Tree cavity availability across forest, park, and residential habitats in a highly urban area

Tree cavities are used by a wide variety of species for nesting, food storage, and cover. Most studies on cavity availability have been conducted in forests, and little is known about urban areas. With urbanization, species that excavate cavities may be less abundant, natural tree-decay processes are managed, and tree densities are reduced, all of which may influence tree-cavity availability. We investigated three questions: 1) What is the prevalence of tree cavities in different habitats in the Chicago area? 2) How do the characteristics of natural and woodpecker-excavated cavities and cavity-trees differ across habitats? 3) How does the urban landscape influence the prevalence of tree cavities? We tested the capacity for large urban parks and residential areas to provide tree cavities at levels similar to forested areas. We surveyed 1,545 trees in these three habitats for excavated and natural (caused by decay) cavities. Cavities were most available in forests, where the density of trees was highest. We found that a similar proportion of trees in forests and parks had excavated cavities, but excavated cavities were rare in residential areas. Trees containing cavities were larger than control trees and had more decay, and excavated cavities were in larger trees with more decay than natural cavity trees. Canopy cover was the main landscape variable influencing excavated cavity availability. Our results suggest that the prevalence of tree cavities may not be a limiting factor for urban wildlife, however that is contingent on the levels of use of natural cavities, which is currently unknown.