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.

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.

The North Cascadia Adaptation Partnership (NCAP) is a science-management partnership consisting of the U.S. Department of Agriculture Forest Service Mount Baker-Snoqualmie and Okanogan-Wenatchee National Forests and Pacific Northwest Research Station; North Cascades National Park Complex; Mount Rainier National Park; and University of Washington Climate Impacts Group. These organizations worked with numerous stakeholders over 2 years to identify climate change issues relevant to resource management in the North Cascades and to find solutions that will facilitate the transition of the diverse ecosystems of this region into a warmer climate. The NCAP provided education, conducted a climate change vulnerability assessment, and developed adaptation options for federal agencies that manage 2.4 million hectares in north-central Washington. In the Pacific Northwest, the current warming trend is expected to continue, with average warming of 2.1 C by the 2040s and 3.8 C by the 2080s; precipitation may vary slightly, but the magnitude and direction are uncertain. This warming will have far-reaching effects on aquatic and terrestrial ecosystems. Hydrologic systems will be especially vulnerable as North Cascades watersheds become increasingly rain dominated, rather than snow dominated, resulting in more autumn/winter flooding, higher peak flows, and lower summer flows. This will greatly affect the extensive road network in the North Cascades (longer than 16 000 km), making it difficult to maintain access for recreational users and resource managers. It will also greatly reduce suitable fish habitat, especially as stream temperatures increase above critical thresholds. In forest ecosystems, higher temperatures will increase stress and lower the growth and productivity of lower elevation tree species on both the western and eastern sides of the Cascade crest, although growth of highelevation tree species is expected to increase. Distribution and abundance of plant species may change over the long term, and increased disturbance (wildfire, insects, and invasive species) will cause rapid changes in ecosystem structure and function across broad landscapes, especially on the east side. This in turn will alter habitat for a wide range of animal species by potentially reducing connectivity and latesuccessional forest structure. Coping with and adapting to the effects of an altered climate will become increasingly difficult after the mid-21st century, although adaptation strategies and tactics are available to ease the transition to a warmer climate. For roads and infrastructure, tactics for increasing resistance and resilience to higher peak flows include installing hardened stream crossings, stabilizing streambanks, designing culverts for projected peak flows, and upgrading bridges and increasing their height. For fisheries, tactics for increasing resilience of salmon to altered hydrology and higher stream temperature include restoring stream and floodplain complexity, reducing road density near streams, increasing forest cover to retain snow and decrease snow melt, and identifying and protecting cold-water refugia. For vegetation, tactics for increasing resilience to higher temperature and increased disturbance include accelerating development of late-successional forest conditions by reducing density and diversifying forest structure, managing for future range of variability in structure and species, including invasive species prevention strategies in all projects, and monitoring changes in tree distribution and establishment at tree line. For wildlife, tactics for increasing resilience to altered habitat include increasing diversity of age classes and restoring a patch mosaic, increasing fuel reduction treatments in dry forests, using conservation easements to maintain habitat connectivity, and removing exotic fish species to protect amphibian populations.

The North Cascadia Adaptation Partnership (NCAP) is a science-management partnership consisting of the U.S. Dept. of Agriculture (USDA) Forest Service Mount Baker-Snoqualmie and Okanogan-Wenatchee Nat. Forests and Pacific NW Research Station; North Cascades National Park Complex; Mt. Rainier Nat. Park; and U. of Washington Climate Impacts Group. These organizations worked to identify climate change issues relevant to resource management in the North Cascades and to find solutions that will facilitate the transition of the diverse ecosystems of this region into a warmer climate. In the Pacific NW, the warming trend is expected to continue and will have far-reaching effects on aquatic and terrestrial ecosystems. Hydrologic systems will be especially vulnerable as North Cascades watersheds become increasingly rain dominated. Coping with and adapting to the effects of an altered climate will become increasingly difficult, although adaptation strategies and tactics are available to ease the transition to a warmer climate as it affects roads and infrastructure, fisheries, vegetation and wildlife. Figures and tables. This is a print on demand report.

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

Throughout many of the world's mountain ranges snowpack accumulates during the winter and into the spring, providing a natural reservoir for water. As this reservoir melts, it fills streams and recharges groundwater for over 1 billion people globally. Despite its importance to water resources, our understanding of the storage capacity of mountain snowpack is incomplete. This partial knowledge limits our abilities to assess the impact that projected climate conditions will have on mountain snowpack and water resources. While understanding the effect of projected climate on mountain snowpack is a global question, it can be best understood at the basin scale. It is at this level that decision makers and water resource managers base their decisions and require a clarified understanding of basin's mountain snowpack. The McKenzie River Basin located in the central-western Cascades of Oregon exhibits characteristics typical of many mountain river systems globally and in the Pacific Northwestern United States. Here snowmelt provides critical water supply for hydropower, agriculture, ecosystems, recreation, and municipalities. While there is a surplus of water in winter, the summer months see flows reach a minimum and the same groups have to compete for a limited supply. Throughout the Pacific Northwestern United States, current analyses and those of projected future climate change impacts show rising temperatures, diminished snowpacks, and declining summertime streamflow. The impacts of climate change on water resources presents new challenges and requires fresh approaches to understanding problems that are only beginning to be recognized. Climate change also presents challenges to decision makers who need new kinds of climate and water information, and will need the scientific research community to help provide improved means of knowledge transfer. This dissertation quantified the basin-wide distribution of snowpack across multiple decades in present and in projected climate conditions, describing a 56% decrease in mountain snowpack with regional projected temperature increases. These results were used to develop a probabilistic understanding of snowpack in projected climates. This section described a significant shift in statistical relations of snowpack. One that would be statistically likely to accumulate every 3 out of 4 years would accumulate in 1 out of 20 years. Finally this research identifies methods to improved knowledge transfer from the research community to water resource professionals. Implementation of these recommendations would enable a more effective means of dissemination to stakeholders and policy makers. While this research focused only on the McKenzie River Basin, it has regional applications. Processes affecting snowpack in the McKenzie River Basin are similar to those in many other maritime, forested Pacific Northwest watersheds. The framework of this research could also be applied to regions outside of the Pacific Northwestern United States to gain a similar level of understanding of climate impacts on mountain snowpack.