GREEN-TREE RETENTION: CONSEQUENCES FOR TIMBER PRODUCTION IN FORESTS OF THE WESTERN CASCADES, OREGON

Abstract

National Forest management in the Pacific Northwest is adopting ecosystem management, in which the entire forest ecosystem is considered, as well as commodity production. Silvicultural prescriptions in ecosystem management often include retention of some live trees following timber harvest (green-tree retention) with the primary goal of maintaining biodiversity. How green-tree retention will affect growth and tree species composition of future forests is an important question. We took a retrospective approach to this question by using past disturbance as an analogue to green-tree retention following timber harvest. We used United States Department of Agriculture (USDA) Forest Service timber inventory plot data from the Cascade Mountains of Oregon and southwest Wash- ington (western hemlock zone). We selected 132 unmanaged stands with a tree cohort of 70-110 yr old (regeneration) or a tree cohort of 70-110 yr old with an overstory of large trees 200+ yr old (remnants), representing clearcuts and stands harvested with green-tree retention, respectively. Regeneration basal area (BA) across species and for Pseudotsuga menziesii tended to decline with increasing remnant density (R 2 = 0.51 and 0.60, respec- tively). but only after remnant densities reached 15 trees/ha; the relationships were roughly sigmoidal. Part of the effect of remnants on regeneration results from remnants' occupation of space, making it unavailable to the regeneration. After adjusting regeneration BA to account for remnants' space occupation, the relationship between regeneration BA and remnant density weakened by about half, implying that remnant effects resulted from both space occupancy and other factors associated with remnants. Total-stand BA was relatively constant across remnant densities. Remnant density was not related to tree-species diversity in the regeneration.