Fire-driven changes to tree drought vulnerability and carbon storage in temperate broadleaf forests
Shifts in rainfall patterns due to climate change are expected to increase drought stress and mortality in forests. Concurrent changes to natural and anthropogenic fire regimes underscore the potential for large-scale impacts on forest ecosystems and carbon cycling. The sensitivity of forests to these changes will require an understanding of tree vulnerability to both drought and fire, yet their interaction is rarely considered. This dissertation explores the vegetation-mediated effects of fire disturbance on soil carbon storage and the importance of drought-fire interactions to tree growth. Through the use of observational, experimental and dendroecological methods, I explore the abiotic and biotic mechanisms by which fire influences the drought vulnerability of juvenile and adult oaks (Quercus spp.). Specifically, I explore linkages between fire-driven shifts in plant functional traits and soil organic carbon stocks. To evaluate drought-fire interactions on juvenile trees, I experimentally imposed drought on existing and planted seedlings to compare the importance of post-fire resprouting from environmental-mediated effects of fire on drought vulnerability. I then examined the impact of fire on climate-growth patterns of adult oaks by sampling increment cores from trees within a long-term fire manipulation experiment. Fire primarily influenced soil carbon stocks directly, likely through combustion, but fire was also shown to indirectly influence soil carbon through shifts in understory plant functional traits. I demonstrate that planted and small, existing seedlings in burned plots had greater drought-induced declines in assimilation and growth rates than individuals in unburned plots, likely due to the warmer, drier conditions promoted by burning. The susceptibility of adult tree growth to drought, however, was unaffected by fire history and instead, periodic fires reduced annual growth by 9.2% during wet periods favorable for growth, an effect attributed to an observed 26% decline in soil nitrogen availability with fire. Overall, this research highlights that fire-driven changes to forest structure and nitrogen (N) cycling have important consequences for tree recruitment, growth and forest carbon storage under climate change. Specifically, my work finds that climate and fire interact to produce distinctly different impacts on adult versus juvenile tree growth. These results suggest that managing forests with frequent fire does little to mitigate drought stress and may instead negatively affect C storage.
drought, density-dependent competition, fire disturbance, microclimate, nitrogen dynamics, ontogeny, Quercus spp., resprouting, soil carbon, temperate deciduous forest
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign