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.Résumé : La perte d'habitats critiques associés aux forêts de fin de succession a encouragé la mise au point d'options d'aménagement qui peuvent accélérer le développement d'une structure forestière de fin de succession dans de jeunes peuplements. Nous avons étudié des traitements d'éclaircie commerciale à l'échelle opérationnelle effectués sur sept stations pour évaluer si l'éclaircie pouvait accélérer le développement de structures forestières de fin de succession dans des peuplements de douglas vert (Pseudotsuga menziesii (Mirb.) Franco) âgés de 40 à 60 ans. Les traitements d'éclaircie incluaient un témoin non traité et la rétention d'arbres à densité forte, à densité modérée et à densité variable. Tous les traitements d'éclaircie comportaient des îlots non traités et les traitements densité modérée et densité variable incluaient des trouées créées par la coupe. Comparativement aux témoins, les unités éclaircies, particulièrement les traitements densité modérée et densité variable, avaient une plus grande variabilité spatiale de la densité d'arbres, supportaient des branches vivantes plus basses, avaient plus de régénération, une meilleure croissance des arbres et légèrement moins de mortalité. Les trouées dans la canopée augmentaient l'étendue de la densité des peuplements et la croissance des arbres situés en bordure. Toutefois, les éclair-cies ont eu peu d'effet sur la croissan...

Forest Restoration Using Variable Density Thinning: Lessons from Douglas-Fir Stands in Western Oregon

“…As expected, the lower post-treatment densities resulted in increased average tree diameter growth [15,29]. Closer inspection showed that the smaller trees had the highest diameter growth increase.…”

“…In contrast, diameter growth of larger trees was still higher than those of smaller trees, but increased less or not at all after thinning, unless densities surrounding the trees were very open [15,29]. The spatial variability created, e.g., by including gaps, was also reflected in diameter growth responses, with trees adjacent to gaps showing higher increases in growth [15,30]. Thus, our results suggest that typical thinning treatments are not efficient at achieving tree sizes typically found in old-growth stands (e.g., [31]).…”

“…Closer inspection showed that the smaller trees had the highest diameter growth increase. In contrast, diameter growth of larger trees was still higher than those of smaller trees, but increased less or not at all after thinning, unless densities surrounding the trees were very open [15,29]. The spatial variability created, e.g., by including gaps, was also reflected in diameter growth responses, with trees adjacent to gaps showing higher increases in growth [15,30].…”

“…Most of these studies [8] were established to compare the effects of a limited set of treatments, i.e., replicated treatments designed to be compared with an analysis of variance. However, the high variability inherent in large treatment units (e.g., [15]) limits the statistical power of traditional comparisons with ANOVA. Even during setup, the assumption that differences in initial conditions are not sufficiently large to influence vegetation responses is harder to justify when treatments are applied to larger areas, e.g., 40 hectares where inherent variability in e.g., topography and associated factors, cannot be avoided.…”

“…The large spatial scale of the treatments allowed insights into the variability itself, for example as influenced by initial conditions [15]. Harvesting operations can be responsible for high variability in residual stand density or structure.…”