Quantifying the synergistic effects of impervious surface and drought on radial tree growth

The global trend of increasing urbanization is generating isolated patches of vegetation that could be strongly influenced by the surrounding landscape. Simultaneously, many of these urban forests will be exposed to the increasing temperatures and higher risk of drought conditions brought up by climate change. The heat island effect associated with an increase in impervious surface cover and its influence on urban vegetation has been document mostly for large urban centers. However, how the effects of urbanization may interact with tree resilience to drought have been much less studied, especially in middle size cities like the majority of those populating eastern North America. In this study, we analyzed 30 years of tree radial growth data collected from individuals of three common native species in remnant forest patches distributed along a gradient of urbanization in the Midwestern USA. Acer saccharum and Quercus rubra show reduced growth rates in years with lower water availability. Our results also indicate that the detrimental effects of impervious surface cover eliminated any of the beneficial effects of growing in a wet year for A. saccharum and Q. rubra, and these effects of increase impervious surface cover on growth were more detrimental than those of droughty years. Carya ovata, the third species, did not show differential growth along the urbanization gradient or among years with varying water availability. This study illustrates how the combination of drought events and increasing impervious surface cover could have a differential impact on coexisting species in urban forests, which could further alter the species composition and functioning of these ecosystems.