The hydraulic capacitance of biomass: opportunities and challenges in observation and modeling

Abstract:

Vegetation couples above and below ground water cycling, and for many locations is the largest component of the terrestrial hydrologic cycle via transpiration. The dynamic controls exerted by vegetation over both transpiration and carbon uptake complicate measurement and process-based modeling of each. Differences in plant traits and hydraulic strategies at the individual, species, and population levels are deterministic of how vegetation mediates these biosphere-atmosphere exchanges. Hydraulic capacitance, a key property within a plant’s hydraulic strategy, serves to buffer the transpiration stream by preventing excessive xylem tension, however its dynamics are not well understood. We use 3 years of hydraulic capacitance data for 5 temperate forest tree species in conjunction with sap flux, eddy covariance, and micrometeorological data to analyze seasonal patterns of variation in landscape-scale water storage and use.

Our results demonstrate the ability of tree trunks to store water, and species-specific patterns of use and depletion of above ground water storage. These 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 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. These results emphasize the necessity of hydraulic capacitance within process-based plant hydraulics models in order to capture the temporal dynamics of ecosystem-scale water and carbon fluxes. The newest wave of plant hydrodynamics models and their incorporation into land-atmosphere exchange models is set to provide the required framework for this type of advanced land-surface and vegetation modeling technique.