Exploring Environmental Factors That Drive Diel Variations in Tree Water Storage Using Wavelet Analysis

Ryan E. Harmon,Frederick D. Day-Lewis,Holly R. Barnard,Deqiang Mao,Kamini Singha

doi:10.3389/frwa.2021.682285

Abstract

Internal water storage within trees can be a critical reservoir that helps trees overcome both short- and long-duration environmental stresses. We monitored changes in internal tree water storage in a ponderosa pine on daily and seasonal scales using moisture probes, a dendrometer, and time-lapse electrical resistivity imaging (ERI). These data were used to investigate how patterns of in-tree water storage are affected by changes in sapflow rates, soil moisture, and meteorologic factors such as vapor pressure deficit. Measurements of xylem fluid electrical conductivity were constant in the early growing season while inverted sapwood electrical conductivity steadily increased, suggesting that increases in sapwood electrical conductivity did not result from an increase in xylem fluid electrical conductivity. Seasonal increases in stem electrical conductivity corresponded with seasonal increases in trunk diameter, suggesting that increased electrical conductivity may result from new growth. On the daily scale, changes in inverted sapwood electrical conductivity correspond to changes in sapwood moisture. Wavelet analyses indicated that lag times between inverted electrical conductivity and sapflow increased after storm events, suggesting that as soils wetted, reliance on internal water storage decreased, as did the time required to refill daily deficits in internal water storage. We found short time lags between sapflow and inverted electrical conductivity with dry conditions, when ponderosa pine are known to reduce stomatal conductance to avoid xylem cavitation. A decrease in diel amplitudes of inverted sapwood electrical conductivity during dry periods suggest that the ponderosa pine relied on internal water storage to supplement transpiration demands, but as drought conditions progressed, tree water storage contributions to transpiration decreased. Time-lapse ERI- and wavelet-analysis results highlight the important role internal tree water storage plays in supporting transpiration throughout a day and during periods of declining subsurface moisture.

Highlights

Water potential gradients between the subsurface and the atmosphere control transpiration rates; there are physiological mechanisms, such as the use of internal water storage, which trees can use to modify water potential gradients
The XWT and CWT deconstructed time series data into frequencies that ranged from 1.5 h to 32 days; we focus on the 1-day frequency to determine how tree water storage affects the diurnal pattern of water use
Diel fluctuations measured in 10-cm sensors in profiles one and three were strongly influenced by temperature, an evaporation/transpiration signal could not be parsed from these data

Summary

Introduction

Water potential gradients between the subsurface and the atmosphere control transpiration rates; there are physiological mechanisms, such as the use of internal water storage, which trees can use to modify water potential gradients. Transpiration estimates in many hydrologic models are often guided by empirical relations (e.g., Jarvis et al, 1976; Feddes et al, 2001) that link stomatal conductance directly and instantaneously to measurements of shallow subsurface moisture and VPD, ignoring the role of other variables such as tree water storage (Mirfenderesgi et al, 2016). This model simplification can misrepresent transpiration dynamics at sub-daily time scales because internal water storage can buffer variability in transpiration induced by decreasing subsurface water availability and/or increasing evaporative demand (Matheny et al, 2014; Mirfenderesgi et al, 2016)

Methods

Results

Conclusion