Fire mortality caused by the Dixie Fire on an old-growth red fir forest, southern Cascades California

Keywords: Red Fir, Wild Fire, Sierra Nevada, Fire Mortality, Swain Mountain
Abstract Type: Poster Abstract

Abstract

In this study I examine fire mortality and effects from the Dixie Fire in an old-growth red fir forest in Lassen National Forest, California. Red fir forests are widespread in the southern Cascades, where wildfire extent has increased in recent decades. Fire exclusion over the last century has altered red fir forest fire regimes, increasing fuels, and altering the effects of fire on forest structure. The 2021 Dixie fire in California burned nearly a million acres at varying severities including the Swain Mountain Experimental Forest. In Swain Mountain the fire burned through a 1-hectare permanent plot where all trees were tagged and have been measured annually since 1989. Plot data include tree diameter (DBH), number of trees dying each year and ingrowth from the sapling size-class. Using these data, I determined the rates of tree mortality in the plot before and after the fire and whether mortality caused by the fire was size dependent. Tree mortality increased significantly in 2021 after the Dixie fire in compared to earlier years. Moreover, tree diameter was an important factor influencing mortality. Smaller diameter trees were more killed more frequently than larger diameter trees.