Water use efficiency and its influencing factors of typical greening tree species in Beijing region

  Objective  Due to the water scarcity and the improvement in urban greening, the tension has grown from the struggles for water between industries, urban life, and urban afforestation in Beijing. Thus, it is the top priority to explore the response mechanism of urban greening tree species to arid habitats and to screen the low-water consumption and high-water utilization landscaping tree species for high-quality urban forest landscape.

  Method  Typical greening tree species including Ginkgo biloba, Koelreuteria paniculata, Sophora japonica, Platycladus orientalis, Pinus tabuliformis, and Pinus bungeana in Beijing were selected and subjected to three groups of soil water stresses such as slight drought (SLD, 50%−70% field capacity (FC)), moderate drought (MD, 30%−50% FC) and extreme drought (ED, lower than 30% FC). The interspecific differences in the instantaneous water use efficiency (WUE_i) and mean water use efficiency (WUE_L) of these tree species under water stresses and their relationship to eco-physiological factors were investigated based on the stable isotope technology and the observations on tree physiological traits.

  Result  The photosynthetic capacities and WUE_i in G. biloba, K. paniculata, S. japonica and P. orientalis decreased significantly compared with control (CK, 90%−100% FC) when potted soil volumetric water content (SWC) was less than 70% (P < 0.05), whereas there was no significant difference in WUE_i of P. tabuliformis and P. bungeana subjected to continuous drought (P > 0.05). The WUE_Ls of evergreen tree species were significantly higher than those of deciduous tree species under MD and ED (P < 0.05). The WUE_Ls of three deciduous tree species differed in response to extreme drought (P < 0.05), and the sequence of WUE_Ls in three deciduous tree species was S. japonica > K. paniculata > G. biloba. The WUE_Ls of G. biloba and K. paniculata in SLD, and P. orientalis and P. tabuliformis in MD reached their respective peaks and then decreased along with the increases in soil water stress; while those of S. japonica and P. bungeana increased with potted soil drying, and were 44.19% and 30.35% higher than control, respectively at severe drought condition (P < 0.05). Comparing the correlations between photosynthetic parameters, WUE_i and WUE_L of different tree species, it was found that the stomatal conductance (g_s) of G. biloba, K. paniculata and S. japonica significantly affected its photosynthesis and transpiration processes (P < 0.01) and hence exerted a strong influence on WUE_i, while those in evergreen tree species such as P. orientalis, P. tabuliformis, and P. bungeana were insensitive to the variation in soil moisture and had little impact on WUE_i (P > 0.05). The WUE_L of evergreen tree species was higher than that of deciduous ones under moderate and extreme water stress.

  Conclusion  Therefore, considering the limited water resources of Beijing, evergreen tree species have stronger abilities to conserve water content and optimize photosynthetic performance to improve their water use efficiency, to adapt to arid habitats than those of deciduous trees in Beijing.