Retention forestry maintains green trees and legacy structural elements to buffer sensitive site features and facilitate colonization of regenerating stands by mature forest communities. However, remnant patches may be too small to provide refugia forest understory vegetation. More research is needed to identify thresholds for different organisms in different forest and landscape types, and to understand the relationship between these organisms and the amount and arrangement of retained trees in recently harvested stands. We examined change in plant dynamics between retention treatments after applying five retention prescriptions with variable retention aggregation in 10 geographically distinct blocks in southwest WA and western OR, USA. We examined vegetation species total and proportional richness and cover, and community composition, using sample grids established in each treatment block and paired with a grid in the adjacent harvested area.
Results/Conclusions
Understory species total cover and richness were affected more by location (patch or harvested area) than by the specific treatments. Proportional vegetation responses varied: forbs were higher in cover and richness in harvested areas for treatments with greater; graminoids differed between patches and harvested areas only in treatments with fewest trees per patch; pioneer species had higher cover and richness in harvested areas in treatments with greater retention, and mid- and late-seral species had higher cover and richness in retention patches. Community composition varied by treatment, location, and interaction. In summary, treatments with aggregated retention (one or two large remnant patches) behaved similarly, and seemed to provide refugia for forest understory vegetation. Treatments with smaller patches (fewer retained trees) were less effective at maintaining mature forest habitat. Retention pattern directly affects understory plant communities remaining after harvest. This, in turn, will likely affect species in other taxonomic groups that utilize mature forest habitat provided in retention patches.