Graduate Thesis Or Dissertation
 

Historical forest succession and disturbance dynamics in Coastal Douglas-fir forests in the southern western Cascades of Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sb397g692

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  • Coastal Douglas-fir (Pseudotsuga menziesii var menziesii) forests are extensive west of the crest of the Cascade Mountains in Oregon and Washington and are renowned for their productivity, biodiversity, and the ecosystem services they provide. Increasing wildfire activity in recent years including the extensive 2020 Labor Day fires, and a warming climate have raised concerns about the resistance and resilience of Coastal Douglas-fir forests. At the same time federal land managers are now tasked with restoration of natural successional and disturbance processes including fire to maintain biodiversity and ecosystem services given natural and anthropogenic stressors. However, we lack information that precisely describes historical fire regimes and successional dynamics in Coastal Douglas-fir forests sufficient to guide reintroduction of wildfire, in the context of restoration and adaptation of forest conditions and successional dynamics that are resilient to climate change. This gap in information exists because rigorous dendrochronological research methods have been sparsely used in Coastal Douglas-fir forests due to the challenges of sampling intact fire-scarred trees and the expectation that historical fires were infrequent and often severe. This dissertation provides the first extensive annually resolved dendrochronological reconstruction of historical (1600-1910 CE) fires and forest development in Douglas-fir forests of the southern western Oregon Cascades. Reconstruction sites covered an ecological gradient where Douglas-fir is the dominant tree species from low elevation warm-dry Douglas-fir forests to high elevation cool-wet Pacific silver fir (Abies amabilis) forests. In Chapter 2, I used multiple lines of evidence to reconstruct historical fire events. A frequent mixed-severity fire regime historically influenced tree establishment and successional dynamics across the broad warm-dry to cool-moist climatic gradient that I sampled. The majority of fire intervals prior to the early 19th century were < 25 years, even in the cool-moist western hemlock (Tsuga heterophylla) and Pacific silver fir forest types. However, there were also some fire intervals that were > 50 years long in both warm-moist and cool-moist microclimates, predominantly in the 19th century. Fire extent and frequency varied at fine spatial and temporal scales with small isolated fires occurring at approximately the same frequency as extensive fires. Most (94%) of historical fires were reconstructed from crossdated fire scars. Although the majority of tree establishment cohorts (90%) were associated with fire, tree establishment cohorts provided evidence of only 26% of historical fires evidenced by fire scars because historical fires did not always provide an opportunity for tree establishment or tree cohorts were killed by subsequent fires. Chapter 2 demonstrates the importance of crossdated fire records for quantifying fire history in these forest types, providing more nuanced and precise understanding of tree establishment and the development of old-growth forest conditions than previously known in Douglas-fir forests with a mixed-severity fire regime.. In Chapter 3, I quantified annual fire-climate relationships, identified variability in fire frequency over time, and evaluated how fire occurrence and frequency are related to climate, biophysical setting, people, and forest succession. I found no evidence that historical fires occurred preferentially in drought conditions prior to European settlement in 1830. After 1830 fires were significantly associated with drought. The increased fidelity of fire to drought years occurred at approximately the same time that fire frequency declined at most sites in my study area, and these declines in fire frequency occurred 50 - 100 years before fire suppression efforts began in the early 20th century. These unexpected results suggest that Native America traditional burning and changes in the flammability of Douglas-fir forests that occur with succession may have historically been important drivers of fire frequency. For example, there was strong evidence of traditional burning at one of my reconstruction sites where extremely frequent historical fires (mean fire interval 2.9 years) regularly occurred in years with wet spring and summer conditions. Declines in fire frequency in warm-dry forests coincided with disease epidemics and the disruption of Native America lifeways brought by European settlement. Decreases in fire frequency occurred several decades earlier across cool-moist western hemlock and Pacific silver fir forests at two of my study sites. These decreases in fire at cool-moist forest sites coincided with a cool-wet climatic period in the early 19th century and the development of mature Douglas-fir forest structure that resists burning. Younger early successional Douglas-fir forests developing after high-severity fire in the 19th century continued to burn frequently until fire suppression in the early 20th century. Chapter 3 demonstrates that fires historically burned under a broader range of climatic conditions, and this means that fire occurrence and frequency were likely mediated by fine scale endogenous drivers including topography, fuel moisture and structure, and the timing and distribution of ignitions. In Chapter 4, I use forest structure and composition, tree establishment records, and fire records to identify forest types and fire-mediated development types and precisely illustrate how fire frequency and severity historically contributed to diversity in forest development, structure, and composition. I identify how forest types and fire-mediated development types are uniquely related to biophysical environment to determine if forest types are indicative of a distinct historical fire regime, and to refine our understanding of how fire influenced forest development in different parts of the landscape. Old-growth forest conditions (composition and structure; forest type) varied at broad scales with microclimate. In contrast, forest development history (development types) varied at finer scales with slope and heatload, and even-aged, two-aged, many-aged, and multi-aged development types occurred across xeric, warm-dry, and cool-moist microclimates. Many-aged stands with continuous tree establishment over several decades to centuries developed almost exclusively on gentle slopes. Even-aged, two-aged, and multi-aged forests with distinct tree establishment cohorts that establish after moderate- to high-severity fire were usually located on steep slopes. Overall, the influence of fire on forest successional dynamics was partitioned at fine scales by topography, and this created a mosaic of distinct forest ages and development histories across warm-dry to cool-moist forest types in Douglas-fir forests in the southern western Cascades. This dissertation provides a uniquely detailed and precise characterization of frequent mixed-severity fire regimes in Douglas-fir forests in the southern western Cascades. A mixture of frequent low-, moderate-, and high-severity fire created the mature and old-growth forests that land management aims to protect today. Removing fire from these Douglas-fir forests has altered successional dynamics, forest structure and composition, and may have intensified drought and wildfire effects at the same time as drought severity and frequency are increasing across forests of the western United States. This research can inform future dendroecological reconstruction in Douglas-fir forests, guide reintroduction of fire, contribute to conservation and land planning, and inform response to contemporary wildfires as we adapt to a warmer and drier climate in the Pacific Northwest.
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