The socio-ecology of dying trees: cavity nesting birds, hazard trees, and declining urban forest canopies

Background/Question/Methods

Precipitous losses of urban trees have raised concerns in cities around the United States over effects on climate, water quality, and neighborhood vitality. Less attention has been paid to implications for biodiversity. At both broad and local scales, the decline of urban forests has clear impacts on wildlife. For species dependent on dead wood, such as cavity nesting birds, those the effects may be complex. At broad scales, cavity nesting birds are likley associated with greater tree canopy cover. At the scale of individual trees, however, cavity nesters may benefit from the acceleration of decay in urban trees caused by stresses of urban life. Standard arboricultural practice typically removes deadwood from places that could impact people or property. We conducted studies at both scales. Using standardized hazard ratings by certified arborists, we assessed the degree of overlap between tree hazards for people and deadwood habitat for birds in locations spanning a wildland to suburban gradient in Massachusetts. In addition, we surveyed birds in the city of Baltimore, Maryland using point counts as part of long-term monitoring with the Baltimore Ecosystem Study (BES LTER). We measured forest canopy structure in 100m transects around these survey points and conducted hazard ratings for 10 randomly selected trees per transect.

Results/Conclusions Trees occupied by cavity nesting birds in Massachusetts exhibited higher hazard ratings than nearby randomly chosen trees of the same size and species, particularly in the category of “probability of failure” (probability of falling). In addition, the wildland location in Massachuestts retained proportionally greater numbers of trees with both high hazard ratings and suitable nesting habitat than trees in residential transects in suburban and urban areas of both western Massachusetts and Baltimore, MD. Management for habitat for cavity nesting birds and other deadwood dependent species requires attention to both broad scales (restoration of declining urban tree canopies) and local scales (maintenance and care of individual trees). At the local scale in particular, best management practices must be derived with reference to both wildlife ecology and legitimate human concerns over risk. Future directions include ongoing survey research to assess how landowners rank these competing priorities when making environmentally relevant decisions.