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The role of disturbance in vegetation distribution, composition and structure at the landscape scale for two western US ecosystems


Disturbance plays a key role in determining the structure, composition and function of ecosystems. Understanding disturbance regimes and their impacts on ecosystems is critical to understanding and managing these systems. This research examines how disturbance structures ecosystems at the landscape scale and how different disturbance agents interact. It is focused on two western US ecosystems: scrub and mixed evergreen forests of coastal northern California, USA and conifer forests of the western slope of the Cascade Mountains, Oregon, USA.

Fire is one of the most important disturbances in western US ecosystems. Variations in the frequency, intensity and spatial scale of fire strongly influence patterns of plant community regeneration. However, because of the unpredictable nature of fire events, fire-vegetation dynamics are not well understood in some ecosystems. For example, the impacts of fire on landscape scale vegetation patterns in coastal northern California have previously not been documented. The first chapter documents landscape scale changes in vegetation communities at Point Reyes National Seashore following the 1995 Vision Fire. Following fire, I found substantial areas had transitioned from coastal scrub to ceanothus scrub (Ceanothus thyrsiflorus Eschsch.) or bishop pine (Pinus muricata D. Don) forest. Transitions from shrub to tree vegetation following fire have rarely been documented in this region. Logistic regression analysis was used to examine the factors influencing the post-fire distribution of bishop pine and ceanothus scrub. Proximity to pre-fire bishop pine stands and pre-fire vegetation type were the most important predictors of post-fire bishop pine regeneration. Pre-fire vegetation type, burn severity and topography were the most important predictors of post-fire ceanothus scrub distribution.

Fire also has the potential to interact with other disturbance agents. In the Douglas-fir (Pseudotsuga menziesii Mirb. Franco) and redwood (Sequoia sempervirens (D.Don) Endl.) forests of Point Reyes National Seashore, introduction of the non-native pathogen Phytophthora ramorum (S. Werres, A.W.A.M. de Cock), which causes the disease Sudden Oak Death (SOD), has led to landscape scale mortality of tanoaks (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H. Oh). As tanoaks die and fall to the forest floor, they not only change forest structure and composition, but also change fuel loads and potentially fire behavior. The second chapter documents increases in fuel loads over time in long term monitoring plots in Sudden Oak Death infested forests. Throughout the study, I observed a significant positive relationship between dead tanoak basal area and surface fuels. I used the fire behavior modeling program BehavePlus to compare potential fire behavior between diseased and healthy stands. Model outputs indicated the potential for longer flame lengths, higher rates of spread and more intense surface fire in diseased stands. Analysis of the relationship between dead tanoak basal area and understory composition indicated that non-native forb cover is increasing in response to increasing SOD-mortality.

The third chapter focuses on the role of fire at the landscape scale in the conifer forests of the western Cascades at Crater Lake National Park, Oregon, USA. The west side forests of Crater Lake National Park are unique in that they represent one of the few places in the Cascade Range where an elevational gradient from low-elevation mixed conifer to high-elevation mountain hemlock forests remains intact and has never been logged. This presents a unique opportunity to study fire ecology in a place where fire can still function at the landscape scale. I examined stand structure, demography and reconstructed fire history using tree cores and fire scar data across an approximately 7000 hectare study area. Our plots were located in mountain hemlock (Tsuga mertensiana (Bong.) Carr), red fir (Abies magnifica A. Murr.), lodgepole pine (Pinus contorta Loudon) and mixed conifer forest types. Stand demography data from high elevation mountain hemlock forests showed continuous regeneration since the early 1600's with no fire scars present which is characteristic of very infrequent and/or low severity fire. Red fir forests showed a combination of both continuous and episodic regeneration over the past several centuries providing evidence for a mixed severity fire regime. Lodgepole pine stands were even-aged with no fire scar evidence and likely established following high severity fire events. Mixed-conifer forests were uneven-aged with the majority of trees established between 1880 and 1920. The median point fire return intervals for red fir and mixed conifer forests was 37.5 years.

Taken collectively, these chapters illustrate the important role of disturbance, and specifically of fire, in shaping the two ecosystems studied here. This work also demonstrates the potential for other disturbance agents, in this case a non-native pathogen, to impact fire behavior and fire effects. Understanding the ecological role of disturbance is critical to land management and conservation, particularly in the context of climate change. As land managers move from concepts of "historic range of variability" to more sophisticated guiding principles, such as resilience, a strong mechanistic understanding of ecosystem function, including disturbance ecology, will be more critical than ever.

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