Early responses to thinning treatments designed to accelerate late successional forest structure in young coniferous stands of western Oregon, USA

Abstract

The loss of critical habitat provided by late successional forests has prompted the search for management options that can accelerate the development of late successional forest structure in young stands. We examined operational-scale commercial thinning treatments at seven sites to evaluate if thinning could accelerate development of late successional forest structures in 40–60 year old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests. Thinning treatments included an untreated control, high density, moderate density, and variable density retention. All thinning treatments had leave islands, and moderate density and variable density included harvest-created gaps. Thinned units, especially moderate density and variable density, had greater spatial variability in tree density, supported lower live branches, had greater tree regeneration and growth, and had slightly lower mortality relative to the control. Canopy gaps extended the range of stand densities and increased growth of trees immediately along gap edges. However, thinning had little effect on growth of the largest Douglas-fir trees and did little to provide large snags or coarse woody debris. These results suggest that thinning treatments can accelerate some aspects, e.g., spatial variability, of late successional forest structures. Other attributes, such as large trees and snags, may prove less responsive to thinning treatments, at least in the short term. Including tree retention levels lower than typical management applications and formation of canopy gaps provide the wide range of conditions that appears beneficial for developing late successional forest structure.