Functional adaptations and trait plasticity of urban trees along a climatic gradient

In urban environments, long-term tree survival and performance requires physiological tolerance or phenotypic plasticity in plant functional traits. Knowledge of these traits can inform the likely persistence of urban forests under future, more severe climates. We assessed the plasticity of morphological and physiological traits of tree species planted along an urban climatic gradient in the Greater Sydney region during a severe, multi-year drought in eastern Australia. We selected four sites along a ∼55 km east-west transect, ranging from the cool/ wet coast to the warm/dry inland. We assessed five tree species (four natives, one exotic) with different predicted climatic vulnerability based on climate-origins, estimating functional traits indicative of drought tolerance: carbon isotope composition (δ13C), Huber value (HV), specific leaf area (SLA), wood density (WD), and leaf turgor loss point (πtlp). Broadly, trees planted in warm/dry sites had more negative πtlp, higher WD, δ13C and HV, and lower SLA than cool/wet sites, indicating phenotypic plasticity to drought. The leaf-level traits πtlp, δ13C and SLA were more strongly correlated with temperature and precipitation, compared to HV and WD. Species differed in the extent of their trait shifts along the transect, with greater plasticity evident in the exotic Celtis australis and the more temperate cool-climate Tristaniopsis laurina, compared to the more tropical, warm-climate Cupaniopsis anacardioides, which showed limited plasticity and lower drought tolerance. Our findings reveal adaptive capacity of urban trees to climate via plasticity in drought tolerance traits, which can direct species selection to improve urban forests resistance to climate change.