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 strategy, generally a species-level characteristic, governs responses to water availability and ultimately drought. It has been demonstrated that in ecosystems ranging from wet to arid and tropical to boreal that sympatric species employ opposing hydraulic strategies for the regulation of water uptake and loss. We demonstrate that these differences in water use result in diverging patterns of ecosystem-scale water flux during drought.
Using leaf-level measurements of water potential and tree-level measurements of sap flux and stem hydraulic capacitance, coupled with plot-level eddy covariance, meteorology, and soil water potential data, we demonstrate seasonal patterns of variation in landscape-scale water use in a mesic and an arid forest that are characteristic of the species dominating the transpiration flux at any given point in the growing season.
Results/Conclusions
Our results also 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 strategy and species composition as required constraints on process-based 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.