Giant sequoias face new threats: severe fires, bark beetles and hotter droughts

Giant sequoias are known for resisting insects and disease and for a fire-adapted life cycle, and before the recent drought scientists almost never saw one die standing without crown scorch from fire. A National Park Service article says the hotter drought of 2012 to 2016 appears to have been a tipping point for giant sequoias and other Sierra Nevada mixed-conifer forests, and three newly observed threats followed.

Scorched giant sequoias seen from a helicopter over Homer's Nose Grove, Sequoia National Park, November 2020. NPS photo.

In a hotter drought, unusually high temperatures worsen the effects of low precipitation, so trees lose more water and have less available. The article treats this as an emerging climate-change threat to forests.

Wildfire

Over 85 percent of all giant sequoia grove acreage in the Sierra Nevada burned between 2015 and 2021, compared with only a quarter in the century before. Thousands of large sequoias, those with trunks four feet across or more, are estimated to have died in six recent wildfires, most of them in the 2020 Castle Fire and the 2021 KNP Complex and Windy fires. From helicopter surveys, fire severity estimates and grove maps, 13 to 19 percent of the world's large sequoias are estimated to have died in those three fires combined, or 8,431 to 11,897 trees. Fieldwork was planned to give more precise figures.

Footprints of six wildfires that significantly affected sequoia groves: Rough (2015), Railroad and Pier (both 2017), Castle (2020), and KNP Complex and Windy (both 2021). Groves in or near the fires are in red. NPS figure.

Several things made the fires severe:

• Fire history. Groves with high sequoia mortality typically had not burned in decades, so fuel loads were heavy.
• Drought-killed trees. The 2012–2016 drought killed many pines, incense-cedars and firs, adding fuel, including at the mid elevations where sequoias grow.
• Extreme fire weather. Very low humidity and above-average temperatures left fuels unusually dry, so fires burned actively day and night. Extreme fire behavior, smoke and rugged terrain hampered firefighting.
• Topography. Some groves with heavy losses sat on warm, dry south-facing slopes.

Bark beetles

During and after the drought, USGS scientists and park managers in Sequoia and Kings Canyon National Parks documented 33 giant sequoias that died standing in several groves. All had been through recent fire, and many had some fire scarring at the base. Most grew in very wet areas, and a native bark beetle of the genus Phloeosinus appears to have killed them. Beetle kill of giant sequoias is newly reported and is believed to result from drought impacts.

Foliage dieback

Many sequoias in the two parks lost foliage abruptly as a short-term response to the drought, most of all in 2014, the most severe drought year in the parks' 122-year record. Dieback on individual trees ranged from none to more than 75 percent and varied between and within groves.

Giant sequoia foliage dieback in the severe drought year of 2014, Sequoia National Park. USGS photo.

Most affected trees recovered quickly: by summer 2015 the dead foliage had been shed and new foliage grown. Dieback was highest at low elevations, probably because of higher temperatures, less snowpack and earlier snowmelt; where adult sequoias were sparse, which probably reflects more stressful sites; and on steep slopes, probably because less water was available. Vulnerability to hotter droughts may therefore vary widely across space, and some of that variation can be explained by measures of a site's water balance.

What changed, and the response

Before 2014 scientists recorded only subtle, long-term changes in forest health. During and after the drought they saw large, abrupt, novel changes: unprecedented numbers of large sequoias killed by severe fire, sequoias killed by bark beetles, and acute foliage dieback.

The Park Service says responding takes coordination among the agencies that manage sequoias, scientists and non-governmental organizations. With partners it is working on more shared learning in sequoia management, more efficient planning and carrying out of actions that improve forest resilience, and stronger public communication on sequoia conservation. Even though severe wildfire is now a threat, prescribed burning remains one of the most important tools for reducing fuels, helping young sequoias regenerate and making groves more resilient to climate change and fire.

Sources

• National Park Service, Sequoia & Kings Canyon National Parks, "Giant Sequoias Face New Threats" (https://www.nps.gov/articles/000/giant-sequoias-face-new-threats.htm); the photograph of researchers credited to a university is left out, as are the reference list and the video's text; rewritten in hubnx's own words.