Dead giant sequoia, believed to be drought-related mortality, Grant Grove (left); giant sequoia with upper 20 percent of crown browning, 2017 (center); and base of tree shown in center showing extensive basal fire damage from 2015 Rough Fire (right). NPS photos / Tony Caprio

October 11, 2020 - Giant Sequoias are known for their resistance to insects and disease and their fire-adapted life cycle. Prior to the recent severe drought, research about giant sequoia mortality suggested that large sequoias typically died by falling or having >90-95% of their crowns scorched by fire. Standing death unrelated to crown scorch was rarely observed by scientists and managers who had spent decades working in the Sierra Nevada.

During California’s 2012–2016 hotter drought, local USGS scientists and park managers observed widespread foliage dieback, most pronounced during 2014. They also documented 30 sequoias that died standing in Sequoia & Kings Canyon NP in the years during and following the drought, associated with native bark beetle activity and fire-related damage around the bases of trees.

Beetle kill in giant sequoias is a newly reported phenomenon, associated with hotter drought. Beetle-killed trees usually had recent severe basal fire scarring. Most of these trees also grew in moist locations.

Managers and scientists are gathering information to better understand responses of giant sequoia to drought and to characterize spatial patterns of vulnerability to hotter drought. The Sierra Nevada Network I&M Program is working with park staff and USGS scientists to help quantify sequoia mortality at a larger spatial scale. Forest managers may be able to enhance tree survival in the face of future hotter droughts. For example, reducing forest density by prescribed burning or mechanical thinning can reduce competition for water among the remaining trees. But prioritization will be needed, deciding where on the landscape such efforts will be best applied. Mapping vulnerability of giant sequoia groves to severe drought and warming temperatures will help managers more strategically target treatments.
Source: NPS