Improving models of stand and watershed carbon and water fluxes with more accurate representations of soil-plant-water dynamics in southern pine ecosystems

Improving models of stand and watershed

watershed

An area of land that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually flows to the lowest point in a basin.

Source: USGS Source

carbon and water fluxes

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

with more accurate representations of soil-plant-water dynamics in southern pine ecosystems

ecosystem

A natural community of plants, animals, and other living organisms and the physical environment in which they live and interact.

Source: A student's guide to Global Climate Change Source

            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, we currently lack several key components necessary for a predictive understanding

predictive understanding

Predictive understanding refers to the ability of a system, typically an artificial intelligence or machine learning model, to anticipate future events or outcomes based on historical data and patterns.

Source: Source

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, we will employ a Model-Data Experiment (MoDEx) design informed by our project team’s previous field experiments and numerical model development.

            To improve our mechanistic understanding of coupled carbon-water processes and to collect necessary data to parameterize and test models, we will conduct an intensive set of field measurements at existing AmeriFlux sites operated by the project team. While a wealth of past work has examined the role of plant-mediated effects on ecosystem water cycling, particularly in arid ecosystems, less is known about these mechanisms in the southeastern U.S., a humid subtropical region characterized by high productivity but experiencing increasing temperature and drought frequency and intensity. Climate change,

climate change

A change in the average conditions — such as temperature and rainfall — in a region over a long period of time.

Source: NASA Climate Kids Source

combined with large-scale structural and compositional changes in vegetation, limit our ability to use existing observational data to predict future responses. 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. These savannah-like systems consist of an evergreen pine overstory and grassy understory and are generally found on well-drained, sandy soils with highly variable water tables. While usually characterized as drought tolerant, carbon sequestration

carbon sequestration

The process of capturing and storing atmospheric carbon dioxide.

Source: USGS Source

rates are dependent on interactions between climate, soil type, hydrology,

hydrology

The science that deals with water, its properties, circulation, or distribution on Earth's surface, and in the atmosphere through evapotranspiration.

Source: NASA earth observatory Source

and plant composition, such that predictions based on simple, empirical responses to environmental drivers are not adequate. Our work will examine plant-level hydraulic coordination of groundwater

groundwater

Water that occurs below Earth's surface. When rain falls to the ground, a portion percolates (sinks) into the ground and collects in spaces in the underground rocks.

Source: A student's guide to Global Climate Change Source

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, we 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. We 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 Energy Exascale Earth System Model (E3SM)

Energy Exascale Earth System Model (E3SM)

The Energy Exascale Earth System Model (E3SM) project includes Earth System Models, simulations, and predictions to investigate energy-relevant science using code optimized for DOE's advanced supercomputers.

Source: NVCL Web Page Source

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. The advances in mechanistic understanding of ecohydrological processes and model development generated from this project will be applicable to a broader set of ecosystems and will help to direct future experimental field and modeling efforts.