MIXLIGHT: a flexible light transmission model for mixed-species forest stands

We describe the calibration and structure of a multi-species, two-scale light transmission model, and demonstrate its effectiveness for predicting instantaneous light availability at the stand scale across a wide range of forest stand compositions. The model, MIXLIGHT, calculates light transmission through the forest overstory at the stand or microsite scale using standard forest inventory data and two other parameters, foliage area density and foliage inclination. Since MIXLIGHT simulations on both scales are based on a list of individual tree characteristics, it allows for simple manipulation of stand structure to study the effects of silvicultural options on light availability. The two scales allow the input of various kinds of data, allowing predictions from data of different sources and completeness. A simple and rapid method of calibrating the foliage parameters for the species of interest is presented, using measurements of direct-beam light transmission measurements made in the shadows of newly isolated trees. In an independent validation, MIXLIGHT predicted light transmission at the stand level closely for 17 forest inventory plots with a wide range of density and species composition, during leaf-on and leaf-off seasons and under sunny and cloudy conditions. A sensitivity analysis indicated that the influence of the parameters in the model on stand level light transmission predictions was (highest to lowest): foliage area density, crown radius, crown length, and foliage inclination. Nonetheless, a test of the common assumption that the foliage is spherically inclined caused significant underestimation of light transmission. With this flexibility and demonstrated accuracy, we believe MIXLIGHT will provide an accessible and effective tool for forest stand management and regeneration modeling.