Interaction between mountain pine beetle-caused tree mortality and fire behavior in subalpine whitebark pine forests, eastern Sierra Nevada, CA; Retrospective observations

Abstract

The 1351 ha Walker Fire burned in late August 2015 on the eastern Sierra Nevada, California escarpment from sagebrush steppe and pinyon pine woodlands (2150 m) through mid-elevation forests into subalpine whitebark pine (WBP; 3310 m). Fuels were particularly dry due not only to the season but as a result of a record multi-year drought (2012–2015). The fire burned into stands heavily impacted by a mountain pine beetle outbreak (MPB) during a prior drought (2007–2010). Using plot data, we report 100% mortality due to fire of understory vegetation and trees in pinyon pine, aspen, white fir, and lodgepole pine forest types. Fire-caused tree mortality was lower in Jeffrey pine forests (80%), although 100% of the understory was killed by the fire. For WBP plots in areas that had not been affected by MPB, 100% of understory and tree layers also burned. Where the fire burned through plots within the former MPB-outbreak area, 2 of 15 plots had 100% understory vegetation and trees burned by the fire, while the remaining 13 had varying levels of survivorship. In those plots, pines dead from prior beetle-kill as well as live trees remained unburned, as did understory vegetation. Whereas such “gray-stage” stands have been found to reduce fire severity in other regions and forest types, the observed reduction of surface fire was unexpected. In that fires in the subalpine WBP zone and bark beetle-fire interactions have not been studied in California, these preliminary observations suggest that subalpine forests are vulnerable to fire where continuous fuels exist below them. However, in xeric, Mediterranean landscapes, MPB outbreaks in WBP may reduce subsequent burn severity in unexpected ways. These findings may be important given current concerns for WBP’s rangewide vulnerability.