Reading Rings: Subfossil Wood Reveals a Glacier’s Climate History

In 2023, research ecologist Jeremy Litell, along with students and staff from the Juneau Icefield Research Program, skied miles across glaciers and ice ridges to follow up on a report of a five-needle pine tree dwelling in the gnarled, patchy, treeline, known as an alpine ecotone, that lined the Llewellyn Glacier. A finding of this species, known as the whitebark pine, would be biogeographically uncharacteristic to British Columbia’s Juneau Icefield. The nearest known whitebark pines reside nearly 400 miles away. While crevasse hopping did not unveil the pine, the team stumbled upon something astounding, a preserved graveyard of weathered, centuries-old tree stumps.

The gnarled wood reveals information about the advance and retreat of tree lines across hundreds of years. Tree lines respond to changing climate and temperatures, inching forward and drawing back at the edges of their survival conditions. Using the preserved wood, the team could investigate past climatic conditions of the Juneau Icefield region, presenting the possibility of better understanding future glacier retreat.

When Littell and his team first set out to locate the whitebark pine, they weren’t prepared to sample dead wood. The scientists, first calling themselves the “Treeple” in 2023, revisited the site in 2024 as the Treeple 2.0, collecting 34 samples of both dead subfossil wood and live wood. A question soon arose: What happened to these trees?

Using a combination of dendrochronology, also known as tree ring science, and carbon-14 dating, the team roughly calculated the ages for their trees. C-14 dating works by using the predictable rate of decay of the isotope C-14. Trees soak up C-14 while alive and stop absorbing the isotope once they die. Comparing the amount of C-14 to the ratio of other carbon isotopes, namely carbon-12 and carbon-13, allows scientists to estimate the age of the wood. Using dendrochronology, the scientists analyzed how external factors, such as temperature or precipitation, may have impacted tree growth. Over the course of a year, most trees build a single ring that consists of a light early wood and dark late wood band. The team found temperature to be the primary limitation for growth for the currently living near the terminus of the glacier. Higher summer maximum temperatures corresponded to greater growth.

Littell’s team dated some subfossil wood samples to be around 1,000 years old. These trees would have existed during the Medieval Warm Period, a time where warmer temperatures permitted trees to establish themselves at slightly higher altitudes. The Little Ice Age disrupted the warmth with abrupt and rapid cooling. During this time, new seedlings probably could not germinate reliably at the higher altitude tree lines. The adult trees at these heights eventually died, littering their skeletons on the landscape. Trees didn’t recolonize that altitude until around the 19th century, and then only sparsely, leaving the land unoccupied for several hundred years. Littell searched within the rings to identify a tree that bridged the two periods. However, the team didn’t have a sample that convincingly overlapped the living and dead wood.

The search for this overlapping ring will continue. Twenty years ago, other scientists visited this icefield and noted six trees that could fit the bill. If their records are correct, the next generation of Treeple will have the opportunity to build a 1,000 year chronological documentation of tree rings for the region. But first, the Treeple will have to find those trees.