Giant Sequoias and Climate

Climate strongly influences giant sequoias. When a sequoia seed lands on soil, and a seedling takes root, its success depends on having enough sunlight and moisture to survive and grow. Climate is a key factor in determining where sequoias grow on the landscape. Currently, their range is a narrow mid-elevation band in the Sierra Nevada – historically the elevation range where half of the precipitation falls as snow. Throughout their long lives, giant sequoias may experience a range of climate conditions, which affects their growth and their resilience to disease, insects, and fire. Climate also affects how often fires occur and how they burn – giant sequoias depend on fire for release of large numbers of seeds from their cones and to open up gaps in the forest where young sequoias can grow.

Within our human time scales, giant sequoias appear to withstand many challenges, including variations in climate, fires, air pollutants, and disease. However, research suggests that past climate-driven shifts caused sequoia declines, reducing the area where they could grow. They were rare during a warm period about 10,000 to 6,000 years ago. Their present level of abundance in the Sierra Nevada dates back only about 4500 years!

Today, giant sequoias face new challenges from warming temperatures and resulting hotter droughts.

Hotter Drought and Giant Sequoias

During California’s 2012–2016 hotter drought – the most extreme in the instrumental record and perhaps in the last several centuries – local scientists and park managers observed widespread giant sequoia foliage dieback, most pronounced during 2014. They also documented 33 sequoias that died standing in the years during and following the drought, associated with native bark beetle activity and fire-related damage around the bases of trees.

Apparent beetle kill in giant sequoias is a newly reported phenomenon, associated with hotter drought. While the Phloeosinus bark beetles have been noted in dead fallen branches of giant sequoias, they were not identified as an apparent cause of death until recently. Dead trees associated with beetles usually also had recent severe basal fire scarring. Most of these trees also grew in moist locations (meadow edges or moist draws). These trees may have been more adapted to moist conditions, making them more vulnerable when the severe drought reduced soil moisture availability.

Learning more about hotter droughts and their impacts on giant sequoias is critical to help park managers prepare for the future.

Understanding Change

Scientists are gathering information to better understand responses of giant sequoia to drought and to characterize landscape-scale spatial patterns of vulnerability to hotter drought. Their Leaf to Landscape project collects and evaluates information at three spatial scales –

1. Leaf-level sampling of individual tree branches – Scientists climb selected giant sequoias to collect foliage samples that are analyzed in a lab for leaf water content and other variables that illustrate or influence the tree’s response to drought.
2. Ground-level observations – Scientists conduct binocular surveys to estimate the proportion of dead foliage visible in large giant sequoias along trail corridors in eight giant sequoia groves.
3. Landscape-scale area that encompasses 38 sequoia groves – Pilots and scientists fly in a specially instrumented Global Airborne Observatory aircraft to map giant sequoia distribution, canopy water content, and live/dead status at the scale of individual tree crowns.

Brief Summary of Results

Giant sequoias showed leaf- and canopy-level responses that were effective in sustaining water availability within the tree for the vast majority of individual sequoias. Very few giant sequoias died during the drought compared with other mixed conifer tree species. But the level of sequoia drought response varied across the landscape. Local site characteristics related to water balance at least partially explained tree sensitivity to drought. For example, lower elevations, steeper slopes, and grove edges are site characteristics associated with lower water availability. The researchers found that low canopy water content is an indicator of recent foliage dieback (when stress levels are so high that adjustments made by the tree to conserve water are insufficient to maintain water availability in the tree). Future efforts will measure recovery from drought and strengthen capability to interpret maps of drought vulnerability.

Responding to Change

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 there are many sequoia groves over a large area, and prioritization will be needed, deciding where on the landscape limited efforts will be best applied. Mapping vulnerability of giant sequoia groves to severe drought and warming temperatures will help managers more strategically target treatments.