Long-term avian community response to hemlock decline
Open Access
- Author:
- Toenies, Matthew John
- Graduate Program:
- Ecology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 20, 2017
- Committee Members:
- David Miller, Thesis Advisor/Co-Advisor
Matthew Marshall, Committee Member
Margaret Brittingham, Committee Member - Keywords:
- Tsuga canadensis
Empidonax virescens
Setophaga virens
birds
community
forest die-off
Adelges tsugae
decline
occupancy
response - Abstract:
- Forest biomes face increasing rates of disturbance from many causes, including climate change, introduced pests, and shifting fire regimes, as well interactions between multiple factors. In particular, the introduction of exotic forest pests is increasing globally, frequently resulting in decline and die-off of affected forest types. The loss of foundational forest species can drastically change the structure and composition of vegetation communities in forests, a phenomenon that holds strong implications for avian communities. However, little research has focused on the long-term effects of forest die-off at a community-wide scale. To examine the response of bird communities to forest die-off, we studied the community associated with declining eastern hemlock (Tsuga canadensis) forests affected by the introduced hemlock woolly adelgid (Adelges tsugae). We conducted variable-radius point counts to survey bird communities in both hemlock and hardwood stands in 2000, during early stages of adelgid infestation, and again in 2015 and 2016, following significant hemlock decline. We also measured the severity of hemlock decline and associated vegetation variables in the same hemlock stands where avian surveys occurred. We used multi-species occupancy models to examine species-specific and species group responses to hemlock decline. Results showed that hemlock basal area declined to varying degrees across the hemlock stands, and that hemlock decline was correlated with current vegetation structure, including an increased deciduous understory. Hemlock-associated bird species declined between the two time periods, while all other species groups responded positively, with the strongest responses in species associated with the shrub-layer, forest edge, and mature deciduous habitat. Species composition in hemlock and hardwood stands became more correlated over time, highlighting a trend toward homogenization of the avian community as the unique species assemblages in hemlock stands give way to the avian community of the surrounding hardwood landscape. Where hemlock-associated species persisted, their presence was correlated with the combined effect of greater hemlock basal area and better hemlock condition. Our results demonstrate that the decline and die-off of foundational forest species can restructure vegetation communities and their associated avian communities over time, with strong implications for forest type specialists. Due to regional variation in patterns of forest die-off and community composition, understanding impacts on forest-dependent communities requires examining response at broad spatial and temporal scales. To determine how the responses to forest die-off that we observed in the avian community may vary across a large regional scale, we examined bird communities in hemlock stands at four sites across the range of hemlock woolly adelgid in the northeastern/mid-Appalachian regions of the United States. We surveyed the avian community in earlier stages of infestation and again following hemlock decline, and analyzed this data using a multi-species occupancy modeling approach. Despite variability among sites in characteristics of the avian community and of the die-off itself, changes in species richness over time were similar across all sites. Hemlock-associated bird species declined at all sites, with concurrent increases in other species groups, especially those associated with forest edge and shrub-layer habitats. Finer-scale variation in the magnitude of avian response coincided with the varying stages of hemlock decline across sites. Species-specific trends showed that Acadian Flycatcher declined most consistently across this large regional scale. Understanding these patterns is critical to predicting and preparing for changes to not only forested landscapes affected by the loss of hemlock, but also for those that will experience similar die-offs as forest pest introductions increase globally.