Variable-density thinning (VDT) is a management option designed to increase structural heterogeneity in second-growth conifer stands. This study examined subcanopy tree growth response to two variations of VDT. At the Forest Ecosystem Study in western Washington, thinning intensity was found to have a significant effect on height growth of Douglasir (Pseudotsuga menziesii (Mirb.) Franco) seedlings which established following the thinning. At the Olympic Habitat Development Study in western Washington, basal area growth response was examined for residual midcanopy western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata ex. D. Don). Both species retained the ability to respond to thinning. The results of this study suggest that non-uniform thinning practices, like VDT, can lead to variation in growth response of residual subcanopy trees and new regeneration. This may accelerate the development of more structurally diverse forests than traditional management practices.

The global carbon (C) balance is vulnerable to disturbances that alter terrestrial C uptake and loss. Moderate disturbances that kill or defoliate only a subset of canopy trees such as insect defoliation, drought, and age-related senescence are increasing in extent and frequency; yet, little is known about the effect of moderate disturbance on forest production and the mechanisms sustaining or supporting the recovery of the C cycle across a range of moderate disturbance severities. We used a broad plot-scale gradient of upper canopy tree mortality within a large manipulation of forest disturbance to: 1) quantify how aboveground wood net primary production (ANPPw) responds to a range of moderate disturbance severities and; 2) identify the primary mechanisms supporting ANPPw resistance or resilience following moderate disturbance. We found that ANPPw was highly resistant to moderate disturbance, with production levels sustained following the senescence of 9 to > 60 % of the upper canopy tree basal area. As upper canopy gap fraction increased with rising disturbance severity, greater light availability to the subcanopy enhanced leaf-level C uptake and the growth of this formerly light-limited canopy stratum, compensating for upper canopy production losses. As a result, whole-ecosystem production efficiency (ANPPw/LAI) increased at high levels of disturbance severity and leaf area loss. These findings provide a mechanistic explanation for sustained ANPPw across the disturbance gradient, in which the physiological and growth enhancement of undisturbed vegetation was proportional to the level of disturbance severity. Our results have important ecological and management implications, showing that moderate disturbances may minimally alter ecosystem functions such as C storage.