Spruce Beetle outbreaks guide American Three-toed Woodpecker Picoides dorsalis occupancy patterns in subalpine forests

摘要

American Three-toed Woodpeckers Picoides dorsalis are considered a sensitive species by the United States Bureau of Land Management and are on the United States Fish and Wildlife Service's Watch List. In Idaho, Oregon and Washington, they are of conservation concern due to low abundance and an apparent reliance on disturbed, old-growth forests. This species is strongly associated with Spruce Beetle Dendroctonous rufipennis epidemics, yet their occupancy relation with epidemic conditions have not been described. We studied Three-toed Woodpecker occupancy patterns in spruce-fir forests experiencing varying degrees of beetle infestation between 2013 and 2016. Accounting for detection probability, we found a strong positive relationship between occupancy and the density of currently infested trees. Estimated occupancy was 0.57 (Bayesian credible interval 0.49, 0.64) for 75-m-radius survey points with zero infested spruce trees vs. 0.99 (Bayesian credible interval 0.99, 1) for points with 235 recently infested stems per ha. In contrast, we found no relationships with density of trees infested at least 3 years prior to sampling, density of older snags (i.e. & 10 years dead) or quadratic mean diameter of healthy or recently infested trees. These results provide evidence of the importance of active Spruce Beetle infestation for Three-toed Woodpecker habitat. Conserving Spruce Beetle-infested trees for at least 3 years following the onset of a beetle epidemic would benefit Three-toed Woodpecker populations and other species that depend on woodpecker-excavated cavities. We suggest managers consider these results when planning logging activities aimed at Spruce Beetle mitigation.