ALTERNATIVE SILVICULTURAL REGIMES IN THE PACIFIC NORTHWEST - SIMULATIONS OF ECOLOGICAL AND ECONOMIC EFFECTS

摘要

New silvicultural strategies to sustain both ecological and human communities are being developed and implemented on federal forest lands in the Pacific Northwest (PNW) United States. Two important stand-level components of the new silviculture regimes are rotation age and retention level of live trees in harvest units. Ecologists have suggested that canopy tree retention and longer rotations will create patterns of stand structure in managed forest that are similar to those in natural forests, and promote long-term ecological productivity and biodiversity. Forest economists, however, are concerned that canopy tree retention and long rotations may reduce wood production, although the high value of large logs produced by these new silvicultural regimes may compensate for reduced growth rates. We used the forest model ZELIG to perform a factorial simulation experiment on long-term responses of ecological and economic variables to nine retention levels and four rotation lengths. ZELIG output on forest structure and composition was input to a forest economics model that calculated net value of wood products in 1989 dollars. The simulated stand data were also linked with regression equations to predict the densities of 17 bird species as a function of tree size class distribution. Five replicates of each treatment were run for the 240-yr simulation period. Results indicated that stand structure under each of the canopy tree retention levels was more similar to the pre-treatment natural forest than following clear-cutting. Variation in tree size under intermediate levels of retention, however, did not reach the level of the natural forest during the simulation period. Tree species composition was strongly related to retention level and rotation age. Shade-intolerant Douglas-fir (Pseudotsuga menziesii) lost dominance to shade-tolerant species under intermediate retention levels and longer rotations. Wood production decreased significantly with increasing retention level and rotation age, with a notable threshold between retention levels of 0 and 5 trees per hectare. Net wood products value did not decrease as rapidly with retention level, and did not differ much among rotation ages, because of the high value of large logs. Bird species responded individualistically to retention level and rotation age. Some had peak densities under short-rotation clear-cutting, but most were associated with structurally complex, closed-canopy forest. Consequently, bird species richness increased significantly with retention level and rotation age. Within the assumptions and limitations of our models, this application provided knowledge on trends and thresholds that can help land managers to choose silvicultural regimes that best balance their management objectives. We concluded that retention level and rotation age strongly influence ecological and economic responses in PNW forests; efforts are needed to reduce uncertainty about these effects. [References: 59]