Indirect measures for characterizing light along a gradient of mixed-hardwood riparian forest canopy structures

Understanding the effect of canopy structure on the understory light environment is useful in the design of silvicultural strategies that facilitate sustainable tree recruitment. To address this need, a study was designed to quantify the relationship between forest structure and light along a gradient of 50–100% canopy cover. The gradient was created by applying 1 of 4 midstory removal treatments to each of fifty 0.05 ha plots located within a mixed-hardwood riparian forest corridor. The light environment was directly quantified with a linear ceptometer and regression analysis was used to examine the relationship between photosynthetically active radiation (PAR) and various metrics of vertical and horizontal structure. Vertical sighting tube estimates of canopy cover (R2 = 0.73), light estimates derived from hemispherical photography (R2 = 0.70), and spherical densiometer estimates of canopy closure (R2 = 0.68) were the best single predictors of understory light transmittance. Including top height and tree density improved the fit of canopy cover based models (R2 = 0.80). Canopy closure estimates derived from hemispherical photography generally showed a weaker relationship with PAR than other measures of stand structure. Further, the strength of this relationship depended upon photo analysis angle. In general, the vertical component of stand structure seems to influence light transmittance through the forest canopy. This vertical complexity must be addressed when evaluating structure–light relations.