DROUGHT TOLERANCE OF CANDIDATE URBAN TREE SPECIES

Abstract

Global climate change is increasing the frequency and intensity of extreme events such as drought and the effect is exacerbated in urban environment, threatning urban tree health and survival. In this study, we conducted a pot experiment using 15 candidate urban tree species (267 trees in total) diverse in origin but broadly categorized as native and exotic. Three treatment groups were established: control (regular watering), moderate drought (short-term water withholding followed by rewatering), and extreme drought (water withholding until visible leaf damage, then rewatering). Key physiological parameters were measured every week at regular intervals throughout the experiment: photosynthetic rate, stomatal conductance, chlorophyll fluorescence, and predawn and midday leaf water potentials. We examined whether physiological traits decline in parallel or independently during drought, assessed recovery capacity at different drought intensities, compared drought tolerance between native and exotic species, and identified species-specific drought tolerance characteristics. Results revealed parallel declines in all physiological parameters during both moderate and extreme drought, with decline severity increasing with drought intensity and duration. Recovery patterns exhibited threshold behavior, with physiological functions failing to recover after predawn leaf water potentials dropped below -2.3 Mpa and Fv/Fm below 4, indicating permanent/irreversible damage after the point. Thus, substantial recovery was observed in moderately stressed trees while extremely stressed trees didn’t show any signs of recovery. Native species were less sensitive to moderate drought than exotic species while the opposite was true under extreme drought. Species-specific responses revealed Ostrya carpinifolia (European Hop-hornbeam) as consistently drought-tolerant, while Koelreuteria paniculata (Goldenrain tree) demonstrated remarkable recovery capacity despite intermediate drought sensitivity. Nyssa sylvatica (Black gum) and Acer saccharinum (Silver maple) were consistently the most drought-sensitive species. Our findings highlight the complex, coordinated nature of physiological responses to drought and the importance of considering both drought resistance and recovery capacity when selecting urban tree species for climate resilience, particularly as extreme drought events become more common under climate change.