Influence of Neighborhood Competition on Douglas-Fir Growth Is Not Altered by Local Environmental Conditions and Climate

To evaluate impacts of competitive conditions in tree neighborhoods on growth responses as influenced by moisture availability and local environmental conditions, we sampled 102 codominant 40- to 70-year-old coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) in regions with relatively high and low moisture availability. We quantified local environmental conditions (topographic position index, heat load, and soil depth), and the annual basal area increment and climate moisture deficits during two growth periods: a five-year period prior to commercial thinning and years six to 10 after thinning. In both regions and growth periods, tree growth was higher for trees growing in local neighborhoods with lower competition. The density/growth relationships differed by region and by growth period in the moist regions, but they were not influenced by climate moisture deficit. Furthermore, including topographic position index, heat load, or soil depth did not improve model support. Our results highlight the importance of managing local competition and indicate that environmental factors such as soil depth, heat load, and topography may be less likely to warrant consideration when developing thinning prescriptions. This could allow foresters to accommodate other ecosystem services when designing density management treatments, at least within typical growing conditions. Study Implications: Concerns about climate change have led to questions whether existing management practices, such as current thinning prescriptions, need to be modified to ensure sustainable provision of ecosystem services. Our results highlight the prominent role of local competitive conditions and indicate that fine-scale differences in topography and soils within our study region are not useful criteria for modifying thinning prescriptions to alter how trees responds to climate conditions, at least under typical growing conditions. Thus, foresters can focus their prescriptions on other aspects when developing thinning or other partial harvesting operations, such as timber production or wildlife habitat.