Species-specific plant water status responses to land surface temperature vary across three cities with different precipitation patterns

Elizeth Cinto Mejia, Alessandro Ossola, Moreen Willaredt, Mia K. Lippey, Oliver Roper, Samridhi Chaturvedi, Elsa Youngsteadt, Emily K. Meineke

As urbanization intensifies and global climate warms, people are exposed to high urban temperatures. Mature trees can reduce human exposure to extreme heat; however, urban forests also experience hot temperatures that may affect their physiological function. To investigate how urban heat, specifically land surface temperature (LST), vapor pressure deficit (VPD), and city-level precipitation regimes affect tree function, we measured stem water potential and stomatal conductance on street trees (eight species) in three U.S. cities with comparable annual temperatures but different precipitation profiles; Sacramento, CA, Raleigh, NC, and New Orleans, LA. LST negatively affected water potential and stomatal conductance in Sacramento, and only for certain species, with these effects shifting across the growing season: early-season trees showed larger declines in stem water potential under heat without restricting stomatal conductance, while late-season trees restricted conductance sharply under heat and maintained stable water potential. Neither Raleigh nor New Orleans showed comparable heat effects on stem water potential, though a few species in Raleigh showed a positive stomatal conductance response to LST. VPD was linked to both water potential and stomatal conductance, though the direction and strength of these responses varied by species. Species differed considerably in their sensitivity: U. parvifolia showed no heat effects in any city, while P. calleryana was the most heat- and drought-sensitive species. Our findings linking tree physiological function to urban heat exposure can inform urban planning decisions on species selection and tree placement in the context of increasingly warm and dry urban climates.