Measuring plant hydraulic conductance in situ: Methods and implications for the continuous monitoring of water transport in four woody species

Kerri Mocko, Joseph M. Michaud and H. Jochen Schenk, Department of Biological Science, California State University Fullerton, Fullerton, CA, Miriam M. Catalán, Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, Kathy Steppe, Laboratory of Plant Ecology, Ghent University, Ghent, Belgium

Background/Question/Methods
One of the largest ecological processes on Earth, water flow through plant stems, is influenced by a myriad of environmental factors such as weather, drought, irrigation, and flooding, and plants’ responses to these conditions. While this process is well studied, it’s commonly assessed at snapshots in time, using measurements of hydraulic conductance on harvested stem segments in a lab, which obscures water transport and storage fluxes over the long-term. We developed a non-destructive method to continuously measure plant hydraulic conductance in situ, in intact stems of woody shrubs and trees, where we simultaneously measure sap flux density with heat ratio sap flow sensors and water potential gradients from the soil to upper stems, using stem psychrometers. Coupled with measurements of stem dendrometry and stem volumetric water content, we establish the timing and extent that water moves radially in and out of internal stem water storage pools. We calculate apparent hydraulic conductance as sap flux density divided by the water potential gradient driving the flux under midday conditions when there’s minimal radial flux, and incorporate hydraulic capacitance to estimate actual hydraulic conductance by accounting for radial storage fluxes. We demonstrate different techniques for determining hydraulic conductance in situ using three experiments.

Results/Conclusions
We used this method to assess water transport in three different experimental settings in southern California: one in the field on native chaparral shrubs during a seasonal dry-down, a second in the Fullerton Arboretum using mature Fuerte avocado trees, and a third in a greenhouse dry-down experiment using potted Eucalyptus grandis trees. We determined hydraulic conductance using soil and stem water potentials in chaparral shrubs and Eucalyptus trees, and in avocado, we used two stem psychrometers, one on basal and one on distal stems. We calculated hydraulic capacitance using stem dendrometers in Eucalyptus and stem volumetric water content in avocado. We found that across species we measured, strategies for water transport were diverse: at midday, hydraulic conductances varied more than six-fold, stem water potentials varied nearly two-fold, and hydraulic capacitances varied over ten-fold depending on drought status. Lastly, we constructed vulnerability curves to illustrate the diversity of responses. This method advances our ability to monitor plant hydraulic function through fluctuating environmental conditions and is useful for constructing plant vulnerability curves, the gold standard for comparing how diverse species transport water. Furthermore, this technique provides a valuable tool for a diversity of ecological applications, from irrigation scheduling to mechanistic tree modelling.