The Role of Biomass Hydraulic Capacitance in Forest Transpiration

Water transport and storage through the soil-plant-atmosphere continuum is critical to the terrestrial water cycle, and has become a major research focus area. Biomass capacitance plays an integral role in the avoidance of hydraulic impairment to transpiration at both short and long time scales. New high temporal resolution measurements of dynamic changes in the hydraulic capacitance of large trees are facilitating new insights into vegetation water use strategies. Diurnal withdrawal from water storage in leaves, branches, stems, and roots significantly impacts sap flow, stomatal conductance, and ultimately transpiration. The ability to store and use capacitance varies based on soil- and root-water availability and uptake depth, tree size, wood vessel anatomy and density, and stomatal response strategy (i.e. isohydricity). Advances in measurement and modeling techniques have increased the viability of stem water storage as an assessment tool for forest function and particularly for drought response and recovery.

We present results from a three-year long study of stem capacitance dynamics in five species in a mixed deciduous northern forest in lower Michigan. Stem capacitance was observed as volumetric water content using frequency domain reflectometry-style sensors. The site receives ~800mm of rainfall annually, but water potential in the well-drained sandy soil nears the permanent wilting point several times during a hydrologically regular year. We demonstrate radical differences in stem water storage use, or storage withdrawal for transpiration, between drought tolerant and intolerant species. Red maple, a drought intolerant, isohydric species, showed a strong dependence on stem capacitance for transpiration during both wet and dry periods. While red oak, a more drought hearty, anisohydric species, was much less reliant on water storage during all conditions. During well-watered conditions, withdrawal from storage by red maple was ~10 kg day^-1, yet storage withdrawal from similarly sized red oaks was ~1 kg day^-1. Red oaks only drew strongly upon stem water storage during periods of extremely dry soils (< 4% volumetric water content).

In addition to species-specific biotic controls, stem capacitance exhibits dependence on abiotic factors of vapor pressure deficit and soil water content. Metrics of vegetation hydration status derived from dynamic changes in biomass water content can provide a means to explore forest health and specifically the response and recovery periods during and after droughts. For example, declines in maximum diurnal stem capacitance between consecutive days can indicate when a plant is unable to completely replenish depleted capacitance due to low soil water potentials, and can be used to mark the onset of hydraulic stress. Capacitance dynamics can likewise be used after drought to directly quantify the recovery period for hydraulic function as the time it takes for stem water content to return to observed pre-drought volumes. Analysis of stem-water storage withdrawal and depletion behaviors exhibits a clear threshold response to declining soil water availability.

The newest generation of advanced vegetation hydrodynamics models mechanistically simulates water flow through the xylem system of trees for the purpose of more accurate predictions of transpiration. Many of these new models can resolve the dynamics of stem water storage. This skill enables hydrodynamic models to resolve the fast dynamics transpiration at a level currently unattainable through the semi-empirical formulations within land-surface models. Direct measurements of stem storage dynamics and tree and plot scales provides a critical source of data to evaluate hydrodynamic models. The availability of these new modeling and measurement technologies will improve our understanding of species’ water status regulation in response to drying soil and drought.