Growth and Mortality of Residual Douglas-Fir after Regeneration Harvests under Group Selection and Two-Story Silvicultural Systems

Abstract

Complex management objectives for many publicly owned Douglas-fir forests have prompted renewed interest in silvicultural systems other than clearcutting. The College of Forestry Integrated Research Project at Oregon State University was implemented to test for differences in economic, biological, and human responses to group selection and two-story silvicultural systems. Three separate blocks were established and treated between 1989 and 1991. Trees were measured immediately after the harvest treatments (1991 or 1992) and after the 2004 growing season. Responses of residual overstory trees to initial group selection and two-story treatments were tested relative to untreated controls units in terms of (1) gross basal area and stem volume growth of all residual trees and of the 10 largest trees per acre; (2) gross basal area and stem volume growth conditional on initial basal area and stem volume, respectively, of all residual trees and of the 10 largest trees per acre; and (3) mortality of all overstory trees. Basal area and volume growth were greatest in the control and least in the two-story treatment, but volume growth conditional on initial volume did not differ significantly among treatments. Mortality was significantly greater in the two-story treatment. Overstory growth release in residual Douglas-fir may require 10 years or more to appear after regeneration cuts on some sites, and the possibility of increased overstory mortality complicates attainment of desired long-term structure under two-story silvicultural systems.