Land-surface and ecosystem models classify trees into functional types by phenology, leaf traits, and bioclimatic limits, while excluding their hydraulic properties. Frequently, trees are grouped within the same plant functional type, despite having opposing hydraulic strategies. Errors in the prediction of transpiration and carbon uptake by land-surface models have been linked to the coarse resolution of plant functional types. We pair a field and modeling study comparing tree species typically classified together within the same functional class to highlight their divergent responses to drought and disturbance, which result, in part, from contrasting hydraulic strategies.
We measured sap flux, stem water storage, stomatal conductance, photosynthesis, rooting depth, and bole growth in tree species in disturbed and undisturbed field sites in Michigan from 2010-2015. Within our research site, two of these species represent opposing extremes of the proposed whole-plant hydraulic safety-efficiency spectrum. Red oak employs an efficient but high-risk hydraulic strategy (i.e., anisohydric stomatal regulation, highly conductive xylem, deep roots) while red maple relies on an `ultra’ safe strategy (i.e., isohydric stomatal regulation, less conductive xylem, shallow roots). Species-specific differences significantly influenced temporal patterns of stomatal conductance and overall transpiration responses to both drought and disturbance. Use of emergent tree-level hydraulic traits has the potential to improve model predictions of ecosystem-level transpiration and growth, particularly during periods of drought and disturbance. The implementation of such functional properties could be accomplished through either the recasting the plant functional type classification system to include whole-plant hydraulic traits, or explicitly representing plant hydrodynamics within land-surface models. Databases of species-specific hydraulic traits, such as the TRY Global Plant Trait Database, provide biologically relevant constraints for the governing hydraulic properties and will facilitate the implementation of both methods.
Here, we propose a framework to incorporate whole-tree hydraulic strategies into land-surface models through the Finite-difference Ecosystem-scale Tree-Crown Hydrodynamics model version 2 (FETCH2). FETCH2 incorporates plant hydraulic traits at the root, stem, and leaf levels to mechanistically link stomatal conductance to dynamically resolved xylem water potentials. We use hydraulic traits from TRY and our field measurements to conduct a sensitivity analysis of FETCH2 parameters defining stomatal response, xylem conduit properties, and rooting depth. Through these properties, we are able to capture the effects of (an)isohydric stomatal regulation, xylem conductance and stem water storage representative of different xylem architectures, and rooting depth on simulated transpiration. Inter- and intra-daily dynamics of simulated transpiration vary for each tested parameter combination, emphasizing the necessity of including traits at all three levels (roots, stems, and leaves) in the framework of a whole-plant hydraulic strategy. Incorporation of these functional traits into FETCH2 allows us to replicate the disparate patterns of water acquisition and use of species with contrasting hydraulic strategies. Holistic plant hydraulic representation in models, through informed plant functional groupings or the incorporation of plant hydrodynamic models like FETCH2, will not only improve predictions of transpiration, growth, and mortality, but also simulations of the surface energy budget and the global carbon and water balances.