Do Residual Trees Increase Structural Complexity in Pacific Northwest Coniferous Forests?

Green-tree retention has been promoted as an alternative management tool to create structurally complex forests that would resemble late-successional forests faster than single-aged stands planted after clearcuts. To evaluate if and how overstory residual trees were associated with a higher structural complexity, nine pairs of naturally regenerated stands, initiated between 55 and 110 yr ago with and without residual trees, were investigated. The new Structural Complexity Index (SCI) was used to relate the observed structure to residual trees and other structural components such as tree density, tree size variation, species composition, and environmental variables such as aspect, slope, and elevation. The SCI-ht, which is based on tree height variation, was negatively associated with residual trees, indicating that residual trees reduced vertical structural complexity. The SCI-dbh, which is based on the tree diameter variation, was positively associated with residual trees up to a density of 40 residual trees/ha, indicating that at low densities residual trees increased horizontal structural complexity. Generally, stands with intermediate densities of the young cohort and a mixture of about equal proportions of Pseudotsuga menziesii and shade-tolerant tree species such as Tsuga heterophylla, had the highest structural complexity. The most structurally complex stands were generally on east aspects. In addition to green-tree retention, species mixture and differences among aspects need to be considered when managing stands for structural complexity.