Development of old-growth structure and timber volume growth trends in maturing Douglas-fir stands

Abstract

Interest in the contributions to biological diversity of old-growth forests has increased in many regions of the world. In the Pacific Northwest of the United States, concern for the contributions has lead to proposals to extend the rotation between timber harvests from the conventional 40–80 years to 150 years and longer. However, the implications of such a change for both development of old-growth structure and timber production are unknown. We examined long-term records (up to 82 years) from permanent plots established in 20 stands of Pseudotsuga menziesii (Mirb.) Franco in western Oregon and Washington that are approaching these proposed rotation ages. Similarity to old-growth structure was assessed by comparing the observed values of four structural variables to published mean values for young and old-growth forest. The assessment of similarity to old-growth structure was limited to characteristics of the live forest stand, due to the lack of measurements of snags and downed logs at initiation of the permanent plots. Timber production was assessed by examining trends in mean annual increment (MAI) of cubic volume. Development towards old-growth structure was rapid up to about age 80 years, and gradual thereafter. About half of the transition from young to old-growth forest structure occurred by age 100 years. Stands least similar to old-growth in early observations had relatively high tree densities and relatively small trees of uniform size. In later observations, stands most similar to old-growth structure were those with higher densities of large (>100 cm DBH) P. menziesii. In general, MAI declined gradually, averaging about 0.3% per year. Thus, longer rotations may not result in large declines of timber growth while providing for forest structure similar to old-growth. Early control of tree density may serve to hasten development of old-growth structure. Our approach to understanding the development of old-growth forest structure could be applied to long-term plot data from forests in other regions, as long as there is adequate information on old-growth and other forest stages.