Green-tree retention: consequences for timber production in forests of the western Cascades, Oregon

National Forest management in the Pacific Northwest is adopting ecosystemmanagement, in which the entire forest ecosystem is considered, as well as commodityproduction. Silvicultural prescriptions in ecosystem management often include retention ofsome live trees following timber harvest (green-tree retention) with the primary goal ofmaintaining biodiversity. How green-tree retention will affect growth and tree speciescomposition of future forests is an important question. We took a retrospective approachto this question by using past disturbance as an analogue to green-tree retention followingtimber harvest. We used United States Department of Agriculture (USDA) Forest Servicetimber 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 of70-110 yr old (regeneration) or a tree cohort of 70-110 yr old with an overstory of largetrees 200+ yr old (remnants), representing clearcuts and stands harvested with green-treeretention, respectively. Regeneration basal area (BA) across species and for Pseudotsugamenziesii tended to decline with increasing remnant density (R2 = 0.51 and 0.60, respec-tively). but only after remnant densities reached 15 trees/ha; the relationships were roughlysigmoidal. Part of the effect of remnants on regeneration results from remnants' occupationof space, making it unavailable to the regeneration. After adjusting regeneration BA toaccount for remnants' space occupation, the relationship between regeneration BA andremnant density weakened by about half, implying that remnant effects resulted from bothspace occupancy and other factors associated with remnants. Total-stand BA was relativelyconstant across remnant densities. Remnant density was not related to tree-species diversityin the regeneration.
Key words: Cascade Mountains: ecosystem management: forest management; green-tree reten-tion; new forestry; Pseudotsuga menziesii; remnant trees: retrospective study; Tsuga heterophylla.