Tree level hydrodynamic approach for resolving aboveground water storage and stomatal conductance and modeling the effects of tree hydraulic strategy

Abstract

The finite difference ecosystem‐scale tree crown hydrodynamics model version 2 (FETCH2) is a tree‐scale hydrodynamic model of transpiration. The FETCH2 model employs a finite difference numerical methodology and a simplified single‐beam conduit system to explicitly resolve xylem water potentials throughout the vertical extent of a tree. Empirical equations relate water potential within the stem to stomatal conductance of the leaves at each height throughout the crown. While highly simplified, this approach brings additional realism to the simulation of transpiration by linking stomatal responses to stem water potential rather than directly to soil moisture, as is currently the case in the majority of land surface models. FETCH2 accounts for plant hydraulic traits, such as the degree of anisohydric/isohydric response of stomata, maximal xylem conductivity, vertical distribution of leaf area, and maximal and minimal xylem water content. We used FETCH2 along with sap flow and eddy covariance data sets collected from a mixed plot of two genera (oak/pine) in Silas Little Experimental Forest, NJ, USA, to conduct an analysis of the intergeneric variation of hydraulic strategies and their effects on diurnal and seasonal transpiration dynamics. We define these strategies through the parameters that describe the genus level transpiration and xylem conductivity responses to changes in stem water potential. Our evaluation revealed that FETCH2 considerably improved the simulation of ecosystem transpiration and latent heat flux in comparison to more conventional models. A virtual experiment showed that the model was able to capture the effect of hydraulic strategies such as isohydric/anisohydric behavior on stomatal conductance under different soil‐water availability conditions.

Ecohydrological modelling with EcH2O‐iso to quantify forest and grassland effects on water partitioning and flux ages

“…Historically, most hydrological models conceptualize vegetation as a static element with prescribed constants that parameterize the physical processes of evapotranspiration, disregarding the strong coupling between evapotranspiration and the physiological processes that drive plant phenology and water use (Fatichi et al, ; Speich, Lischke, Scherstjanoi, & Zappa, ; Wegehenkel, ). Over the past 15 years, various ecohydrological models have explicitly included dynamic vegetation parameterization to overcome such limitations (e.g., RheSYSS [Tague & Band, ], EcH 2 O [Maneta & Silverman, ; Kuppel, Tetzlaff, Maneta, & Soulsby, ; Simeone et al, ], tRIBS‐VEGGIE [Ivanov, Bras, & Vivoni, ], Cathy [Niu et al, ], Tethys‐Chloris [Fatichi, Ivanov, & Caporali, ], and FLETCH2 [Mirfenderesgi et al, ]). However, the verification of these models is often focused on short‐term to midterm hydrologic (e.g., streamflows and soil moisture) and ecological dynamics (e.g., seasonal phenology), and rarely are these models are compared with long‐term direct metrics of vegetation dynamics (e.g., biomass production and transpiration) that affect the water balance.…”

Stomatal conductance increases with rising temperature

“…to save water when it was pointless to keep stomata open for the photosynthesis) but we observed the contrary response: stomata opened even more. Many models, 26 on the scale from leaf through plant and ecosystem 27,28 and even global circulation models 29 relationship holds over the wide range of environmental conditions. This assumption worked in our experiment at the range of temperatures close to temperature optimum of photosynthesis.…”

Plant Hydraulic Trait Covariation: A Global Meta-Analysis to Reduce Degrees of Freedom in Trait-Based Hydrologic Models

“…Plant hydrodynamics models (PHMs) allow for enhanced prediction of vegetation-water use dynamics, and thereby carbon uptake dynamics, of individual plants on the basis of observable hydraulic traits [18,[41][42][43]. Such PHMs are being rapidly adopted into dynamic vegetation and land-surface modeling (LSM) platforms in an effort to improve the simulation of forest function, in terms of carbon and water exchange and their combined influence on expectations of plant growth and mortality [44][45][46].…”

“…Process-based modeling of water transport through the soil-plant-atmosphere continuum has advanced significantly within the last few years, with the development of new PHMs and their ongoing incorporation into LSMs as replacements for the traditional empirical methods to predict transpiration by calculating stomatal conductance [42,44,45,48]. These models simulate the transport of water in the soil-plant-atmosphere continuum as flow through a porous media, using the Darcy or Richards equations for saturated or unsaturated flow, respectively, and restrict stomatal conductance on the basis of leaf and branch water potentials.…”

“…Though this approach has been effective for simulating how drought affects tropical forests at large scales [45], it is incapable of simulating the role of biomass water storage capacity and dynamic changes to capacitance; which have been shown to be equally if not more critical in other ecosystems [41,52]. A more sophisticated set of models tackles this problem by modeling dynamic changes in xylem capacitance as a function of xylem water potential by assuming a relationship between the amount of water stored in the plant and the water potential [42,53].…”