A comparison of tree cavity availability across three habitat types in a highly urban area
Tree cavities are used by a wide variety of species including birds, mammals, and reptiles as nesting habitat, for food storage, and cover. With urbanization, natural decay processes are managed and natural tree densities are reduced, both of which may influence the availability of tree cavities. We tested the capacity for large urban parks and residential areas to provide tree cavities at levels similar to natural areas within the Chicago city limits and two suburbs. Natural areas consisted of forest preserves, large city parks were >28ha. Residential sites were based on randomly selected coordinates within the study area. Sites within a habitat types were >2km apart. In natural areas and large parks, at least 50 trees with a diameter breast height (dbh) >12cm were examined for cavities, and for residential sites all trees >12cm dbh adjacent to the roadway on one city block were examined for cavities. We classified cavities as ‘natural’ (branch break or crack) or ‘excavated’ (by woodpeckers), recorded the tree decay class (1 = 100% alive to 5 = snag), size, height and orientation of cavities. We compared the prevalence of cavities across habitats, and the characteristics of cavity trees with cavity-free (control) trees.
Trees with one or more cavities were significantly less prevalent in residential areas compared to the other habitat types, with twice as many cavity trees present in large urban parks and natural areas. Total tree cavity availability was similar in large parks (45 cavities per 100 trees) and natural areas (37 per 100 trees), and lowest in residential areas (20 per 100 trees). However, tree densities were the greatest in natural areas. Trees with cavities had a significantly larger dbh than control trees and had more decay. Furthermore, cavities excavated by woodpeckers were significantly more likely to be found in trees with a larger dbh, and in trees with more decay compared to natural cavities, and tended to be in natural areas or large parks. Excavated cavities were rare in residential areas. Pruning of residential trees may create opportunities for fungal entry into trees leading to the formation of natural cavities, but the prevalence of woodpecker excavated cavities was largely limited to natural areas and large parks.