Spatial distributions of understory light along the grassland/forest continuum: effects of cover, height, and spatial pattern of tree canopies

The understory light environment is a key determinant of vegetation pattern and ecosystem processes, and varies spatially perhaps more than any resour ce used by plants. Understory light varies along gradients of vegetation st ructure that range from grassland with no woody canopy cover to forest with nearly complete woody canopy cover. Spatial variability in understory ligh t is largely determined by several characteristics of overstory plants - sp atial pattern, height, and cover - which vary concurrently along the grassl and/forest continuum. Using a spatially-explicit ray-tracing model, we quan tified trends in mean and variance of understory light along the continuum. We modeled understory light over a glowing season for two types of plots: (1) generated plots in which cover, spatial pattern, and height of trees we re varied systematically, and (2) three actual plots using stand data from pinon-juniper woodland sites for which cover, spatial pattern and height va ried concurrently. Mean understory light decreased with increasing canopy c over and was sensitive to changes in height, as expected, but was not sensi tive to spatial pattern. Variance in understory light was maximum at an int ermediate value of cover that was dependent on both spatial pattern and cov er - maximum variance occurred at lower values of cover as height increased and as spatial pattern progressed from regular to random to aggregated. Th ese trends in the overall patterns of understory light were also examined w ith respect to changes in understory light in canopy and intercanopy locati ons. Variance in understory light for intercanopy locations was less than t hat for canopy locations at low canopy cover? but exceeded that for canopy locations as canopy cover increased. The value of canopy cover at which var iance in intercanopy locations exceeded that in canopy locations was sensit ive to variation in height but not in spatial pattern. The distributions of understory light for the actual plots were generally similar to those for corresponding generated plots, with dissimilarities attributable to differe nces in cover and height. The general trends highlighted by our simulations are broadly applicable to sites along the grassland/forest continuum. (C) 2000 Elsevier Science B.V. All rights reserved.