FIRE IN WESTERN CONIFER FORESTS: GEOMORPHIC AND ECOLOGICAL PROCESSES AND CLIMATIC DRIVERS

Major postfire debris flow and flood events in mountain basins result from two primary effects on slope hydrology and stability: (1) Reduced infiltration in severe burns results from water-repellent burn residues and/or surface sealing by ash and mud, and smooth runoff paths allow rapid, pervasive overland flow. Devegetation also promotes dry ravel of noncohesive soils and channel loading. Herbaceous revegetation can dramatically reduce runoff within a few years, especially where rhizomatous plants were present before fire. Patchy burns generate much less runoff because discontinuous flow paths allow infiltration. (2) Decay of root systems within several years after fire reduces shear strength in slope sediments, and along with reduced evapotranspiration increases the potential for saturation-induced failures for up to a few decades. Downstream effects of debris-flow events depend on connectivity with higher-order channels, sediment size, and geomorphic setting. Consequences for aquatic ecosystems range from brief pulses of turbidity and fine-sediment infilling of channels, to nutrient transfer from slopes to floodplains, and long-term habitat structuring by boulders and woody debris.

Fire regimes vary dramatically with climate, over both space and time. Cool-moist forest ecosystems typically experience long intervals between large, high-severity fires, with some species well-adapted to regenerate after fire mortality (e.g. lodgepole pine). Sedimentation occurs in similarly low-frequency, high-magnitude events. In contrast, dry forest species like ponderosa pine are adapted to survive frequent, low-severity fires that produce minor sediment pulses. Over the Holocene, however, insolation changes and shorter millennial-scale climatic cycles caused large variations in fire regimes and geomorphic response. Severe droughts (e.g., in the Medieval Warm Period 900-1200 AD) promoted catastrophic fire and debris-flow activity in a variety of ecosystems. Some severely burned forests require prolonged recovery or are replaced by new communities as environmental conditions change. Fire-induced sedimentation is strongly nonstationary, with frequency-magnitude distributions dependent on changing climate and vegetation, a fundamental concern in ecosystem management for a warmer future.