Year of Award

2026

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Systems Ecology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Cory C. Cleveland

Commitee Members

Benjamin Colman, Matthew Church, Alan Townsend, Monica Turner

Keywords

disturbance ecology, free-living, nutrient cycling, soil

Abstract

In recent decades, northern conifer forests have experienced larger, more frequent and more intense wildfires. Severe, stand-replacing wildfire substantially reduces forest nitrogen (N) stocks, a critical nutrient that often limits net primary productivity on land. Continued increases in wildfire activity may exacerbate forest N limitation in the future. Yet, while research shows N stocks in northern conifer forests recover following severe wildfire, the processes that contribute to rapid increases in N during post-fire regeneration are unclear. Atmospheric deposition of N and known sources of biological N fixation cannot explain the observed post-fire N accumulations. This paradox raises the question: How do northern conifer forests recover N following severe wildfire? While our understanding of atmospheric N deposition inputs and symbiotic N fixation is well developed, we lack a detailed understanding of other cryptic mechanisms that allow forests to accumulate and store N.

This dissertation explores various sources and sinks of N in subalpine conifer forests in the Greater Yellowstone Ecosystem (GYE). Using a series of observational and experimental studies, I found (1) high rates of free-living N fixation in regenerating forests are limited by asynchrony in biophysical conditions (temperature and moisture), yet N fixation still has the potential to add significant quantities of N; (2) nutrient controls on free-living N fixers play a subtle, secondary role in limiting N fixation compared to biophysical controls; (3) surface soil is the dominant zone of plant N-acquisition, despite subsurface soil containing up to half of total soil N; (4) vertical patterns of soil N cycling are strongly controlled by organic matter distribution, yet hot spots of subsurface N fixation may amplify total soil N fixation to rates that rival atmospheric N deposition. Overall, the results of my research indicate that resolving uncertainty in northern conifer forest N budgets most likely hinges on a more comprehensive and precise understanding of known N sources, sinks and losses. However, my research shows that cryptic N sources provide an important flux of N that may provide substantial biogeochemical resilience to these critical ecosystems.

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© Copyright 2026 Robert Edward Heumann