Growth reduction in harvest-age, coniferous forests with residual trees in the western central Cascade Range of Oregon

Abstract

To evaluate the relationship of overstory residual trees to the growth of unmanaged young-to-mature understory Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco) and western hemlock ( Tsuga heterophylla (Raf.) Sarg.), the basal area and volume of 14 paired plots with and without residual trees were examined in the Willamette National Forest, Oregon. Residual trees were large survivors of the fires that initiated the understory between 55 and 121 yr ago. Understory stands were naturally regenerated and not managed in any way. High residual tree and understory densities were negatively associated with understory volume. The relation of density of residual trees to total understory and Douglas-fir basal areas and volumes was best described by a negative logarithmic function. The rate of decrease in total understory and Douglas-fir basal areas and volumes per individual residual tree became smaller with increasing residual-tree density. Predicted total understory volume reduction was 23% with five residual trees/ha and 47% with 50 residual trees/ha, averaging 4.6% and 0.9% per residual tree, respectively. After including the estimated volume growth of residual trees since initiation of the understory, stand volume was still 19% lower with five residual trees/ha and 41% lower with 50 residual trees/ha than in stands with no residual trees, averaging a reduction of 38% and 0.8% per residual tree, respectively. In mixed stands of Douglas fir and western hemlock, predicted Douglas-fir basal area and volume declined more rapidly than did total understory basal area and volume when residual-tree densities exceeded about 15 trees/ha. This difference was probably due to the relative shade-intolerance of Douglas fir. Predicted Douglas-fir volume reduction was 13% with five residual trees/ha and 75% with 50 residual trees/ha, averaging 2.6% and 1.5% per residual tree, respectively. The southern aspects had more than 150% the total understory basal area and volume and more than 200% the Douglas-fir volume and basal area of the northern aspects. Lower density and basal area of understory trees, particularly of dominant and codominant Douglas fir, were associated with increasing residual-tree densities. Given the same diameter at breast height (DBH), heights of Douglas fir were not related to residual trees. Regardless of understory age, understory volume was greatest in stands with the lowest understory densities. These results suggest that timber production in unthinned green-tree retention units may be reduced and may depend on the density of leave-trees. Thinning of understory trees is recommended to reduce growth loss from intraspecific competition.