Date of Award
Master of Science
Prolific oak regeneration occurred over past centuries from anthropogenic cutting, grazing, and fire, and has declined over the past century due to decreased disturbance. Mesophication within closed-canopy forests of the eastern deciduous region has resulted. Oaks are a keystone species and provide abundant resources for forest wildlife; the ability of late-successional tree species to provide similar resources is relatively understudied. To determine the importance of oak-hickory stands for forest birds, we examined two habitat-selection hypotheses that influence avian abundance and distribution: (1) Habitat heterogeneity (i.e. differences in forest structure) and (2) Availability and distribution of food resources (i.e. index of arthropod biomass). We examined avian response across a gradient of oak-hickory to non-oak tree-species dominance. Non-oak stands were largely dominated by sugar maple, American beech, and yellow poplar (Liriodendron tulipifera). We predicted that migratory breeding-species of concern would respond positively to oak-dominated stands and associated vegetation (e.g. oaks and hickories) because they provide more heterogeneity and likely more food than non-oak stands and associated vegetation (e.g. yellow poplar, maples and beech). We hypothesized that oak and hickory species contain more arthropod biomass and diversity than late-successional species. We conducted breeding bird surveys from 30-April to 15-July 2013-2014 in the Shawnee National Forest, Illinois at 22 study sites. We characterized study sites by dominant canopy vegetation. We used a modified version of the branch-clipping technique to sample arthropods on tree species that represented a gradient of succession. We used generalized linear mixed models to examine differences in arthropod metrics. We estimated detection probabilities for PIF-concern, breeding species and modeled density as response to a priori habitat models using hierarchical distance sampling in R package unmarked. We compared best-fit habitat models for each species with a model of our derived index of food availability (i.e. mean total arthropod biomass (g/m)) for each study site. Habitat heterogeneity was higher in oak-hickory dominated sites. Aerial foragers and foliage gleaners responded predominantly to forest composition and structure, whereas ground foragers responded largely to microhabitat. Density estimates from five of seven species that included percent oak-hickory composition in top models exhibited positive responses. All species that included canopy tree diversity (n=6) in top models responded positively to increasing canopy diversity, and canopy diversity was significantly higher in oak-hickory sites than non-oak sites. Yellow poplar, oak and hickory species yielded more total arthropod biomass (g/m), Lepidopteran biomass (g/m), and guild diversity and richness than late-successional beech and maples. Heterogeneity and food biomass both influenced avian abundance of aerial foragers and foliage gleaners. Our results imply that oak-hickory stands are ecologically important for migratory forest birds of concern because heterogeneity and food resources increase as oak-hickory canopy composition increases. Oak regeneration is a challenging process to manage because oaks require frequent disturbance to achieve the high levels of light needed by this genus. Therefore, it may be beneficial to manage stands for yellow poplar dominance because this species grows rapidly. Moreover, our results suggest that yellow poplar yields comparable and higher estimates of arthropod biomass and diversity on hickories and oaks, respectively. However, foraging opportunities are likely restricted on yellow poplar because of limited structure, attributed to excurrent branching.
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