Dynamic responses of ecosystem hydraulics and gas exchange to water supply and demand

Jeffrey D. Wood, University of Missouri, Columbia, MO, Lianhong Gu, Oak Ridge National Laboratory, Oak Ridge, TN and Lawren Sack, Department of Ecology & Evolutionary Biology, University of California Los Angeles, Los Angeles, CA

Background/Question/Methods
Plant-environment interactions determine the productivity and water-use dynamics of terrestrial ecosystems, but these responses, especially during droughts, have remained a “black box”. Increasing drought frequency and intensity threaten forests across the globe, thus it is urgent to identify the factors that control plant carbon (C) and water flux dynamics at the ecosystem scale. At leaf and whole-plant levels, the plant hydraulic system controls stomatal opening and thus gas exchange. Recent modeling studies have inferred hydraulic control of whole ecosystem gas exchange, but this has not been tested directly with measurements.
We synthesized eddy covariance observations of ecosystem fluxes and leaf water potential measurements to study the ecosystem scale hydraulic behavior of a temperate Quercus-Carya forest in mid-Missouri, USA. Specifically, we (i) test for the emergence of ecosystem scale hydraulic vulnerability and the coordination of gas exchange and hydraulic conductance, and (ii) examine gas exchange and water supply/demand dynamics during the extreme drought of 2012.
Results/Conclusions
The mid-day leaf-specific ecosystem hydraulic conductance (Keco; the quotient of transpiration and mean soil-to-leaf water potential difference), over two growing seasons ranged from 0.2–1.7 mmol m−2 leaf s−1 MPa−1. Keco responded dynamically to the environment, with soil and leaf water status the most important drivers, which was also the case for surface conductance (Gs) to water vapor. We found ecosystem scale hydraulic vulnerability, with both Keco and GS declining exponentially with canopy mean predawn leaf water potential. Furthermore, Keco was strongly coordinated with both GS and gross primary productivity (GPP).
The drought of 2012 was characterized by unusually hot and dry conditions, and a rapid period of intensification over 8−10 weeks. As leaf area index peaked in spring, ecosystem dry-down was underway, with community predawn leaf water potential (Ψpd) dropping to −0.5 MPa over about one month. This was followed by the most striking dry-down, which was biphasic and characterized by fast and slow declines in Ψpd. During the fast phase, Ψpd fell from −0.5 to −1.8 MPa over 14 days, with a further decline of Ψpd to −3.8 MPa over 65 days during the slow phase. Ecosystem evapotranspiration and GPP dynamics were highly coherent with Ψpd, showing similar patterns of downregulation that were also biphasic. Invoking an ecosystem scale pressure-volume analogy, we found an ecosystem wilting point of −2.0 MPa.
Taken together, these results demonstrate a hydraulic basis for seasonal dynamics of ecosystem gas exchange and drought responses.