Hydraulic Redistribution in Forests: Spatial and Temporal Drivers of Variation and Consequences for Climate Feedbacks

Authors

Elin Jacobs¹* (ekarlsson@purdue.edu), Zoe Cardon², Jeffrey Dukes³, Yilin Fang⁴, Theresa Hudson¹, Indira Paudel³, Lisa Welp-Smith¹

Institutions

¹Purdue University, West Lafayette, IN; ²Marine Biology Laboratory, Woods Hole, MA; ³Carnegie Institute for Science, Palo Alto, CA; ⁴Earth System Science Division, Pacific Northwest National Laboratory, Richland, WA

Abstract

The Earth system models used for climate projections are still limited in their ability to accurately simulate observed soil moisture and its control on transpiration and photosynthesis. One reason for this may be that these models do not realistically represent a process known as hydraulic redistribution (HR), where roots transport water from wet to dry soils. To better inform these models, this project seeks to: (1) understand drivers of spatial and temporal variation in rates of HR across the landscape; (2) pilot the implementation and realistic parameterizations of these drivers in models; and (3) test the new parameterizations against observations to better understand the consequences of their inclusion for climate feedbacks. Researchers plan to: (1) quantify the prevalence and magnitude of HR across tree species, tree sizes, and drought durations in mixed temperate forest species with contrasting hydraulic strategies by measuring soil and plant hydrologic/hydraulic properties and fluxes; (2) use stable isotope techniques to trace the origins of water sources in the soil and in water fluxes within the potentially lifting trees; and (3) compare the field data to model simulations to help improve model realism. Most studies of HR to date have focused on vegetation in arid to semi-arid regions or coniferous species where water is limited either annually or seasonally. The study sites are located in Indiana, a humid region where droughts historically are episodic events.

Climate projections, however, suggest that warmer conditions will consistently increase water stress during the growing season, which may exacerbate droughts and increase the importance of HR in buffering periods of water limitation. This provokes the question: to what extent does HR alleviate water stress and contribute to maintaining photosynthesis rates during droughts? Researchers will use experiments to quantify the prevalence and magnitude of HR to test parameterizations in two models that couple with the E3SM Land Model (ELM): (1) FATES-HYDRO, a plant hydrodynamics model that builds on the FATES model by adding multiple plant hydraulic traits linked to vegetation dynamics and carbon/water fluxes; and (2) ELM with a new plant hydraulic stress (PHS) representation. A key difference between these models is that FATES-HYDRO, unlike ELM-PHS, considers plant water storage, which may affect the partitioning of water to plant storage vs. HR.