Hydrodynamic Model Illuminates the Role and Consequences of Tree Hydraulic Traits

Hydraulic performance of plants is governed at the tissue level by traits that define properties of conductive tissues, such as xylem, and control structures, such as stomata guard cells. At a higher level, the effects of these tissue-level traits integrate to emergent whole-tree traits that govern the apparent relationships between the tree hydrodynamics and soil and atmosphere environmental forcing. These emergent tree-level traits could be further upscaled across trees of similar structure to the canopy and forest plot scales. Provided a mechanistic description of plant hydrodynamics, these traits could be characterized using the parameters of the formulations describing the water flow through the integrated tree conductive system. Such traits include the characteristics of embolism resistance and xylem conductivity and capacitance, stomatal closure mechanisms in response to internal and external forcing, hydraulic architecture, and root properties. The diversity of such traits produces a wide range of response strategies to both short-term variation of soil moisture and VPD, and to long-term changes to climate and hydrological cycles which affect water availability. Currently, there is a scale mismatch in our ability to observe hydrodynamic traits and their consequences. Eddy-covariance measurements observe the total consequence of plant hydrodynamics, over a large-scale plot, sap flux and tree storage observations provide an observation of hydrodynamic consequences at the individual-tree scale; while direct observations of xylem and stomata traits provide information in the tissue level that does not directly integrate to the whole-tree level. We propose a set of virtual experiments, using FETCH2, a tree-level hydrodynamic model, to determine the role of different hydraulic trait combinations that govern trees’ vulnerability to limitations in soil water availability. We use a quantitative hydrodynamic modeling framework which allows studying the influence of each suits of plant hydraulic traits independently, and assesses how the different trait groups interact with each other to form viable hydraulic strategies in response to reduced soil moisture availability.

FETCH2 simulates the integrated plant-level transpiration and water capacitance, provided hydraulic traits and environmental forcing, the simulations are structured so that they integrate to the plot level and could be directly evaluated with both tree-level sap and storage measurements, and plot-level eddy-covariance measurements of transpiration. We define a multi-dimensional hydraulic virtual “trait space” by considering a broad continuum of hydraulic traits at each of the leaf, stem, and root levels. We test the consequences of different strategies under a range of environmental conditions based on observations in a research forest in Northern Michigan, USA. We evaluate the degree to which simulated trees suffer hydraulic failure due to cavitation, resulting in loss of xylem conductivity, or carbon starvation, and measure the resulting xylem safety margin for each combination in the trait space. We demonstrate that only some regions in the multidimensional trait-space lead to viable consequences, indicating coordination between traits into distinct whole-tree hydraulic strategies. We demonstrate how the relationship between plant behavior at the iso/anisohydric continuum and hydraulic safety margin, which has been previously hypothesized and observed by others, emerges from this trait coordination and their tree-level outcomes. Finally, our results suggest that hydrodynamic models represent an exciting new possibility to define and study plant traits and hydrodynamics.