Effects of a wide gradient of retained tree structure on understory light in coastal Douglas-fir forests

We characterize understory light of seven stands that varied along a gradient of tree retention. Using hemispherical canopy photographs and digital image, we estimated gap light or solar radiation reaching the understory through the canopy. Using nonlinear regressions, we related gap light to several structural attributes in the examined silvicultural treatments. The silvicultural treatments affected both the median and range of gap light in the understory. As overstory removal increased from uncut second growth to green-tree retention, the median value of light increased from 8 to 68% full sun, while the range of light increased from 3-22% to 26-88% full sun. We found strong, significant, and negative nonlinear relationships between gap light at a particular microsite (0.04 ha) in the understory and the height, diameter at breast height, density, and volume of surrounding retained trees (r(a)(2) = 0.77-0.94). These relationships can aid planning of treatments that retain forest structure, such as variable retention, by allowing predictions of understory light from commonly used field data. These predictions allow forest managers to understand some of the ecological consequences and tradeoffs associated with retaining structure during harvesting.