Promoting late-successional forest structure has become an increasingly important management objective in the Pacific Northwest. Unfortunately, stands previously managed for timber production typically lack several structural attributes that define late-successional forests including the presence of large diameter trees, deep-multi layered crowns, and a heterogeneity of tree size classes. While many of these characteristics will develop naturally through forest succession, there has been considerable interest in whether forest management can accelerate the process.
Variable density thinning (VDT), a silvicultural treatment that increases resource availability by thinning at varying intensities within a stand, is one strategy that may facilitate late-successional structural development. Several studies have found VDT to stimulate individual tree growth. However, to date, evidence validating whether VDT is accelerating the development of late-successional forest structure is lacking. To answer this question, we compared tree diameter growth, crown characteristics, and size class diversity (Shannon index and the Gini coefficient) in five stands partially treated with VDT in the Olympic National Forest of western Washington 14 years post-treatment. We examined biotic (species composition, stand age, and post thinning stand density) and abiotic factors (average temperature, moisture, and elevation) to explain potential differences in development among stands.
Results/Conclusions
Consistent with other studies, diameter growth significantly increased in areas thinned compared to the unthinned areas. Thinned areas also contained a greater amount of large diameter trees (>80cm). Among dominant trees (>40cm diameter at breast height), VDT had a significantly positive effect on change in crown length (1.7m), width (.08m), and volume (106m3). Diversity in size class structure, however, was not strongly influenced by VDT, as three of the five stands had a lower size class diversity rating in VDT treated areas. Differences in growth rates among stands may partially account for the lack of size class heterogeneity, as stands with greater percentages of Pseudotsuga menziessii, higher mean annual temperature, and lower post-thinning basal area demonstrated superior growth rates. Collectively, our results indicate that VDT will accelerate the development of larger diameter trees and deep-multi-layered crowns. However, the lack of overall size class diversity 14 years post-thinning suggests that late-successional size class diversity does not progress at the same rate. Instead, the rate of size class diversification will likely depend on individual stand characteristics.