Empirical Measurements and Model Representation of Hydraulic Redistribution as a Control on Function of Semiarid Woody Ecosystems

Authors

William Pockman¹* (pockman@unm.edu), Marcy Litvak¹, Yiqi Luo²

Institutions

¹University of New Mexico, Albuquerque, NM; ²Cornell University, Ithaca, NY

Abstract

The strength of Earth System Model (ESM) projections of ecosystem responses to climate change hinges upon robust modeling of biomes that differ widely in climate sensitivity. Drylands, covering more than 40% of the terrestrial surface, strongly influence the trend and year-to-year variability in global sink strength. Yet, the seasonality and magnitude of biogeochemical processes in drylands are challenging to represent in ESMs, partly because capturing plant-available water dynamics and/or ecosystem functional responses to plant- available water is difficult. Plant-mediated hydraulic redistribution (HR; water movement via plant roots between soil compartments that differ in water potential) offers a major avenue for improving the representation of dryland soil moisture dynamics. This project 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, researchers are coupling point measurements of sap flow and soil moisture, with ecosystem-scale measurements of NEE, GPP, and Re, and the use of the data assimilation approach to develop the Terrestrial Ecosystem (TECO) model to address three objectives:

(1) identify the key mechanisms determining the presence and magnitude of HR in species/biomes across a range of semi-arid western woodlands/forests;

(2) 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 them, and;

(3) scale plant-level patterns in HR to ecosystem-level NEE, GPP and Re in response to precipitation anomalies.

Researchers use parallel measurements of HR in dominant species in the fetch of AmeriFlux towers in three key biomes (ponderosa pine forest, piñon-juniper woodland, and juniper savanna) that vary in elevation, climate, and deep soil moisture availability. Researchers supplement existing instrumentation to measure root sap flow and soil water potential revealing the timing/magnitude of HR in each dominant species (Objective 1). Researchers measure the timing of HR and the local and ecosystem conditions under which it is observed (Objective 2). Researchers use these data and the flux tower record (2009 to present) for 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, researchers compare tree-level measurements of HR with measurements of ecosystem function to correlate the HR patterns associated with variation in GPP, Re, and NEE using empirical data for comparison with the TECO-HR model (Objective 3).