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This article is part of the Fall 2021 issue of the Earth Science Matters Newsletter. 

Giant sequoias are among the world’s largest and most long-lived trees, an iconic species that only occurs naturally in a narrow elevational band in the Sierra Nevada mountains of California. Sequoias have long been a target for conservation and were one of the motivating factors for the creation of both Sequoia and General Grant (now Kings Canyon) national parks.

From 2012 to 2016, California suffered one of the most severe droughts on record, and while sequoias did not die in large numbers, they exhibited strong responses to the drought including substantial physiological changes, unprecedented foliage dieback, and—for a small set of trees— beetle-caused mortality. With forecasts indicating that severe droughts may become more commonplace in the future, park managers are considering various treatment options, such as prescribed burning, thinning, and even raking around individual sequoias to minimize the impacts of any fire. Given limited resources, we need to know the precise locations of the most vulnerable sequoias in order to best place treatments.

In a new paper, university, USGS, and U.S. Forest Service scientists worked with National Park Service managers to combine several high-quality remote sensing datasets (data on forest structure and condition collected from a plane using lasers and advanced cameras) with a ground-based dataset containing the precise locations of giant sequoias in order to develop maps of sequoia drought vulnerability for Sequoia and Kings Canyon national parks. Researchers used remote-sensing-derived estimates of canopy water content in the sequoias to detect the level of vulnerability of individual trees, with vulnerable trees defined as those with consistently low water content or those showing a trend of substantial water loss from 2015 to 2017. USGS Researchers found that 9% of the 7408 sequoias in their dataset showed signs of high drought vulnerability.

classified remotely sensed imagery showing water content of tree crown
Remote imagery reveals a pronounced spatial gradient in the whole-crown water content of individual giant sequoias during the drought. Warmer colors indicate lower water content. Credit: G. Asner, Carnegie Institute of Science.

After classifying trees in their dataset by their vulnerability, researchers developed a statistical model that predicted how likely a tree was to be highly vulnerable based on factors such as topography and climate. This model was then used to extrapolate sequoia vulnerability across entire groves. Trees classified as highly vulnerable tended to occur at lower elevations, in locations with higher water deficits, and, for a subset of trees, close to meadows. The latter finding may be the result of trees near meadows having shallower root systems because of sequoia’s inability to grow roots in saturated soils.

In a world where sequoias are facing novel threats from a warming climate and from wildfires that are burning with never-before-seen intensity, efforts to conserve sequoias are receiving heightened emphasis, and tools to enable that conservation are becoming increasingly necessary. Ongoing USGS forest research will help land and fire managers pinpoint the sequoias most at risk from drought and allow them to prioritize management treatments to mitigate that risk.

The paper, "Mapping the vulnerability of giant sequoias after extreme drought in California using remote sensing" was recently published in Ecological Applications.

<< Back to Fall 2021 Newsletter

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