Long-term changes in boreal forest occupancy within regenerating harvest units

Many observational and short-term experimental studies have examined how boreal bird communities respond to forest harvesting. However, there are virtually no longitudinal studies that have assessed long-term effects of forest harvesting on boreal birds. The Calling Lake Fragmentation Study is a long-term experiment in northern Alberta's boreal forest, established in 1993. We have 24 years of avian point count data (1993-2016) from this study site, giving us the opportunity to study how the boreal songbird community has responded over 23 years to forest regeneration after harvesting. We ran multi-season occupancy models of 51 species surveyed at 72 stations (8 stations per 40-ha sampling area) within 3 unharvested forest sites, 3 harvest units that were cut in 1984 and treated to enhance conifer survival, and 3 harvest units that were cut in 1994 and left untreated to regenerate naturally. Eight of 21 species associated with older boreal forests (≥60-80 years old) used harvest units 23-33 years post-harvest. Eleven of 16 species associated with shrublands or younger forests used harvest units, before declining as harvest units regenerated, while four of 16 such species increased their probability of occupying harvest units over the monitoring period. Increased occupancy by 5 Neotropical migrants and 2 short-distance migrants was associated with increased El Nino weather events preceding a given breeding season. We also found evidence that increased occupancy of 3 species within harvest units was preceded by and correlated with increased occupancy by those species in unharvested forests, while occupancy of 4 species in unharvested forests was preceded by and correlated with increased occupancy within harvest units. Our results indicate the importance of unharvested forests for some boreal songbird species, the utility of longitudinal studies with controls for assessing changes in occupancy due to forest regeneration in harvest units versus factors operating at larger spatial scales, and the potential of harvest units to serve as habitat for boreal birds up to 33 years post-harvest.