Urbanization reduces tree growth and drought resilience in remnant forest patches
Anthropogenic drivers of global change, ranging from habitat fragmentation to changing climatic regimes, are affecting forest composition and function across the landscape. The combined effects of these changes impact forest resilience through edge effects, altered hydrologic and nutrient cycling, and invasive species pressure. In this study, we examined the annual tree growth response to forest fragmentation, land use change, and drought in remnant forest patches using a combined field and GIS approach. We assessed the growth response to both historic climate variation and recent landscape change along an urban to rural gradient in Southeastern Michigan. We chose three common deciduous species with variable levels of drought tolerance: Acer saccharum (lowest), Quercus rubra (moderate), and Carya ovata (highest). We sampled in-situ basal area, soil nutrient-levels, soil texture, and annual tree growth data in 16 forest patches. We calculated adjacent impervious surface coverage, distance from forest edge, distance from roads, and distance from cities. We also obtained regional temperature, precipitation, Palmer Drought Severity Index data. We used a mixed-effects, hierarchical Bayesian model to evaluate the effects of urbanization on tree growth resilience to drought.
Sampled trees (121) ranged in ages from 22 to 156 years and diameters from 16.5 to 61 cm. C. ovata exhibited higher growth at larger diameters while A. saccharum and Q. rubra exhibited higher growth at smaller diameters. A. saccharum had higher correlation with all climate variables than C. ovata or Q. rubra. Growth for all species was most correlated with May drought and June precipitation. These results indicate that A. saccharum will be most negatively affected by increasing early season drought and decreasing summer precipitation. Closer to the forest edge, we found higher basal area, higher soil nitrogen concentration, and higher annual tree growth in all species. Growth decreased for all species with increased distance from edge. We also found that in areas with a higher percent cover of impervious surface, trees had lower growth rates with A. saccharum growth most reduced. Distance from roads and cities did not significantly impact growth for any species. These results indicate that of the species we studied, A. saccharum will be most negatively affected by urbanization in remnant forest patches.