Year of Award

2023

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

Philip Higuera

Commitee Members

Ashley Ballantyne, Cory Cleveland, Kendra McLauchlan, Carl Seielstad

Keywords

biogeochemistry, climate change, forest ecosystem, paleoeecology, wildfire

Abstract

Warmer and drier climate conditions over recent decades are contributing to widespread increases in fire activity across western North America. The combined impacts of changing climate and fire activity threaten to undermine the longstanding resilience of forest ecosystems to wildfires, potentially leading to ecological transformations. This context creates a pressing need to understand the direct effects of climate and wildfire on ecosystem processes, and how longer-term changes in climate, fire activity, and ecosystem processes interact to determine ecological trajectories.

This dissertation advances our understanding of the causes and ecosystem consequences of wildfire and changing fire regimes in northern Rocky Mountain forests over years to millennia. In Chapter 1, I utilized extensive field sampling in two recent wildfires in western Montana to document how fires alter microclimatic conditions in biologically meaningful ways for regenerating and surviving vegetation. Through a subsequent study that tracked conifer seedling demography over the first three years following these wildfires, Chapter 2 highlights how spatial heterogeneity in fire effects can facilitate tree regeneration following wildfires, provided suitable post-fire climate. Overall, forests are exhibiting early signs of resilience to these two fire events.

Chapters 3 and 4 utilized paleoecological methods to address questions of long-term ecosystem change, and relationships among climate, fire activity, and ecosystem processes. In Chapter 3 I used a high-resolution, multiproxy lake-sediment record from the northern Rockies to reconstruct fire activity and ecosystem change over the past 4800 years. Changes in vegetation and fire activity coincident with increased effective moisture c. 2000 years ago highlight the overarching impact of climate on shaping ecosystem processes. Past wildfires had diverse, detectable impacts on biogeochemical processes, and evidence of ecosystem resilience to wildfires was consistent across variations in climate, vegetation, and fire activity over the late Holocene. Chapter 4 addressed variability in fire activity across a broader landscape, much of it burned during the historic 1910 fire season. I developed a network of 12 lake-sediment records to reconstruct a landscape-scale fire history over the past 2500 years and quantify the historical range of variability. This perspective reveals that contemporary burning, including the 1910 fires, remains within the historical range of fire activity in subalpine forests in this region.

My findings imply that northern Rocky Mountain subalpine forests are currently experiencing fire activity and ecological dynamics broadly consistent with historical variability. However, each chapter also highlights the sensitivity of multiple forest ecosystem processes to climate conditions and climate variability. Given the rapid rate of ongoing climatic change, subalpine forests will become increasingly vulnerable to widespread shifts in composition and structure. Future research could focus on identifying thresholds to change and early warning signals.

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© Copyright 2023 Kyra Diane Clark-Wolf