Multiple-Temporal Scale Variations in Nighttime Sap Flow Response to Environmental Factors in Ficus concinna over a Subtropical Megacity, Southern China

With ongoing climate change and rapid urbanization, the influence of extreme weather conditions on long-term nocturnal sap flow (Q(n)) dynamics in subtropical urban tree species is poorly understood despite the importance of Q(n) for the water budgets and development plantation. We continuously measured nighttime sap flow in Ficus concinna over multiple years (2014-2020) in a subtropical megacity, Shenzhen, to explore the environmental controls on Q(n) and dynamics in plant water consumption at different timescales. Nocturnally, Q(n) was shown to be positively driven by the air temperature (T-a), vapor pressure deficit (VPD), and canopy conductance (expressed as a ratio of transpiration to VPD), yet negatively regulated by relative humidity (RH). Seasonally, variations in Q(n) were determined by VPD in fast growth, T-a, T/VPD, and meteoric water input to soils in middle growth, and RH in the terminal growth stages of the trees. Annual mean Q(n) varied from 2.87 to 6.30 kg d(-1) with an interannual mean of 4.39 +/- 1.43 kg d(-1) (+/- standard deviation). Interannually, the key regulatory parameters of Q(n) were found to be T-a, T/VPD, and precipitation (P)-induced-soil moisture content (SMC), which individually explained 69, 63, 83, and 76% of the variation, respectively. The proportion of the nocturnal to the total 24-h sap flow (i.e., Q(n)/Q(24-h) x 100) ranged from 0.18 to 17.39%, with an interannual mean of 8.87%. It is suggested that high temperatures could increase transpirational demand and, hence, water losses during the night. Our findings can potentially assist in sustainable water management in subtropical areas and urban planning under increasing urban heat islands expected with future climate change.