Comparing Specie-Specific Water Flux and Storage Dynamics Using in Situ measurements

Increasing evidence has shown diurnal variations in the hydraulic capacitance of stems due to changes in water availability. Tree trunks’ swell overnight and shrink during the transpiration period, because water from internal storage plays a direct role in maintenance of the daily transpiration stream. Trees rely on the internal storage of water since it affects their ability to sustain transpiration, photosynthesis, and growth during periods of drought and whenever water transport occurs within the biomass. These dynamics of water depletion, storage, and replenishment modify trees’ water potential and cell turgor thus affecting total stem circumference. Therefore, investigating the interactions between trees’ internal water storage, trunk shrink and swell dynamics, moisture availability and demand is an important step in understanding vegetation responses to changing environmental conditions.

For this study, we compared half-hourly data for direct (TDR probes) and indirect measurements (trunk circumferential fluctuations using micron-scale dendrometers) of trees’ water storage to define trees’ individual responses to changes in water availability. We pair these measurements with continuous time-series of meteorological data (e.g., soil water content, PAR, precipitation, temperature, and VPD), and individual-scale Granier-style sap flow sensors measurements as a proxy for transpiration. This experiment was conducted in two different species of trees with dissimilar hydraulic strategies: Pinus strobus (white pine) and Acer rubrum (red maple). Our results suggest that soil moisture plays an important role in maintaining the internal water reserves particularly for red maple. For white pine, changes in internal water dynamics were readily detected using dendrometers due to the pine’s softwood and their trunk-elastic capacity. By contrast, changes in water storage in red maple were more visible in the observations made using capacitance sensors than dendrometers given the relative inelasticity of the wood. These storage-water dynamics information can be used for detecting water stress in trees, and within process-based plant hydraulics models to capture the temporal dynamics of ecosystem-scale water and carbon fluxes. Incorporating differences in species-specific hydraulic controls over transpiration and wood water storage dynamics into models may improve the understanding and representation of the impacts of drought stress on ecosystem carbon and water fluxes.