Environmental control on transpiration and its cooling effect of Ficus concinna in a subtropical city Shenzhen, southern China

Abstract Climate models predict rising temperatures and more frequent and prolonged urban heat islands (UHI) in southern China. The urban vegetations have become a focal point to mitigate the detrimental effects of UHI and to provide cooling through transpiration. However, transpiration in trees such as Ficus concinna in relation to extreme long-term conditions of UHI is scarcely documented. Here, we investigated the transpiration dynamics and its cooling effects in F. concinna induced by changes in site environmental variables in a subtropical megacity, Shenzhen over five consecutive years (2015–2019) based on continuous sap flow measurements. Seasonally, the transpiration (Tr) and its cooling effect (i.e., heat energy absorbed (Q) and temperature reduction (ΔT) by Tr) were highest in summer, peaking in the month of July with the mean values of 1.98 mm d−1, 4.91 MJ m−2 d−1 and 3.93 °C m−2 d−1, respectively. The highest cooling effect was shown during warmer and wet years. Daily Tr had a positive linear relationship with shortwave radiation (Rs) in May-June, air temperature (Ta) and volumetric soil water content (SWC30) in July-August and vapor pressure deficit (VPD) in September, respectively. Furthermore, the main regulatory variables of Tr within each season were found to be the spring Rs, summer Ta and SWC30, and autumn Rs and VPD, which explain 49, 82 and 74% of the variation, respectively. The influence of these variables (i.e., Ta, Rs and VPD) on Tr was modified by the effect of SWC30. Interannually, Ta, SWC30 and precipitation (PPT) were responsible for most of the observed variation in Tr, individually explaining 72, 81, and 66% of the variation. Furthermore, multiple regression model indicated that together Ta, SWC30 and PPT explained 89% of the variation in Tr. The diminished sensitivity of Tr to environmental variables and enhanced sensitivity to SWC30 during dry years point to species acclimatization to soil dryness. Our findings clearly indicate the temporal dynamics in Tr and its cooling effectiveness for F. concinna over longer timescales. It is suggested that F. concinna could be better suited in response to increasing temperature in subtropical urban areas, as the species may be capable to provide efficient cooling based on its high Tr rate and has the potential to mitigate the UHI effect.