The Klamath-Siskiyou region of northwestern California and southwestern Oregon supports globally outstanding temperate biodiversity. Fire has been important in the evolutionary history that shaped this diversity, but recent human influences have altered the fire environment. We tested for modern human impacts on the fire regime by analyzing temporal patterns in fire extent and spatial patterns of fire severity in relation to vegetation structure, past fire occurrence, roads, and timber management in a 98,814-ha area burned in 1987. Fire severity was mapped by the U.S. Department of Agriculture Forest Service as low, moderate, and high based on levels of canopy scorch and consumption. We found (1) a trend of increasing fire size in recent decades; (2) that overall fire-severity proportions were 59% low, 29% moderate, and 12% high, which is comparable to both contemporary and historic fires in the region; (3) that multiaged, closed forests, the predominant vegetation, burned with much lower severity than did open forest and shrubby nonforest vegetation; (4) that considerably less high-severity fire occurred where fire had previously be absent since 1920 in closed forests compared to where the forests had burned since 1920 (7% vs. 16%); (5) that nonforest vegetation burned with greater severity where there was a history of fire since 1920 and in roaded areas; and (6) that tree plantations experienced twice as much severe fire as multi-aged forests. We concluded that fuel buildup in the absence of fire did not cause increased fire severity as hypothesized. Instead, fuel that is receptive to combustion may decrease in the long absence of fire in the closed forests of our study area, which will favor the fire regime that has maintained these forests. However, plantations are now found in one-third of the roaded landscape. Together with warming climate, this may increase the size and severity of future fires, favoring further establishment of structurally and biologically simple plantations. Patrones de Severidad de Fuego y Condiciones del Bosque en las Montañas Klamath Occidentales, California Resumen: La region Klamath-Siskiyou (Noroeste de California y Suroeste de Oregon) sostiene una biodiversidad templada globalmente sobresaliente. El fuego ha sido importante en la historia evolutiva que moldeó a esta diversidad, sin embargo, influencias humanas recientes han alterado el ambiente del fuego. Probamos los impactos humanos modernos sobre el régimen de fuego analizando los patrones temporales de la extensión del fuego y los patrones de severidad del fuego en relación con la estructura de la vegetación, incidencia de fuego en el pasado, caminos y manejo de madera en unárea de 98,814 ha quemada en 1987. La severidad del fuego fue clasificada en el mapa por el Servicio Forestal de EE. UU. como baja, moderada o alta tomando en cuenta el grado de chamuscado y consumo del dosel. Encontramos (1) que hubo una tendencia hacia un aumento del tamaño del fuego en décadas recientes; (2) que las proporciones totales de severidad de ...

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