Retained structures and bird communities in clearcut forests of the Pacific Northwest, USA

摘要

The retention of mature forest habitat elements (e.g., snags, downed wood, green trees) in clearcut forest stands is used to mitigate the negative impacts of timber harvesting on wildlife. We measured retained habitat elements and surveyed forest birds in recently (<15years old) clearcut forest stands within 4 locations across the Pacific Northwest of the United States, ranging from central Washington to northern California. Our objectives were to quantify the structural characteristics of retained habitat elements and identify how retention practices have shaped local breeding bird communities. We sampled 2022 harvest units in each location. We used a multi-species, multi-scale occupancy model to estimate probabilities of occurrence at large (stand level) and small (within stand) scales for all observed bird species, including those that were rarely detected, in addition to generating estimates of species richness at the stand level. We compared local occupancy with an index of regional occupancy to identify species that were selecting or avoiding clearcuts with retention. Average green-tree diameters ranged from 33 to 42cm while snag diameters ranged from 34 to 74cm; single trees were often larger on average than trees retained in patches, depending on the region. Downed wood volumes ranged from 129 to 650mpdhap#, with the majority of volume (o6076%) contributed by larger logs (>50cm in diameter). We observed 71 songbird and woodpecker species using clearcuts during 20082009. Stand-level richness varied within and across study regions (mean range: 3038 species). We found species with considerably higher and lower probabilities of occurrence than predicted by regional occurrence, suggesting that clearcuts with retention were valued differently by members of the forest bird community in each region. Upper canopy/bark foraging species generally occurred within clearcuts less often than expected, while cavity-nesting species exhibited a range of positive and negative relationships that sometimes differed by region. Our results provide context for understanding the contribution of retained habitat elements to forest birds and can potentially guide future monitoring activities pertaining to conservation of forest species in managed landscapes of the Pacific Northwest.