Date of Award

Winter 2024

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Serita D Frey

Second Advisor

Stuart Grandy

Third Advisor

Richard G Smith

Abstract

Forests, the largest active cycling reservoir of carbon (C) in the terrestrial biosphere, mediate the exchange of CO2 between the atmosphere and the biosphere in large enough quantities to affect Earth’s climate. Presently, forests are significant C sinks and mitigate one-third of all anthropogenic C emissions by storing more C in biomass and soils than they exchange with the atmosphere. Anthropogenic activities and climate change disrupt the balance between forest C uptake and loss. This raises concerns about the sensitivity of the forest C sink. Traditionally, forest C exchange between trees and soils under climate change has been studied separately, owing to the spatial and temporal scales needed to assess forest C cycling accurately. Moreover, the belowground components of trees have traditionally been overlooked, limiting our full understanding of tree C exchange. My dissertation focuses on addressing these uncertainties by evaluating the trade-off between whole tree growth and soil C loss in response to soil warming (Ch. 1), and the exchange of C between the belowground component of trees and soils under warming × nitrogen (N) addition (Ch. 2 and 3). I demonstrate that tree species' nutrient use strategies—specifically, the acquisitive nutrient uptake of maples (Acer spp.) and the conservative nutrient conservation of oaks (Quercus spp.)—play a key role in influencing long-term forest C exchange with the atmosphere under warming (Ch. 1). I find while tree growth can counterbalance warming-induced soil C loss, the magnitude of this diminishes over time owing to declining soil nutrients. In the long term, the forest C sink returns to its former strength due to the attenuation of soil respiration rate mediated by plant-soil interactions. Lastly, I demonstrate supplementing soils with additional N counterbalanced warming-induced soil C loss by increasing root C inputs, returning ecosystem C balance to control levels. The unique root traits of Acer and Quercus spp. play a central role in the formation and loss of soil C under heat × nitrogen addition (Ch. 2 and 3). Specifically, I show that greater inputs of root-derived C do not necessarily translate to soil C gains if microbial and autotrophic respiration rates are also increased. Together, my three chapters provide a comprehensive understanding of the capacity of forests to exchange C with the atmosphere under warming × nitrogen addition, reducing the uncertainty of the future forest C sink.

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