Improving Models of Stand and Watershed Carbon and Water Fluxes with More Accurate Representations of Soil-Plant-Water Dynamics in Southern Pine Ecosystems

Authors

Tom O’Halloran^1,2* (tohallo@clemson.edu), Jean-Christophe Domec³, Jamie Duberstein¹, Cheng-Wei Huang⁴, A. Chris Oishi⁵, Brian Viner⁶, Tom Williams¹

Institutions

¹Baruch Institute of Coastal Ecology and Forest Science, Georgetown, SC; ²Department of Forestry and Environmental Conservation, Clemson University, Clemson, SC; ³Nicholas School of the Environment, Duke University, Durham, NC; ⁴Portland State University, Portland, OR; ⁵Coweeta Hydrologic Laboratory, Southern Research Station, U.S. Forest Service, Coweeta, NC; ⁶Savannah River National Laboratory, Aiken, SC

Abstract

Plant responses to water limitations involve a complex set of interactions with soil, the atmosphere, and other plants. While there is strong fundamental knowledge about the key processes through which plant hydraulics affect productivity, researchers currently lack several key components necessary for a predictive understanding of ecosystem response to future climate conditions. These components include: (1) mechanistic understanding of plant-mediated hydraulic processes in under-studied systems, and; (2) representations of biophysical factors affecting coupled water-carbon cycles in models. To address these challenges, researchers will employ a Model-Data Experiment (ModEx) design informed by the project team’s previous field experiments and numerical model development. To improve the mechanistic understanding of coupled carbon-water processes and to collect necessary data to parameterize and test models, researchers will conduct an intensive set of field measurements at existing AmeriFlux sites operated by the project team. This project will focus on longleaf pine ecosystems, once a dominant forest type in the region that is undergoing large-scale efforts to restore it through much of its native range. The work will examine plant-level hydraulic coordination of groundwater and soil water uptake, hydraulic redistribution (HR), plant water storage (PWS), transpiration, and leaf-level conductance, as well as competition among plants and the combined effects of hydrologic processes on ecosystem carbon dynamics. To address mechanisms missing in current land surface models, researchers significantly expand the functionality of an existing numerical model, developed by members of the project team, by adding components to resolve dynamic groundwater-root-hydraulic interactions and ecosystem respiration. The result will be a novel model that can resolve fully coupled interactions between groundwater, soil moisture, plants, and the atmosphere. Researchers use the extensive field measurements to parameterize and validate the expanded functionality of the new model and use it to test hypotheses that isolate the processes that compete for plant-stored water and quantify the resulting effects on ecosystem water and carbon fluxes. Finally, a series of simulations driven with E3SM future climate scenarios will predict the ability of HR and PWS to buffer longleaf pine productivity under projected extremes of the hydrologic cycle, including higher vapor pressure deficit and periods of drought. To date, in the first project year, researchers have installed a series of groundwater and soil moisture sensors provided by the AmeriFlux Management Project Year of Water project and have begun collecting new observations of tree sap flow and hydraulic conductance parameters.