Empirical measurements and model representation of hydraulic redistribution as a control on function of semiarid woody ecosystems

Earth System Models

earth system model (ESM)

A model that simulates how chemistry, biology, and physical forces work together. These models are similar to, but much more comprehensive than, global climate models.

Source: U.S. Department of Energy Source

(ESMs) allow us to simulate how 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

worldwide absorb and release carbon dioxide

carbon dioxide (CO2)

An important heat-trapping gas, also known as a greenhouse gas, that comes from the extraction and burning of fossil fuels (such as coal, oil, and natural gas), from wildfires, and natural processes like volcanic eruptions.

Source: NASA Source

and how these processes respond to earth’s changing climate. The strength of these model projections depend on how well ESMs match the response of biomes (forests, grasslands, shrublands, etc) that each are different in their sensitivity to climate and their relative contributions to the global carbon cycle.

carbon cycle

The movement and exchange of carbon through living organisms, the ocean, the atmosphere, rocks and minerals, and other parts of the Earth.

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

Drylands cover more than 40% of the global terrestrial

terrestrial

The geographical and biological features that are associated with the Earth's solid, rocky surface, residing on or resembling the land or terrain found on our planet.

Source: NASA Source

surface and are a major driver of both the long-term trend and the year-to-year variation in total carbon uptake by terrestrial ecosystems. Yet, both the seasonality and magnitude of biogeochemical processes

biogeochemical processes

The complex interactions and transformations of chemical elements and compounds that occur throughout Earth's ecosystems, such as nutrient and element cycling between living organisms, the atmosphere, hydrosphere, lithosphere, and pedosphere (soil).

Source: Source

in drylands are challenging to represent in ESMs. Moreover, recent studies suggest that the inability of ESMs to simulate dryland ecosystem function is, in part, due to a failure to represent in the models the fluctuation of plant-available water dynamics and/or to simulate the response of ecosystem carbon exchange to plant-available water. 

Plant-mediated hydraulic redistribution (HR) is defined as the flow of water between soil compartments that differ in water content via plant roots connecting parts of the soil profile that differ in water content. Incorporating the role of HR offers a major opportunity for improving the representation of dryland soil moisture dynamics that is thought to limit the ability of ESMs to accurately describe carbon exchange in these ecosystems. The proposed research tests the idea that soil moisture dynamics and ecosystem function in dryland biomes cannot be well understood or modeled without a better representation of HR. To improve the representation of HR in ESMs, we will combine measurements of water flow through roots and the water content of different parts of the soil, with ecosystem-scale measurements of ecosystem carbon exchange, and the use of an approach called data assimilation to improve ability of the Terrestrial Ecosystem (TECO) model to address three objectives:

Identify the key factors that determine whether or not HR occurs and the amount of water moved by HR when it occurs in species/biomes across a range of semi-arid western woodlands and forests.

Quantify seasonal patterns of HR across dryland species and biomes to determine when HR has the biggest impact on soil moisture dynamics and the mechanisms driving these patterns.

Use plant-level patterns in HR to estimate ecosystem-level NEE, GPP and Re in response to precipitation anomalies (periods of drought and unusually high water availability).     To address these objectives, we will measure HR in dominant dryland species at existing sites instrumented to sensitive and continuous measurements of carbon exchange with the atmosphere in three key biomes (ponderosa pine forest, pinon-juniper woodland, and juniper savanna) that vary in elevation, climate, deep soil moisture availability and the presence of experimental manipulations. At each site, we will supplement existing instrumentation to add measurements of root sap flow, and soil water potential to measure the timing and magnitude of HR in each dominant species (Objective 1) and we will measure root sap flux

flux

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

Source: NRC Library Source

patterns on a broader group of dominant and subdominant woody species to characterize the broader extent of HR regionally. Across sites, we will establish the differences in timing of HR and the conditions under which it is observed (Objective 2). Using newly-collected data and data from the flux towers (2009-present), we will employ data assimilation to improve model structures and parameterization in TECO to provide important advances in the ability of ESMs to capture HR and predict dryland ecosystem function. Finally, we will compare the tree-level measurements of HR with measurements of ecosystem function to correlate the HR patterns associated with variation in carbon exchange using empirical data for comparison with the TECO-HR model.  TECO-HR will be used to test hypotheses about the role of HR in soil moisture dynamics under future 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

scenarios to identify tipping points when HR will either fail or be insufficient to sustain roots in drying surface soil.