Giant sequoia responses to extreme drought

Each year, the iconic giant sequoias of California’s Sierra Nevada attract millions of visitors from around the globe. Travelers come to marvel at, and be inspired by, the world’s most massive trees, all the while infusing hundreds of millions of dollars into local rural economies. As regional temperatures have risen over the last several decades, the National Park Service and U.S. Forest Service managers charged with protecting the big trees have increasingly called for detailed sequoia vulnerability maps. Such maps would help target areas for treatments aimed at increasing sequoia resilience – treatments like prescribed fire or mechanical thinning to reduce non-sequoia tree density which reduce both fire hazard and local competition for sequoias. Understandably, managers want maps of where on the landscape they can get the most “bang” from their limited management dollars.

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In the middle of California’s most extreme drought on historical record, giant sequoias did something in September of 2014 that had never been reported: many of the big trees showed extensive foliage dieback (Figure 1), in magnitudes that varied across the landscape. As the foliage dieback was almost certainly a stress response to drought conditions, USGS scientists stationed in Sequoia and Kings Canyon national parks immediately recognized an opportunity to allow sequoias to reveal where they were most vulnerable to future drought.

This effort would require integrating foliage dieback maps, remote sensing, and physiological measurements. The first step – mapping foliage dieback across portions of as many sequoia groves as possible – was the most urgent. California’s first winter storms would undoubtedly knock most brittle, dead sequoia foliage to the ground, thus ending mapping efforts. Recognizing this urgency, scientists developed and tested a sampling protocol, trained a field crew, and saw the crew systematically recording foliage dieback, all in under two weeks. For both speed and safety, surveys were limited to a 150 m-wide corridor along maintained trails. Before the first big storm ended these surveys, crews hiked 64 km (40 miles) of trails in eight different sequoia groves, recording foliage dieback for 4,278 large sequoias over an area spanning 847 ha (2,093 acres).

Analyses revealed foliage dieback was highest: (1) at low elevations, probably due to higher temperatures, reduced snowpack, and earlier snowmelt; (2) in areas of low adult sequoia densities, which likely reflect intrinsically more stressful sites; and (3) on steep slopes, probably reflecting reduced water availability. However, these variables were only modestly predictive of foliage dieback, suggesting that other factors could also be important. Maps of foliage dieback patterns within groves were consistent with the possibility that subsurface hydrology (invisible to us) may play a role. For example, the central portion of the Giant Forest grove of sequoias showed extremely low foliage dieback, whereas the periphery of the grove showed high dieback (Figure 2). This pattern indicates that the grove’s center may receive more subsurface water than its periphery. To test this and other possibilities, ongoing work will integrate foliage dieback data with both remote sensing and direct physiological measurements from sequoia crowns taken during and following the drought.

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Data integration will proceed over the next few years as part of the broader “Leaf to Landscape” collaboration between USGS scientists, tree physiologists at the University of California, Berkeley, and remote sensors at the Carnegie Institute of Science. In addition to broadly improving our understanding of giant sequoia responses to extreme drought and warmer temperatures, this work should yield giant sequoia vulnerability maps, helping guide future efforts to increase sequoia resilience.

The paper, “Patterns and correlates of giant sequoia foliage dieback during California’s 2012-2016 hotter drought” was published in Forest Ecology and Management.

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