Centuries-Old Trees Offer Clues to the Ongoing Southwest Megadrought

Scientists are using tree rings to inform climate research, but old specimens are increasingly vulnerable in our warming world.

A BRANCHLESS TREE pierces the blue sky from my vantage point below on a near vertical slope, black lines running up its smooth yellowing surface, contouring the tree’s growth. Located near the top of a 11,300-foot mountain slope in the Tushar Mountains of Southwest Utah, the now dead tree has remained rooted in the same remote patch of soil for hundreds of years. As a chainsaw slices its weathered skin, Karen King knows she’s working with an incredibly old tree specimen, something she’s only experienced a handful of times. “The outer parts are pretty degraded, but those inner rings are 400 to 600 years old,” said the University of Tennessee Knoxville assistant professor and recent postdoctoral researcher at Columbia University’s Lamont-Doherty Earth Observatory. With a project that depends on finding the oldest trees possible, “I’m thinking this is going to work out,” she said.

scientists collecting tree ring samples

Research team members Karen King, Danny King, and Justin Maxwell collect a cross-section of a remnant spruce tree in the Tushar Mountains of Southwest Utah. Photo by Meryl Phair.

The cross section — which holds information about how the tree has responded to fluctuating environmental conditions and stressors such as wildfires, avalanches, and insect infestations sealed into its wood — falls into a pile of sawdust. The sample is collected and placed into a backpack by Grant Harley, an associate professor with the University of Idaho earth and spatial sciences department, and Justin Maxwell, a professor of geography at Indiana University Bloomington, before the pair move down the incline to inspect another tree jutting out of the rocky slope.

The rest of the research team — a lively group of graduate students from Harley and Maxwell’s labs, along with United States Forest Service (USFS) Fire Ecologist Brian Van Winkle, Research Assistant Danny King, and Harley’s adventurous dog Sophie — is scattered along the ridge, Blue Lake siting like a turquoise gem in the canyon below them. They are here to take cross sections of remnant Engelman spruces and extract cores from living trees on the alpine slopes surrounding the lake, part of an effort to collect data on the region’s past drought and temperature conditions.

The current Southwest megadrought, which began in 2000, is more intense than anything seen in the region the past 1,200 years, according to a Nature study. Climate change is fueling dry conditions, and heat is also a factor, but the causes of the drought are more complicated to pin down. Exploring this research area is where the team of dendrochronologists come in.

Dendrochronology, the scientific method of tree-ring dating, uses annual growth rings of trees to gather important documentation of past climatic and atmospheric events, including those that pre-date human-recorded weather data. (Records from the US National Weather Service only date back to 1891.) Information about those events can provide insight into what we’re seeing now.

While tree-ring scientists have studied other droughts over the past 1,200 years, conclusive evidence they were caused by heat has yet to materialize. One challenge has been finding trees old enough to extract the necessary data.

“We’re hopeful and optimistic that we’ll be able to use the new samples to augment pre-existing data and push back this temperature reconstruction into the last millennium, or at least prior to 1000 CE,” said King. The existing Blue Lake tree ring record currently spans from 1336 to 2020 CE and only includes data from living trees.

AFTER SETTING UP camp on the ridge just under the tree line, we head down the mile-long access road to Blue Lake that plummets between the bald rocky mountain ridges. As we round the first bend, the dendrochronologists shoot up the slope, breaking off in groups to scout out the clusters of dark green spruce trees covering the incline.

The physical task of coring can only be described as a full-body workout. Thankfully, spruce trees are easier to core than others. “If this was a hickory, forget about it,” said King. Researchers use an increment borer to extract a core without harming the tree. The tool is centered on the trunk and twisted by hand so it pierces the bark and enters the tree. A long metal spoon is inserted into the hollow borer, scooping out a thin cylinder of the tree’s core. The ideal core sample will hit “pith,” the center of the tree, and for a dendrochronologist, it’s the equivalent of winning the lottery. Once extracted, the core is placed in a paper straw so it doesn’t get damaged and tagged with its location and species name.

When establishing chronologies with cores from living trees, the outermost ring is the current year. Dendrochronologists can work their way back in time from the present to find the tree’s age. This is called the “anchor chronology” because it’s tied to a known date. When the samples go back to the lab, they’ll be sanded down and polished so researchers can take high-resolution radial images of the rings. “Standard woodworking but for science,” said King.

extracting a tree core sample

University of Idaho master’s student Ellen Bergan extracts a core sample from the increment borer. Photo by Meryl Phair.

tree core sample

Research Assistant Danny King shows the results of a successful core sampling. Photo by Meryl Phair.

To detect drought, scientists look at the width of the rings: narrower rings indicate years of less precipitation while thicker rings indicate wetter conditions. Using an analysis technique called blue intensity, the researchers can also extract temperature information. The method involves shining light onto the high-resolution images of the rings and measuring how much blue spectrum light is reflected. In hotter temperatures, trees build thicker cell walls, making them denser. Denser rings will reflect less blue light, so this information can be used to estimate temperatures at a specific point in time.

As we hike over the mountain slopes, the dendrochronologists are looking for stand-alone trees, rather than those in clusters, along with trees protected by natural barriers or other fallen trees. These are both easier to core and less likely to be rotten (“punky”) or to have sustained damage from rockslides and avalanches. Many of the trees in the area are punky so we climb higher to get a better view of those below, a typical strategy in alpine coring.

As we head down from the ridge line, it’s clear we hit a jackpot of old trees. The dendrochronologists split up, shouting to each other about the trees they found. The excitement in the air is almost as palpable as the creaking from the increment bores.

FOLLOWING A SAFETY INTRODUCTION to the folding saw the team will be using, the second day of sampling consists of moving through a gray graveyard of fallen trees and cutting cross sections. As we make our way across the slope, the smell of fresh wood shavings and the hum of chainsaws fills the quiet alpine air. Like the core samples, the cross sections are tagged, and the more fragile ones are secured in plastic wrap for protection.

Unlike the samples of living trees, there’s no way to know how long remnant samples have been on the landscape. King explains that for these cross sections, the tree rings essentially act as “barcodes” that help researchers date them. Following the same process of creating digital images that’s used with the cores, the cross-sections will be turned into digital images. Using these, they’ll be able to match and statistically verify the tree rings from the anchor chronology to the rings of the remnant samples to find their age and the years each ring formed.

Closer to the ridge, the trees thin out. White plumes of smoke from the Thompson Ridge Fire look unsettlingly close, the orange ashy undertones from the fire visible on the horizon. “Well, that’s an extreme juxtaposition,” said University of Idaho MS student Ellen Bergan, a blue increment borer peeking out of their backpack. “It really reminds you why we’re doing this work.” USFS Fire Ecologist Van Winkle assures us that while it looks close, the fire would have to burn thousands of acres before we would need to evacuate.

Using tree rings to build a record of past temperatures and drought conditions in the southwest region has the potential to add critical insight into the present causes of drought, as well as other far-ranging impacts of a warming climate. But as scientists work to decipher the climactic clues contained in these trees, wildfires are becoming more frequent and more severe, making these resources more vulnerable.

“Places like national parks are well protected so they’ll have roads and infrastructure that act as natural firebreaks,” said King. “We’re looking for the oldest trees, the most scientifically valuable sources of information, and they might not be in the most protected places. As the climate is changing, we’re seeing higher burn acreage per fire so it’s a concern and an added challenge.”

At the end of the second day, with sore muscles and backpacks laden with sixty-seven core samples, we head back up the steep road to camp. After two full days of traversing the steep mountainside in search of the oldest trees, team members still haven’t lost their humor.

“Hey, what about that tree,” said University of Idaho PhD student Nick Koenig, pointing to another spruce tree up the slope.

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