EGU General Assembly 2020
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Diurnal and inter-day hysteresis of species-specific stomatal conductance from sap-flow measurements illustrates hydraulic-stress responses strategies of trees

Gil Bohrer, Theresia Yazbeck, Ana Maria Restrepo Acevedo, and Ashley M. Matheny
Gil Bohrer et al.
  • The Ohio State University, Civil, Environmental and Geodetic Engineering, Columbus, United States of America (

Modeling of plant hydraulics is at the forefront of development in vegetation and land-surface models.  Numerical tools that consider water flow within the conductive system of plants, and particularly trees, have been developed and used in studies of hydraulic strategy and consequences of hydraulic behavior for drought tolerance. Several established land-surface models such as ED2, CLM, and E3SM have recently developed “hydro” versions and are ready to extrapolate the consequences of including tree hydraulic behaviors into large scale and global simulations. At the core of any plant hydrodynamic model is the assumption that stomatal conductance is dependent on xylem water potential. Further, “plant hydro” models assume that the effect of soil moisture on stomatal conductance is not direct but cascades through depletion of xylem water content in dry soil conditions.

We use observations of sap flow, soil moisture, and evapotranspiration at a mixed forest in the University of Michigan Biological Station (UMBS) at the footprint of the US-UMd flux tower to characterize the onset and advancement of hydraulic stress and post-stress recovery. We define stress by observing tree-level decrease of stomatal conductance during sunny days as soil-moisture deficit progresses. We use the Penman-Monteith (PM) formulation to calculate stomatal conductance given observed atmospheric forcing: air temperature, humidity, net radiation, soil heat flux, and aerodynamic resistance. Such PM-based approach effectively decouples changes in evapotranspiration due to atmospheric forcing vs. changes due to decreased stomatal conductance. Multiple years of sap-flow measurements in tens of trees of multiple species allow us to identify the species-specific characteristics of the onset of stress, and the hysteretic dynamics of stomatal conductance. The daily hysteresis indicates the severity of stress. Longer-term inter-day hysteresis of the relationship between noon-time stomatal conductance and soil moisture, before and after rain have alleviated the moisture stress, indicates species-specific strategies of hydraulic-stress recovery. Recovery time is related to the degree of stress, and can vary between a nearly reversible state and 1 to 2 days of recovery, to a long recovery of several days. We find large differences between species in the sensitivity to stress and in the strength of coupling between stem water content and stomatal conductance. These are consistent with the hydraulic strategy of the trees along the an/isohydric continuum.    

Identifying the hydraulic characteristics of water stress and direct observations of the coupling between stem water storage, conductance, and transpiration provide key observations with which to tune hydrodynamic models and allow process-based functional-type parameterization of stomatal conductance that accounts for tree hydrodynamics and hydraulic stress recovery.   

How to cite: Bohrer, G., Yazbeck, T., Restrepo Acevedo, A. M., and Matheny, A. M.: Diurnal and inter-day hysteresis of species-specific stomatal conductance from sap-flow measurements illustrates hydraulic-stress responses strategies of trees, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5672,, 2020

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  • AC1: Comment on EGU2020-5672, Gil Bohrer, 05 May 2020

    A quick walk-through my presentation:

    We used EC flux, Meteorology, soil moisture, and sap-flow data from US-UMB in Michigan, USA. We have 4 years of sap-flux data, and it scales well with plot-level EC (slide 3, Matheny et al 2014 JGR),

    We try to determine the drivers of transpiration hysteresis

    We used Penmann-Montheith equation, plugged in all the met data, and solved for stomata conductance (slide 8). We only used observations in days without rain, binned all observation by time of day, and PAR. We split cases to dry vs wet soil and ignored intermediate levels. 

    Clear hysteresis with PAR is apparent in stomata conductance even in wet days (slide 9).

    In dry days, stomata conductance (Gs) was lower after 10 AM, and hysteresis was larger (slide 10)

    Our approach allows estimation of species-specific dynamics of stomata conductance (slide 10). There are strong species-specific differnces in the degree of water availability response of Gs. Oak shows nearly no reduction of Gs in dry conditions. Maple shows intermediate response, and pine radically limits Gs in dry conditions.  

    We are working on incorporating all this in the next generation ELM-Hydro model.