Effect of Urbanization on Tree Growth in Remnant Forest Patches

Abstract

Land conversion and increasing impervious surface cover are affecting forests across the landscape by increasing local temperatures and altering ecosystem processes. In this study, we assessed the impact of impervious surface cover and other environmental, landscape, or climate factors on the growth of trees in adjacent forest patches. Tree cores were collected from 36 Acer saccharum, 40 Carya ovata, and 45 Quercus rubra trees in 11 deciduous forests in Southeastern Michigan and their annual radial growth measured from 1985 to 2014. Soil and stand basal area data were collected in each forest and distance from edge, mean impervious surface percentage within 250 m, and percent slope were calculated for each individual from National Land Cover Datasets. Annual average temperature, total precipitation, and Palmer Drought Severity Index data was also collected from the National Climatic Data Center. Tree growth rate was modeled by species and diameter using a Bayesian framework with non-informative priors as a function of percent impervious surface. Running models with several combinations of explanatory variables (distance to edge, slope, percent sand, total nitrogen, basal area, and climatic variables) did not improve the goodness of fit so random effects terms for individual growth and year were added, in addition to antecedent terms, which helped account for the potential effect of previous years’ growth. The model resulted in R2 values of 0.67 for A. saccharum, 0.71 for C. ovata, and 0.88 for Q. rubra. Higher levels of impervious surface did significantly, negatively affect the growth of small A. saccharum individuals and growth in an individual is strongly dependent of growth from the previous year. The year random effects term showed weak correlations with summer temperatures (-), spring and summer (+) precipitation, and the Palmer Drought Severity Index (-). These observations imply that not all trees species and size classes are affected equally by urbanization. Remnant forest patches typically experience little to no management and largely need to be selfsustaining over time, which highlights the need for a better understanding of how urbanization will affect these ecosystems.