Self-reinforcing patterns of forest fire severity in the southern Cascades, USA

Forest fire severity in dry conifer forests in California has increased in recent decades resulting in more severe fire effects. There is considerable scientific debate on whether this increase is outside the historical range of variability. One of the challenges in making this assessment is that for most places, we don't know what historical patterns of fire severity were and how they compare to contemporary patterns. In this talk, fire severity patterns in 2008 derived from remote sensing are compared to 19th century fire severity patterns identified using dendo-ecology in the same watershed. Since no logging occurred in the watershed, differences in fire severity between the late 19th century and 2008 are related to fire history, vegetation structure, topography, changes in vegetation and fuels since fire suppression, and weather conditions during the fire. Fire severity in 2008 was greatest on upper and mid-slope positions and where the vegetation in the late 19th century was montane chaparral. Fire severity was lowest on lower slope positions. This is the same vegetation type/slope position-fire severity pattern present in the 19th century landscape and suggests that, at least in complex terrain, historic vegetation and topography strongly influence recurring patterns of fire severity. The recurring fire severity pattern in Cub Creek promotes persistence of old multi-layered forests in valley bottoms and lower slopes and young denser forest on upper slopes.