Variable retention harvest is a common practice to maintain biodiversity in managed forests, but the effects of the size and spatial arrangement of these patches on the vegetation community is not well understood. To address how the vegetation community responded to the size and spatial configuration of retention patches in harvest units, we examined community composition dynamics among 5 retention treatments that varied by amount and spatial distribution of mature forest structures across 10 blocks in southwest Washington and western Oregon, USA. We sampled understory species cover and richness in sample grids established in each retention patch and paired with a grid in each adjacent harvest area. Grids were sampled within one to two years after harvest, and again three years later. We examined treatment differences by conducting a permutational analysis of variance, visualizing the compositional analysis using non-metric multidimensional scaling ordination, and conducting Indicator Species Analysis.
Results/Conclusions
Understory community composition varied by treatment, location (retention or harvest area), and time since harvest, although the overall community composition seemed to differ mostly between retention patches and harvest areas. Although changes in the vegetation community may have occurred because of specific losses or gains in retention patches compared to harvests, it appears that these treatments share a common community, as suggested by overlapping in our ordination. As for individual species response, mature forest species like Maianthemum dilatatum, Oxalis oregana, Trillium ovatum, Vaccinium parviflorum, and Tsuga heterophylla seedlings were indicative of some or all retention patches in larger aggregated treatments, but none were indicative of retention patches in the most dispersed treatment. Our results suggest that larger retention patch sizes help to maintain mature forest vegetation communities, but because these patches are dynamic in time, it will be critical to understand the longer-term potential for stability of these vegetation communities, as well as how these remnant vegetation communities assist in the recolonization of forest species in adjacent disturbed areas.