Dynamic Root Foraging and Allocation in E3SM, Exploring Responses and Feedbacks at Four United States AmeriFlux Sites

Authors

Benjamin J. Ahlswede¹ (bswede@uic.edu), Beth Drewniak², Miquel Gonzales-Meler², Max Berkelhammer¹*

Institutions

¹University of Illinois–Chicago, IL; ²Argonne National Laboratory, Lemont, IL

Abstract

Future climate projections predict extreme weather events, such as hurricanes and drought, will increase in frequency and duration over the coming century. These climate changes will stress natural vegetation that are adapted to current climate conditions. Forests respond to stress by allocating resources to where they are most needed, by redistributing fine roots deeper into the soil during times of drought and altering allocation between above and belowground components. These stress responses are not present in many modern Earth System Models, reducing the ability to predict ecosystem responses to stress and how said responses will affect future climate. Here, researchers utilize E3SM in default mode, but also with the addition of two stress dynamics, a dynamic rooting depth module where plants can forage for water and nitrogen, and a dynamic allocation subroutine, allowing redistribution of resources between above and belowground components. Firstly, researchers examined root dynamics globally, and found that by allowing roots to forage, the timetable of recovery from stress events is altered differentially between dry and humid ecosystems. Researchers then use point scale simulations in four forest AmeriFlux sites with long observation records paired with tree ring data, Morgan Monroe State Forest (US-MMS), Missouri Ozarks (US-MOz), and Sylvania Wilderness (US-Syv), and Niwot Ridge (US-NR1). Researchers simulated all four sites in default mode, with dynamic rooting depth, dynamic allocation, and the combination thereof. While there are some small changes in response to rooting depth, E3SM was completely agnostic to the presence or absence of fine root biomass. As such the dynamic allocation subroutine reduced allocation to fine roots by a factor of 10 in some cases.

Currently, there is no function for fine root biomass in E3SM and closely related models, therefore any stress events that might alter allocation to roots is likely misrepresented in E3SM climate projections and may result in an overestimation of future productivity. Ideally, fine root biomass should have a function, such as nutrient and water absorption in such models. Fine roots are extremely difficult to study in situ; therefore, modeling root dynamics are required to develop hypotheses about how changes to root systems will impact ecosystems and the climate. Future work should focus on adding this functionality to E3SM based on observations of fine roots.