Sap Flow Velocity in Fraxinus pennsylvanica in Response to Water Stress and Microclimatic Variables

Abstract

In arid and semiarid regions with water shortage, forestry development is limited by water availability. Understanding how tree sap flow responds to water stress and microclimatic variables is essential for the management of trees and the understanding of the eco-physiological properties of trees in arid areas. In the city of Tianjin in northern China, we measured the sap flow of Fraxinus pennsylvanica, a widely distributed urban greening tree species in semiarid regions of China. We measured the sap flow in four F. pennsylvanica trees over 6 months (April–September 2021), using a thermal diffusion probe method, and simultaneously monitored microclimatic variables and soil moisture. Results indicated that high nighttime sap flow velocity might be produced under the water stress condition. In addition, the nighttime sap flow velocity under the water stress condition was more susceptible to the combined effects of meteorological factors at night. The daytime sap flow velocity exerted a highly significant positive effect on the nighttime sap flow velocity during the whole research period, and the model fit was higher in the early growing season than that in the late growing season (early growing season: R2 = 0.51, P < 0.01; late growing season: R2 = 0.36, P < 0.01). Vapor pressure deficit had a positive effect on daytime sap flow. However, net vapor pressure deficit restrained daytime sap flow velocity when the intercorrelation between the microclimatic variables was removed. Our study highlights that drought areas perhaps have higher nighttime sap flow and that more emphasis should be placed on nighttime sap flow and the response of nighttime sap flow to microclimatic variables. In addition, the influence of other microclimatic variables on vapor pressure deficit needs to be considered when analyzing the relationship between daytime sap flow and vapor pressure deficit. An increase in net VPD can suppress the daytime sap flow.

The dynamics of nocturnal sap flow components of a typical revegetation shrub species on the semiarid Loess Plateau, China

“…Therefore, during the water potential gradient between the soil and plants, En was used for Rn to increase the leaf water potential and for water loss via cortex transpiration and cuticle transpiration ( Chen et al., 2020 ). Before midnight, En was mainly used for Rn because of the difference between the root system and the soil of the tree after sunset; after midnight, En was used for Qn on semiaid and semihumid regions ( Si et al., 2015 ; Su et al., 2022 ). Due to the controlling effect of leaf stomatal conductance on transpiration, Qn was deduced from the control of leaf stomatal conductance on whole transpiration during the day ( Caird et al., 2007 ), while the stomata have different thresholds and sensitivities during the day and night ( Buckley, 2005 ; Ogle et al., 2012 ).…”

An Improved Sap Flow Prediction Model Based on CNN-GRU-BiLSTM and Factor Analysis of Historical Environmental Variables

“…Stone et al used the method of generalized additive modelling to predict the influence of individual climate parameters on sap flow, and the results show a 60% relative humidity (RH) critical threshold of predicted sap flow and the indirect effect that wind speeds have on sap flow under protected cropping systems [26]. Su et al used principal component analysis (PCA) to understand the response of sap flow to water stress and microclimatic variables, and the results showed that there was a positive correlation between vapor pressure deficit and daytime sap flow [27]. Liu et al clarified the characteristics and influencing factors of sap flow in Populus tomentosa Carr.…”