January 04, 2023

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Ecosystem Wilting Point: A Threshold in Forest Response to Drought

Forests have great difficulty breathing when drought pushes them past drought thresholds.

MOFLUX Tower in the forest.

Observations of the breathing of the whole forest at Missouri Ozark AmeriFlux site provided new insights into ecosystem drought responses.

[Courtesy University of Missouri.]

The Science

Like animals, forests breathe; unlike animals, which breathe in oxygen and breathe out carbon dioxide, forests take in carbon dioxide and release water vapor and oxygen through tiny openings on leaf surfaces called stomata. Forests actively regulate the opening and closing of stomata in response to environmental variations. A team of researchers found that this regulation can only be done up to a threshold—the ecosystem wilting point. When drought is so severe that this threshold is passed, forests lose the ability to control their breathing, which will lead to forest decline if sustained.

The Impact

The finding and determination of the ecosystem wilting point provide new insights into how vegetation balances water loss from leaves with water acquisition by roots. Results show linkages between traits of the root system and canopy of leaves. When water supply to leaves no longer matches the demand from the air, the leaves dehydrate. When dehydration is severe enough, the ecosystem wilting response is triggered, which restricts forests’ breathing. As an ecosystem trait, the ecosystem wilting point can be used to test climate models’ ability to simulate drought responses.

Summary

The ecosystem wilting point is a property that integrates the drought response of an ecosystem’s plant community across the soil–plant–atmosphere continuum. The ecosystem wilting point defines a threshold below which the capacity of vegetation to extract soil water and the ability of leaves to maintain stomatal function are strongly diminished. A team of researchers combined eddy covariance and leaf water potential measurements to derive the ecosystem wilting point of an oak-hickory forest using an analogy to the pressure-volume technique that is usually used to study leaves or roots. During severe drought, the forest crossed the ecosystem wilting point, became insensitive to changes in weather, and was a net source of carbon dioxide for nearly all of July and August. After soaking rains, the forest showed rapid recovery responses, but a legacy of drought damage limited the recovery of canopy photosynthesis. Long-term records of plant water status suggest that this forest is commonly only 2–4 weeks of intense drought away from reaching the ecosystem wilting point and thus highly reliant on frequent rainfall to replenish the soil water supply.

Principal Investigator

Lianhong Gu
Oak Ridge National Laboratory
[email protected]

Co-Principal Investigator

Jeffrey Wood
Oak Ridge National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This material is based upon work supported by the Biological and Environmental Research (BER) Program within the U.S. Department of Energy’s (DOE) Office of Science through Oak Ridge National Laboratory’s Terrestrial Ecosystem Science (TES) Science Focus Area (SFA). ORNL is managed by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-00OR22725. Support was also provided by the National Science Foundation under grant #2017949.

References

Wood, J. D., et al. "The Ecosystem Wilting Point Defines Drought Response and Recovery of a Quercus-Carya Forest." Global Change Biology 29 (7), 2015–29  (2023). https://doi.org/10.1111/gcb.16582.