Using GIS to compare street tree performance across a range of urban conditions
A single city can have highly variable distributions of impervious surface and coarse vegetation cover, resulting in a gradient of temperatures throughout the city. Previous research has documented the effects of urbanization on several species of mature trees, with a higher proportion of trees in poor condition in the warmest, most densely paved parts of a city. Here, we test whether this pattern is evident in sapling trees and across multiple tree species on a landscape level. We used GIS to extract temperature, impervious surface, and coarse vegetation measurements for more than 3,900 condition-rated sapling street trees in Raleigh, NC. To account for conditions beyond the immediate planting site, we also included measurements of impervious surface and coarse vegetation cover within 50m and 100m radii of the planting sites.
Tree species differed in how they were affected by urbanization factors such as increased temperature and impervious surface cover. Trees of several species, such as Acer rubrum and Cercis canadensis, were significantly less likely to be in excellent condition in the warmest parts of the city compared to cooler areas, suggesting that these species may not fare well as the climate warms. Alternatively, other tree species, including Magnolia grandiflora and Pistacia chinensis, showed consistent proportions of trees in excellent condition across the range of each urbanization factor tested. As a whole, these results provide additional evidence for the importance of landscape level factors, in addition to site-specific features like soil quality, in influencing tree health in cities. In our continuing analyses, we are considering how herbivore-host interactions on urban trees are affected by similar landscape-level factors.