MVP: a model to simulate the spatial patterns of photosynthetically active radiation under discrete forest canopies

The spatial patterns of photosynthetically active radiation (PAR) under forest canopies, including both its mean and spatial variation, are critical factors to numerous understory ecophysiological processes. Currently, Beerʹs law is the primary algorithm used in ecological models simulating PAR transmission through plant canopies, because more accurate models are too complicated to be used operationally. This study developed a simple and computationally efficient model at a stand scale to simulate both the mean and variation of PAR (MVP) under forest canopies. The model assumes that a forest canopy is composed of individual crowns distributed within upper and lower boundaries with two types of gaps: the between- and within-crown gaps. The between-crown gaps are simulated with geometric optics, and the within-crown gaps are described by Beerʹs law. The model accounts for the covariance of PAR in space through time, making it possible to simulate both instantaneous and daily accumulated variance of PAR. Validation with observed PAR from the boreal ecosystem–atmosphere study (BOREAS) indicates that the model captures the mean and variance of PAR under forest canopy reasonably well. MVP holds the potential to improve simulation of light interception by forest canopies as well as the treatment of canopy rainfall interception in ecological models.