Urbanization intensifies tree sap flux but divergently for different tree species groups in China

In recent years, positive and negative effects of urbanization on forest ecosystem have been reported by many studies, while some uncertainties about the impact of urbanization-induced spatial heterogeneity of environmental factors on forest systems still remain unclear. In this study, we analyzed the urbanization effects on sap flux of a common subtropical evergreen tree species Schima superba along an urban-rural gradient in Guangdong Province, South China, and identified the consistency of these results among different groups (evergreen, deciduous, and coniferous species) using data from 83 previously published studies in China. The mean sap flux density (F_d) of S. superba in Xiaoqingshan (XQS), Heshan (HS), Dinghushan (DHS), and Shimentai (SMT), along the urban-rural gradient was 40.9 g m^-2 s^-1, 32.1 g m^-2 s^-1, 17.0 g m^-2 s^-1, and 17.5 g m^-2 s^-1, respectively, presenting a decreasing trend with the diminishing urbanization. This pattern in F_d tended to enlarge with tree size and was well confirmed by the enhanced leaf transpiration rate (by 119%) and photosynthetic rate (by 8.8%) for the S. superba in another urbanization gradient from the urban (Hangzhou, denoted as "HZ") to rural sites (Jiande, denoted as "JD") in Zhejiang Province, East China, which has similar climatic condition and urbanization with Guangdong Province. We attributed such positive effects to the decreased sapwood density and specific leaf area (SLA), as well as the increased Huber value (sap wood area/leaf area) and the sap wood specific hydraulic conductivity (K_S). We also found that pollutant emission exerted more impact on F_d than climatic factors change, since the variation of the latter was not large enough to cause significant change of F_d under the same climatic zone. In addition, we conducted a principal component analysis (PCA) based on the published 83 studies. Results showed F_d of evergreen tree species was related positively to principle 1 and negatively to principle 2, respectively, whereas the F_d of deciduous broadleaf and coniferous tree species was positively and negatively related to both principles, respectively. This study demonstrated the potential impact of urbanization-related pollutant emission changes on water use of forest trees and the growth among different groups.