revealed that the impact of different SSD schemes is highly variable among replicates. Nonetheless, results revealed that small size disturbances exhibit more heterogeneous impact. A threshold effect was detected around a disturbance size of 1000 m 2 suggesting a relative SSD impact that decreases for large SSD sizes. It was also found that species relationship is consistent between different disturbance schemes.