Spatially explicit modeling of overstory manipulations in young forests: Effects on stand structure and light

Young forests can be manipulated in diverse ways to enhance their ecological values. We used stem maps from two dense, second-growth stands in western Washington and a spatially explicit light model (tRAYci) to simulate effects of five silvicultural manipulations on diameter distribution, species composition, spatial patterning, and light availability. Each treatment removed 30% of the basal area, but differed in how trees were selected for removal. Three primary treatments were thin from below (removing the smallest trees), random thin (removing trees randomly), and gap creation (removing all trees in circles ∼1 tree height in diameter). Two additional treatments combined elements of these approaches: random ecological thin (a mixture of thin from below and random thin) and structured ecological thin (a mixture of thin from below and gap creation).Despite comparable removal of basal area, structure and light distributions differed markedly among treatments. Thin from below greatly reduced the range of tree sizes, decreased the importance of shade-tolerant tree species, increased regularity of spacing, and increased light at the forest floor by ∼140%. Random thin had little effect on size structure, species composition, or spatial patterning, but produced increases in light similar to those in thin from below. Gap creation had little effect on size structure or species composition, but increased the mean light value by >200% and also increased the range of light values. Random ecological thin had little effect on size structure, species composition, or spatial patterning, but increased light to levels comparable to thin from below. Structured ecological thin also had little effect on size structure and species composition, but increased clumping of residual stems, producing light values intermediate between thin from below and gap creation. These comparisons illustrate that different approaches to tree removal can produce very different forest structures and resource gradients. The combination of stem maps, simulated thinning, and a light model provides a useful tool for predicting the initial effects of any number of silvicultural manipulations on forest structure and light.