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

Abstract

With ongoing climate change and rapid urbanization, the influence of extreme weather conditions on long-term nocturnal sap flow (Qn) dynamics in subtropical urban tree species is poorly understood despite the importance of Qn 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 Qn and dynamics in plant water consumption at different timescales. Nocturnally, Qn was shown to be positively driven by the air temperature (Ta), 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 Qn were determined by VPD in fast growth, Ta, T/VPD, and meteoric water input to soils in middle growth, and RH in the terminal growth stages of the trees. Annual mean Qn 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 Qn were found to be Ta, 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., Qn/Q24-h × 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.

Modelling of sap flux density of oak in a humid region in China

“…The nighttime sap flux has different controlling factors with the daytime sap flux (e.g., Hayat et al, 2021Hayat et al, , 2022Xu & Yu, 2020) (Bo et al, 2017;Zeppel et al, 2004;Zha et al, 2017;Zhang, Xu, et al, 2019). Our results further showed that the hysteresis of ASFD to Rsi is relatively larger than the hysteresis of ASFD to VPD (Figure 11), which is consistent with previous studies under wet soil conditions (e.g., Horna et al, 2011;Phillips et al, 1999).…”

“…The nighttime sap flux has different controlling factors with the daytime sap flux (e.g., Hayat et al, 2021, 2022; Xu & Yu, 2020), thus, the modelling effort in our work by separately calculating daytime and nighttime sap flux shows potential to improve the overall sap flux modelling. Our measurements showed that total nighttime sap flux correlates positively with total daytime sap flux, confirming our previous hypothesis for the model development.…”

“…However, during the nighttime, the response of sap flux to meteorological variables established with reference to daytime measurements is not valid, because stomata usually close during the nighttime and the corresponding sap flux is ultimately used to refill the capacitance of trunks and leaves (Siddiq & Cao, 2018; Wang et al, 2011) rather than to transpirate. Previous studies suggested the major controlling factors of nighttime sap flux are the total daytime sap flux (Snyder et al, 2003; Wang et al, 2011) and meteorological variables of VPD (Wu et al, 2020; Xu & Yu, 2020), Ta (Hayat et al, 2022; Wang et al, 2011), and wind speed (Chen et al, 2020; Guo et al, 2021). Though these explorations help understand the characteristics of the nighttime sap flux, a practical and efficient model remains lacking for the nighttime sap flux, motivating the model development effort of sap flux in this study.…”

“…Previous studies suggested the major controlling factors of nighttime sap flux are the total daytime sap flux (Snyder et al, 2003;Wang et al, 2011) and meteorological variables of VPD (Wu et al, 2020;Xu & Yu, 2020), Ta (Hayat et al, 2022;Wang et al, 2011), and wind speed (Chen et al, 2020;Guo et al, 2021). Though these explorations help understand the characteristics of the nighttime sap flux, a practical and efficient model remains lacking for the nighttime sap flux, motivating the model development effort of sap flux in this study.…”

Response of Typical Tree Species Sap Flow to Environmental Factors in the Hilly Areas of Haihe River Basin, China

“…The physiological structure of plants (such as leaf area index, sapwood, crown width, and phenology) determines their potential for transpiration [15][16][17][18]. Environmental factors that impact V include meteorological factors, such as temperature (Ta), total solar irradiance (TSI), relative humidity (RH), vapour pressure deficit (VPD), wind speed (WS), and precipitation (P), which determine the instantaneous changes in transpiration and are the main driving force of canopy transpiration [19][20][21][22][23][24], and soil factors, such as soil moisture content and soil temperature, which determine the peak and total transpiration of plants [25][26][27]. In particular, when soil moisture is limited, the hydraulic resistance of the soil and roots increases, leading to delayed water transport and stomatal closure [28,29].…”