PS 48-245 - Tree foliar traits vary across urban landscapes: utilizing hyperspectral imaging to scale across the city

Abstract: Determining how multiple global change factors may alter forest nitrogen (N) availability can help us conserve and manage plant communities. Foliar morphological and chemical traits provide valuable insights into how plants and plant communities functionally respond to environmental change. Quantifying changes in functional traits over time enables better understanding of plant community dynamics in light of global change factors, such as urbanization, which can alter biogeochemical cycling of nitrogen and other limiting nutrients. Our goal was to determine how foliar LMA (leaf mass per area) and N resorption vary across heterogeneous developed land, and to use remotely sensed hyperspectral imagery to scale plant traits to forests across urban landscapes. In 2019, we collected fresh canopy leaves during peak growing season (late June and July) to determine LMA and senesced canopy leaves in October to determine N resorption efficiency in trees across an urbanization gradient in Madison, WI. Airborne hyperspectral imagery was collected multiple times across this gradient during the 2019 growing season.

We found a range of LMA (35.6 g m^-2 - 147.4 g m^-2) and N resorption efficiency (23.4% - 89.6%) across the forests, demonstrating how tree genera and urban environmental conditions differentially influence leaf traits. Maple trees (Acer spp.) had significantly higher LMA in urban (91.3 g m^-2) compared to rural (60.9 g m^-2; P=0.03) forests. LMA of hickories (Carya spp.) and oaks (Quercus spp.) were not significantly different between urban (hickory=91.0 g m^-2, oak=109.3 g m^-2) and rural (hickory=88.0 g m^-2, oak=102.8 g m^-2; P>0.05) forests, although urban trees exhibited greater LMA. Similarly, maple, hickory, and oak trees demonstrated the same patterns in N efficiency across urban environments with greater N resorption efficiency in urban trees. However, only maple trees had significantly greater N resorption efficiency in urban (75.8%) than rural (58.1%) sites (P=0.03) versus hickories (urban=58.8%, rural=38.7%) and oaks (urban=74.3%, rural=71.9%). These findings suggest that thicker leaves in urban trees may offer protection against water loss from urban heat island effect and/or against air pollutants (e.g., ozone) with heavier structural compounds. Greater N resorption efficiency in cities suggests trees are not responding to greater N deposition or availability in urban environments due to other stressors. Further data analysis on utilizing hyperspectral imaging to scale leaf traits (i.e., LMA) across the city and seasons will provide information as to how trees are responding to urban environmental conditions across larger spatial scales.