Portland State University. Earth, Environment, & Society Ph. D. Program
Term of Graduation
Date of Publication
Doctor of Philosophy (Ph.D.) in Earth, Environment, & Society
Earth, Environment, & Society
1 online resource (19 pages)
Shifting wildfire patterns and climate conditions, magnified by anthropogenic climate change, are threatening the resilience of conifer forests in North America and more specifically, the western US. If native conifer species are functionally maladapted to novel fire patterns and post-fire climate conditions, large-scale shifts in conifer forest structure, composition, and extent may occur as warming intensifies. Forest resilience in the context of fire and climate can be understood and quantified by the survival of trees through fire events and success of trees to regenerate post-fire and maintain population levels. In this dissertation, I use field observations and remote sensing to examine patterns of fire-induced tree mortality and post-fire tree regeneration as proxies of conifer forest resilience in the western US, across a range of environments and forest types, and particularly within the context of expansive high-severity, stand-replacing wildfires.
In Chapter 1, I evaluate the interactions between climate-environment conditions and the spatial, structural, and temporal characteristics of fire refugia as drivers of subalpine forest recovery in the cool and moist Cascade Range of Oregon and Washington. Seed dispersal pressure, captured at the landscape scale by remotely sensed and spatially explicit fine-grain tree cover post-fire, explained the majority of variance in tree establishment responses across fires. Further, the structure and composition of fire refugia interacted with climate-environment conditions to augment tree establishment responses, creating variable post-fire forest recovery trajectories within the interior of large patches of stand-replacing fire. Toward modelling and predicting tree establishment responses and forest state-transitions after large stand-replacing fire(s), my findings demonstrate the importance of accurately capturing and accounting for spatially explicit processes and structural seed source characteristics that affect seed dispersal patterns.
In Chapter 2, I quantify large-scale patterns of post-fire delayed conifer tree mortality across three ecoregions and two broad forest types in the western US using high-resolution satellite imagery, and I evaluate whether post-fire delayed conifer tree mortality is a ubiquitous process across broad geographies, and if so, I ask i) what drives it? and ii) can it meaningfully affect seed dispersal and thus forest regeneration processes? I found that between 1-5 years post-fire, delayed conifer tree mortality responses occurred at ecologically significant rates across fire perimeters, varied with scale (i.e., fire perimeter vs. ecoregion levels), and exhibited the potential to critically reduce long-term seed source availability to severely burned forest patches and patches of non-forest, across landscapes. 1-year post-fire burn severity at a 30m resolution was the strongest predictor of delayed tree mortality responses, indicating early patch-scale vegetation change may be a strong proxy for both tree-level fire injuries and subsequent competition with rapidly recovering understory vegetation. Burn severity also strongly interacted with 30-year average and post-fire climatic moisture deficits, illustrating the powerful effects of productivity gradients and compound disturbances (e.g., drought) on delayed tree mortality probability.
Finally, in Chapter 3, I use an aggregated database of post-fire conifer establishment responses, across over 1800 sites and four ecoregions in the western US, to challenge the generalized notion that conifer species' shade-tolerance dictates their regenerative capacity within exposed early seral post-fire environments. Across ecoregions, I found evidence of strong early seral establishment and dominance (ratio of total conifer establishment) by shade-tolerant conifers. Responses were primarily driven by coarse-scale climatic factors that affect seasonal (i.e., summer) moisture deficits to vegetation, as well as the expression of species' functional traits that contribute to the regeneration niche, or fire resilience. Empirical evidence from this study suggests that conifer species' post-fire recovery responses can be understood across diverse environments via the collective expression of their functional traits, and that individual traits (e.g., shade-tolerance) may be a poor predictor of expected responses when considered in isolation.
© 2021 Sebastian Upton Busby
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Busby, Sebastian Upton, "Post-Fire Tree Mortality and Regeneration Patterns as Proxies of Conifer Forest Resilience" (2021). Dissertations and Theses. Paper 5860.
Available for download on Thursday, December 01, 2022