The majority of fire intervals prior to the early 19th century were 50 years long in both warm-moist and cool-moist microclimates, predominantly in the 19th century. 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. In Chapter 2, I used multiple lines of evidence to reconstruct historical fire events. 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.

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. 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. 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. 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. 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. In fact, post-fire forest development appears resilient to considerable variation in the fire regime and climatic and biotic constraints on tree establishment.Ĭoastal 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. Limited regeneration in a short window did not necessarily lead to persistent shrublands. This study illustrates that post-fire establishment in Douglas-fir/western hemlock forests of the central western Cascades historically was a multi-decadal process. Douglas-fir growth (basal area increment) exhibits high plasticity in relation to different competitive interactions within uni-specific and multi-species cohorts and between cohorts of different age, suggesting wide variation in the structure and dynamics of early-seral ecosystems and an ability to tolerate moderate competition when young. Only 8% of cohorts completed establishment within 20 years and 12% had establishment lasting >80 years. The rate and duration of establishment were surprisingly consistent across variation in topography (elevation, slope position, and aspect), among cohorts initiated from the late 12th to the early 20th century, and regardless of the severity of the cohort-initiating fire or the timing of establishment by shade-tolerant species. On average, it took 43.5 years to reach establishment of 90% of the trees per cohort. The number of post-fire cohorts was estimated per stand, and establishment trajectories were evaluated by cohort. To improve understanding of how post-fire tree establishment and early cohort development have varied in space and over time and elucidate some of the factors contributing to that variation, we analyzed forest structure, tree ages, and Douglas-fir growth across the central western Cascades of Oregon where cohort ages span nearly eight centuries. Previous work in Douglas-fir/western hemlock forests of the Pacific Northwest suggests stands initiate either with abrupt establishment (00 years.

rapid forest development for ecological or commercial objectives.

Therefore, the period of post-fire tree establishment is a key stage in forest stand and ecosystem development that can be viewed in the context of competing management interests in diverse early-seral ecosystems vs. Early-seral ecosystems make important contributions to regional biodiversity by supporting high abundance and diversity of many plant and animal species that are otherwise rare or absent from closed-canopy forests.

Fire in the cascadea