Relationships of green-tree retention following timber harvest to forest growth and species composition in the western Cascade mountains

National Forest management in the Pacific Northwest is shiftingfrom a focus on commodity production to ecosystem management, in whichthe health of the entire forest ecosystem is considered, rather than thatof a few key species. Ecosystem management includes retention of somelive trees following timber harvest (green-tree retention) to preservebiodiversity, imitating the natural fire regime of large, but patchyfires that leave many live trees. How ecosystem management will affectgrowth and species composition of future forests is an importantquestion. This study takes a retrospective approach to this question byusing past disturbance as an analogue to green-tree retention followingtimber harvest. Using USDA Forest Service timber inventory plot datafrom the Cascade Mountains of Oregon and SW Washington, 132 unmanagedstands were identified with a tree cohort of 70-110 years old(regeneration) or a tree cohort of 70-110 years old with an overstory oflarge trees 200+ years old (remnants). All stands were in the Tsueabeterophylla (Raf.) Sarg. zone (Franklin and Dyrness 1973). Single-agedstands represented clearcuts, while two-aged stands served as analoguesto stands harvested with green-tree retention. Regeneration basalarea/hectare (ba/ha) declined when remnant-tree densities exceeded about15 remnant trees/ha (R2.0.51) in a relationship roughly described by a
sigmoidal curve. Conceptually removing remnant-tree space occupancyeffects decreased remnant-tree density's value as a predictor ofregeneration ba/ha by about 500 at management-level remnant densities ( ba/ha in the regeneration increased slightly with increasing remnant-treedensities (R2.4.19). Western redcedar ba/ha in the regeneration wasapparently not related to remnant-tree density (R2.4.02). The degree ofaggregation in remnant trees did not appear to affect regeneration ba/ha,but few stands contained the isolated clumps of remnant trees likelyunder a management scenario. Neither measured site characteristics norregeneration density was related to regeneration ba/ha across species.Remnant-tree density was apparently unrelated to tree-species diversityin the regeneration. Total-stand ba/ha remained relatively constantacross remnant densities.