Contrasting heat tolerance of urban trees to extreme temperatures during heatwaves

Extreme climate conditions, including more frequent and prolonged heatwaves, are increasing in many regions throughout the world. Urban trees can aid in mitigating the adverse impacts of climate extremes; however, their capacity to do so is limited by species differences in ability to maintain function and retain leaves to provide shade under extreme temperatures. To assess tree vulnerability to heatwaves, we used a common garden experiment in Western Sydney, Australia, which included four widely-planted tree species; two native evergreen and two exotic deciduous species. Data were collected during 2019 and 2020, the two hottest summers on local record with maximum air temperatures reaching 45 degrees C. We monitored plant physiological status as stomatal conductance (g(S)) and midday leaf water potential (Psi(mid)). We determined heat tolerance by measuring the leaf critical temperature (T(cri)t) for photosynthesis and leaf turgor loss point (pi(tlp)) as thresholds for loss of function and calculated the thermal safety margin. Plant performance was assessed by measuring tree growth and leaf damage after heatwaves. Species responded dynamically with gS decreasing as air temperature and vapor pressure deficit increased during heatwaves. Exotic species had higher g(S), Tcrit and ptlp than native species. Leaf damage under heatwave conditions was overall lower in native species. Our results highlight the impact of heatwaves on urban trees and the value of physiological metrics to inform tree species selection to create more resilient urban environments.