Evaluation of easily measured stand inventory parameters as predictors of PAR transmittance for use in poplar silvopastoral management

This study compares the ability of several poplar (Populus spp.) stand parameters to predict photosynthetically active radiation (PAR) transmission through the overstorey canopy in a silvopastoral system. As an indirect measure of PAR transmission, diffuse non-intercepted radiation (DIFN) levels underneath fully foliated poplar stands were measured in two climatically contrasting regions of the North Island of New Zealand. The ability of several easily measured stand inventory parameters to predict understorey DIFN was then evaluated using regression analysis. Pasture net herbage accumulation (NHA) was also measured below the fully foliated trees and related to understorey DIFN. A directly inverse relationship between DIFN in the gap between tree crowns and stand canopy closure measured with a standard digital camera was found to be the simplest, most accurate and robust of the stand parameters examined. In comparison, there were site differences in the relationship for this understorey environment for all of the stand parameters based on individual tree metrics. Site differences in the relationship for DIFN directly underneath the tree-crowns were also apparent for both the stand parameters based on individual tree metrics and digital photographs. Inter-site variation in these relationships was attributed to unaccounted differences in stand leaf area. There was a strong straight-line relationship between DIFN and understorey pasture NHA.