Species-specific impacts of urbanization on climate sensitivity of tree transpiration

Background/Question/Methods

The expansion of urban tree canopy is a commonly proposed nature-based solution to combat warming temperatures. Trees can help cities adapt to extreme heat by moving water from the ground to the atmosphere, where it evaporates (or transpires) and creates cooling latent heat fluxes. It is a common practice to predict urban rates of transpiration by assuming trees will respond to climatic conditions in a similar manner as rural trees. Urban trees, however, have unique set of environment conditions (e.g., higher heat loads, nutrient availability, ambient carbon dioxide), and management practices (e.g., irrigation and pruning) that likely alter urban transpiration rates compared to rural canopy trees. Using sap flux sensors we quantified transpiration rates in similar sized urban and rural canopy trees for two different maple species that are native to Northeastern US- Red Maple (Acer rubrum) and Sugar Maple (Acer saccharum). Our urban study trees were located in well-irrigated yards in Boston, MA. Rural canopy trees of A. saccharum were located in Hubbard Brook, NH while rural A. rubrum trees were located at Harvard Forest in Petersham, MA. We used local weather station data to examine the climate sensitivity of transpiration to changes in temperature and vapor pressure deficit (VPD).

Results/Conclusions

We find species-specific responses of tree transpiration to urbanization. A. rubrum showed strong sensitivity to climatic conditions regardless of planting location while A. saccharum only showed strong climate sensitivity when growing in the urban location. A. rubrum showed increases in transpiration with increases in mean air temperature or VPD in rural (r2=0.36, p<0.001 for temperature; r2=0.90; p<0.001 for VPD) and urban sites (r2=0.21, p=0.002 for temperature; r2=0.28; p=0.02 for VPD). Transpiration in A. saccharum did not increase with temperature or VPD when growing in rural sites (r2=0.02, p=0.02 for temperature; r2=0.04, p<0.001 for VPD), but did increase with temperature and VPD when growing in urban locations (r2=0.25, p<0.0001 for temperature; r2=0.63, p<0.0001 for VPD). These findings highlight the need for understanding the response of different tree species to urbanization when predicting rates of transpiration and the associated cooling potential provided by trees for cities.