Variation in urban forest productivity and response to extreme drought across a large metropolitan region

The growth and survival of urban trees and maintenance of urban forest canopy are important considerations in adaptation of urban regions to climate change, especially in relation to increasing frequency of extreme climatic events such as drought. However, urban forest growth and drought response may vary considerably within large urban landscapes across gradients in land use, urbanization, forest composition and structure, and environmental factors. We quantified urban forest growth and resilience and resistance to extreme drought in the greater Chicago metropolitan region based on patterns of annual basal area production from increment core analysis. We evaluated variation in growth and drought response in relation to a broad urban to rural gradient, land-use categories, local-scale environmental predictors, and forest community characteristics. Urban forest growth varied greatly among land-use classes and major genera. Plot-level variation in productivity was predicted most strongly (R-2 = 0.53) by total plot-level basal area, canopy height, species composition, soil and ground-cover characteristics, and position within the urban-rural gradient. Urban forest growth was strongly related to regional meteorological drought. In periods of extreme drought conditions growth declined in the year of the drought (i.e., was not resistant to drought effects), but was highly resilient to drought in the subsequent 5 year period. Drought response did not vary consistently across land-use classes or among major genera, and site or community characteristics had little explanatory power in predicting drought response. Improved understanding of factors driving variation in urban forest growth and drought response could help inform adaptation-focused urban forest management strategies.