Avian response to forest management for oak regeneration
Felicity L. Newell, MS
Advisor: Amanda Rodewald
Today, the majority of eastern deciduous forests in North America are closed-canopy mature second-growth. In contrast, an open forest canopy is thought to have been important in presettlement forest ecosystems. Changes in forest structure are of special concern given that oaks (Quercus spp.) cannot regenerate effectively in closed-canopy stands. Although oaks have historically dominated many eastern forests, forest composition is changing due to anthropogenic impacts on disturbance regimes. Consequently, many forest management efforts now aim to promote oak regeneration and restore forest structure, which ultimately will require a more complete understanding of the ecological processes associated with canopy disturbance. Partial harvesting (e.g. shelterwoods) provides one way to examine the importance of canopy openness. My research examined the link between canopy openness and birds that nest and forage in the forest canopy. Specifically I examined the extent to which (1) avian canopy-nesting species select particular canopy features and (2) canopy structure affects reproductive success. I compared measures of preference (abundance, settlement patterns, site fidelity, and age distributions) and fitness (e.g. nest success) between recent shelterwood harvests and unharvested second-growth stands in mixed-oak forests of southern Ohio. I studied a guild of sensitive canopy-nesting species, including Cerulean Warbler (Dendroica cerulea), Eastern Wood-pewee (Contopus virens), Scarlet Tanager (Piranga olivacea), Yellow-throated Vireo (Vireo flavifrons), and Blue-gray Gnatcatcher (Polioptila caerulea).
My research was conducted at four state forests in southern Ohio from 2007–2009. I compared shelterwoods recently harvested to 50% stocking and reference closed-canopy forest in the same landscape context. Distance-based line transect surveys (56 transects in 18 stands at 4 state forests from 2007–2008) were used to examine changes in the bird community associated with shelterwood harvesting. Midstory and several ground-nesting species were 30–70% less abundant in shelterwood stands compared to reference mature forest. Shrub-nesting species increased >100% several years post-harvesting while overall canopy-nesting species were 50% more abundant in shelterwood stands.
Intensive research focused on canopy songbirds as one late-successional group that might benefit from partial harvesting. Five species were studied intensively at 12 sites in three state forests from 2007–2009. Canopy species generally showed a weak positive association with partial harvesting with abundance of four species 30–100% higher in shelterwood stands compared to unharvested reference stands. For Cerulean Warblers associations with partial harvesting were significant only when controlling for slope and aspect, and abundance was >200% higher in shelterwood stands in one state forest. Similar settlement patterns and site fidelity in shelterwood and unharvested stands suggested limited preferences for stand type. A higher proportion of first-time breeders in shelterwood stands compared to reference stands for the Cerulean Warbler and Scarlet Tanager may have been a post-disturbance response, where young birds colonized newly created (or improved) habitat. For all five canopy species, nesting success was comparable in shelterwood and unharvested reference stands. Partial harvesting did not appear to negatively affect canopy songbirds, although reproductive success was generally low (15–36%) even in a predominantly forested landscape.
An information-theoretic approach was used to compare different models to explain nest-site selection and nesting success of canopy songbirds. Point sampling using a prism and variable radius plot was used to collect data on forest structure for nest and random plots in July and August each year. I identified basal area (m2 ha-1) as the variable most strongly associated with harvesting. Examination of nest-site selection and nesting success indicated that factors such as topography, canopy structure, and floristics may be important in habitat selection for canopy songbirds. Only the Eastern Wood-pewee selected for canopy openness created by partial harvesting, but overall the canopy-nesting guild selected areas with fewer medium-sized trees (23–38 cm dbh). Cerulean Warblers favored productive northeast-facing slopes with abundant grapevines. Eastern Wood-pewees and Yellow-throated Vireos nested in areas with 20% more white and red oak, while Blue-gray Gnatcatchers nested in bottomland areas avoiding areas with red oak. The type of oak appeared to be important and four species nested in Quercus alba twice as much as available while three species avoided Quercus rubra. For the canopy-nesting guild and several individual species, nesting success was negatively associated with red oaks around the nest. Concealment and size of the support branch were important for the Scarlet Tanager, which has a relatively large obvious nest. Tanager nests were more likely to be depredated early in the nesting cycle, whereas Cerulean Warbler nests were more likely to be depredated later in the nesting cycle when feeding young. Nest success was 20% higher late in the season for Eastern Wood-pewees.
Results from this study help to inform current management efforts designed to restore and maintain natural processes in forest ecosystems. Management implications from this study include prioritization of areas for protection, and recommendations on size and species of trees retained in partial harvests. Partial harvesting shifts the bird community from midstory and ground-nesting species to shrub and canopy-nesting species and can provide habitat for a mix of species as part of a balance of disturbance levels on the landscape. Results suggest shelterwood harvests provide at least short-term breeding habitat suitable for canopy songbirds, although managers should carefully consider long-term management of shelterwoods. Strong association between birds and oaks in this study demonstrates the value of current management efforts to promote oak regeneration. My work provides new insight that the species of oak may be important. Patterns of habitat preferences and nesting success suggest that white oaks are more valuable than red oaks for canopy birds. Managers should make special effort to retain Quercus alba in partial harvests and other restoration projects. In addition, harvesting of medium-sized trees (23–38 cm dbh) may be preferable to harvesting large trees (> 38 cm dbh). My results also suggest that older forests with canopy gaps and/or those on northeast-facing slopes should be protected for conservation of the severely declining Cerulean Warbler. In areas lacking steep slopes, creation of canopy gaps could benefit ceruleans, but further work is needed.