Physiological Tolerance to Heat and Drought Are Decoupled Across Common Woody Urban Greening Species in North China

Urban vegetation is increasingly exposed to the compound heat and drought stress due to global warming and urban heat island effects, yet heat tolerance and its linkage with drought resistance or photosynthesis remain unclear for plants in urban environments. We evaluated corresponding traits in nine woody species common to northern China's highly urbanized megacity cluster dominated by temperate continental monsoon climate. Specifically, we assessed: (1) temperature thresholds causing declines in the maximum quantum yield of photosystem II (F_v/F_m), (2) leaf resistance to xylem embolism and turgor loss, and (3) leaf gas exchange and biochemical efficiency of photosynthesis of field-grown, mature plants, but under lab conditions. We also recorded the in situ F_v/F_m and leaf temperature under contrasting air temperatures to assess whether and how plants maintained functional integrity of the photosynthetic apparatus under heat. We showed that stress tolerance and photosynthetic traits differed markedly among species. The overall weak heat tolerance of these plants resulted in significantly decreased F_v/F_m in four species under high air temperature, while species with a wider thermal safety margin, primarily determined by leaf temperature, retained greater functional integrity. Traits associated with leaf heat tolerance, drought tolerance, and photosynthetic efficiency were decoupled. Moreover, correlations were identified between heat tolerance traits and climatic metrics, indicating the variability in climate of species distributional range. Findings of this study add to the limited knowledge regarding the physiological resistance of urban greening plants and may provide reference during the establishment of green infrastructure in this region.