Anthropogenic climate change has shifted forest fire regimes in the Pacific Northwest U.S by increasing wildfire frequency and area burned and the shift is projected to continue during the 21st century if temperature and summertime aridity continually increase. Such changes threaten natural resources in these systems, including drinking water reservoirs, which could see reduced water quality during post-fire recovery. Productive forests with historically flammability-limited wildfire regimes are susceptible to large-scale high-severity events because of large fuel sinks; therefore, as flammability increases with climate change, the frequency of these events is also expected to increase. However, it is unclear how climate change and wildfire will alter long-term fuel availability in these forests; if a strong fuel limitation develops, it could potentially offset increases in fuel flammability. Herein, we apply RHESSys-WMFire, a process-based ecohydrological framework coupled with a stochastic fire-spread model and a post-fire effects model, to explore the long-term coevolution of climate, vegetation, and wildfire in a historically climate-limited forest in the western cascades as it responds to two future climate scenarios: (1) one that enforces extreme fire-weather, and (2) one that is less arid and more suitable for forest production. Both scenarios feature three 525-year climate sequences to capture the co-evolution of vegetation and fire behavior for three stable climate regimes: the present, near future (2040s), and distant future (2070s). Each sequence was constructed from 30 years of climate data from existing CMIP5 GCM using a randomized climate resampling technique. We found both climate storylines forced a fuel limitation that increased during the 21st-century; however, increases in fuel flammability were greater, and resulted in increases in wildfire size, frequency, and area burned in near and distant future relative to the present. The severity of fuel limitation also corresponded with shifts in the fire-size distribution and the fire recurrence interval of different elevations, wherein strong fuel limitation caused relatively smaller fires and lower frequency. We surmise that reduced fuel availability will scale with the severity of climate forcing; however, in forests where fuel flammability is presently low, it will begin to limit wildfire behavior until a certain threshold has been reached.

Climate change has altered wildfire regimes in the Western United States in the past few decades. Fire season is becoming longer and burned area in the Western Cascades is projected to increase 200-400% above contemporary levels by the end of the century. Such changes in fire regimes can have cascading consequences for human and natural systems, including degradation of downstream water quality. Understanding the potential consequences of an altered fire regime will be necessary for managing forested watersheds to protect highly valued resources, especially high-quality drinking water, with the threat of a wildfire occurrence. In this study, we apply the ecohydrologic model RHESSys, coupled with the fire spread model WMFire and a fire effects model, to investigate how climate change and forest management techniques, such as stand thinning, can affect wildfire regimes in the Cedar River Watershed in Western Washington, which provides drinking water for people in the greater Seattle area. We run multiple simulations with different forest management and future representative concentration pathway (RCP) scenarios to assess future changes in fire activity due to climate change and the efficacy of management practices for reducing fire severity in this watershed. Both forest management and climate change alter the fire regime in the Cedar River watershed. With climate change, this basin becomes progressively more fuel-limited, which creates fuel conditions that allow thinning to become an effective method for managing wildfire.

Climate exerts considerable control on wildfire regimes, and climate and wildfire are both major drivers of forest growth and succession in interior Northwest forests. Estimating potential response of these landscapes to anticipated changes in climate helps researchers and land managers understand and mitigate impacts of climate change on important ecological and economic resources. Spatially explicit, mechanistic computer simulation models are powerful tools that permit researchers to incorporate climate and disturbance events along with vegetation physiology and phenology to explore complex potential effects of climate change over wide spatial and temporal scales. In this thesis, I used the simulation model FireBGCv2 to characterize potential response of fire, vegetation, and landscape dynamics to a range of possible future climate and fire management scenarios. The simulation landscape (~43,000 hectares) is part of Deschutes National Forest, which is located at the interface of maritime and continental climates and is known for its beauty and ecological diversity. Simulation scenarios included all combinations of +0°C, +3°C, and +6°C of warming; +10%, ±0%, and -10% historical precipitation; and 10% and 90% fire suppression, and were run for 500 years. To characterize fire dynamics, I investigated how mean fire frequency, intensity, and fuel loadings changed over time in all scenarios, and how fire and tree mortality interacted over time. To explore vegetation and landscape dynamics, I described the distribution and spatial arrangement of vegetation types and forest successional stages on the landscape, and used a nonmetric multidimensional scaling (NMS) ordination to holistically evaluate overall similarity of composition, structure, and landscape pattern among all simulation scenarios over time. Changes in precipitation had little effect on fire characteristics or vegetation and landscape characteristics, indicating that simulated precipitation changes were not sufficient to significantly affect vegetation moisture stress or fire behavior on this landscape. Current heavy fuel loads controlled early fire dynamics, with high mean fire intensities occurring early in all simulations. Increases in fire frequency accompanied all temperature increases, leading to decreasing fuel loads and fire intensities over time in warming scenarios. With no increase in temperature or in fire frequency, high fire intensities and heavier fuel loads were sustained. Over time, more fire associated with warming or less fire suppression increased the percentage of the landscape occupied by non-forest and fire-sensitive early seral forest successional stages, which tended to increase the percentage of fire area burning at high severity (in terms of tree mortality). This fire-vegetation relationship may reflect a return to a more historical range of conditions on this landscape. Higher temperatures and fire frequency led to significant spatial migration of forest types across the landscape, with communities at the highest and lowest elevations particularly affected. Warming led to an upslope shift of warm mixed conifer and ponderosa pine (Pinus ponderosa) forests, severely contracting (under 3° of warming) or eliminating (under 6° of warming) area dominated by mountain hemlock (Tsuga mertensiana) and cool, wet conifer forest in the high western portion of the landscape. In lower elevations, warming and fire together contributed to significant expansion of open (

Summarizes the science of climate change and impacts on the United States, for the public and policymakers.

There has been a significant increase in fire activity in the western United States over the past two decades, attributed to climate change, but much of the data that support this attribution are from fires in frequent, low-severity fire regimes. Recent increases in fires with mixed- and high-severity fire regimes of the Pacific Northwest have highlighted the importance of collecting baseline data and understanding fire-climate interactions in forests with less frequent fire to inform research and guide management. My dissertation focuses on these objectives in three chapters. In the first chapter, I characterized historical fire frequency and severity over 400 years in a dry, mixed conifer forest in Stehekin, Lake Chelan National Recreation Area in Washington state, and used ANOVA and GLM to identify the bottom-up controls on fire in this mountainous terrain. I found that fire frequency was high before the fire suppression era (31-year mean fire-interval), increased significantly during the non-Indigenous settlement period, and was impacted by fire suppression (51-year mean fire interval following suppression). Both fire frequency and severity are controlled by a complex interaction among topography, site, and environmental variables, which could increase resilience to climate change. In the second chapter, I classified and mapped fuel characteristics (fuelbeds) and fire potentials across a low-frequency, high-severity fire regime (Mount Rainier National Park, (the Park)) using a combination of field data, LiDAR, and climate data. Using this examination at high-resolution, I identified higher fuel loadings and fire potentials on the west side of the Park that could eventually indicate greater impacts and changes there, although the effects of climate change are more certain and will come sooner on the east side. In the last chapter, I reviewed bottom-up controls (topography and fuels) on fire frequency across the continuum of moist, high-severity fire regimes to dry, low-severity fire regimes from the west side of the Olympic Mountains to the east side of the north and central Cascades. Using this examination, I identify and describe a corresponding “fuel management continuum” to inform wildfire and forest management strategies.

Both fire and climatic variability have monumental impacts on the dynamics of temperate ecosystems. These impacts can sometimes be extreme or devastating as seen in recent El Nino/La Nina cycles and in uncontrolled fire occurrences. This volume brings together research conducted in western North and South America, areas of a great deal of collaborative work on the influence of people and climate change on fire regimes. In order to give perspective to patterns of change over time, it emphasizes the integration of paleoecological studies with studies of modern ecosystems. Data from a range of spatial scales, from individual plants to communities and ecosystems to landscape and regional levels, are included. Contributions come from fire ecology, paleoecology, biogeography, paleoclimatology, landscape and ecosystem ecology, ecological modeling, forest management, plant community ecology and plant morphology. The book gives a synthetic overview of methods, data and simulation models for evaluating fire regime processes in forests, shrublands and woodlands and assembles case studies of fire, climate and land use histories. The unique approach of this book gives researchers the benefits of a north-south comparison as well as the integration of paleoecological histories, current ecosystem dynamics and modeling of future changes.