Solving a climate puzzle, one tree ring at a time

Patricia Lonergan

Monday, July 10, 2023 - 11:32

Main News

Anyone who has seen a tree stump will have noticed the rings in the wood and, at some point, learned that counting these rings can tell you the age of the tree. This unique growth not only tells us how long a tree stood, but it also holds clues to past weather patterns.

Trevor Porter, a professor in the department of geography, geomatics and environment at U of T Mississauga, is tapping into this natural archive to create a detailed picture of how Canada’s climate has changed over the past 1,000 years. With a network of research sites that span the Yukon and Northwest Territories, his goal is to build a chronology that stretches back long before weather records such as thermometer readings were kept, and to better understand what a future, warmer Arctic may look like.

Trees are sensitive to precipitation and temperature, which affect their annual growth. Porter, who is a paleoclimatologist, analyzes the individual rings, examining their width and the density of the wood and then subtracting the natural pattern of growth to see how the environment has changed. “What we learn from tree rings is valuable,” he says.

While there are records that go back 10,000 years or more from other sources – ice cores, sediment and ground ice, for example – tree rings provide detailed, annual information that is exactly dated. One of Porter’s long-term aims is to create a comprehensive record from the region that goes back a millennium. So far, his team has managed 913 years. “I really want to push this farther back in time,” he says, which means looking for dead trees buried in lakes or mud deposits where the wood is preserved.

Porter describes the process of reconstructing the past climate using tree rings – a field known as dendrochronology – as a painstaking exercise. Going back further in time requires matching the pattern in a sequence of rings from a dead tree with a section of rings of a living tree from the same geographic area. With a match, the dead tree can be accurately dated. “It’s a bit like solving a jigsaw puzzle,” he says.

A record in wood since the first world war

Cross section of tree rings, outer most ring is dated 2019 and inner most ring is dated 1915. Annotations include: Trees have large rings when they start growing. They add a lot of wood each year, proportionate to their size, when they are young. As a tree matures and the trunk expands, its rings are naturally thinner because each year's growth is spread across a larger diameter. Taking natural growth patterns into account, wider rings near the outside of the tree suggest longer and warmer growing seasons.

How core samples are collected:

Porter and his students travel to northern Canada during the summer to collect core samples from trees at multiple sites.

Three illustrations. First of person boring into trunk of a tree. Second of person using chainsaw to cut log and third of microscope. — Left: They use a hand-crank increment bore – basically a hollow drill bit – to pull core samples from living trees without harming them.
Middle: Disk-shaped samples are sliced from dead trees by chainsaw.
Right: The samples are examined under a microscope. Rings are counted and calendar years are assigned to each ring. Dots are added to help find specific decades easily when cross-dating.
(Illustrations by Chris Philpot)

'You can't talk to one tree and get the full story'

To gather data further back in time, researchers aim to include information collected from roughly 50 dead trees. Each tree "remembers" the past differently based on various factors.

Line drawing of map of northern Canada, with a circle in the upper left side of Yukon Territory next to Alaska as the study site. — The tree-ring record Porter is building represents thousands of trees across a large region.

A line graph from 1900 to 2020 showing the average mean temperature May to September has increased from a little over 2.2 degrees celsius to over 6.2 degrees celsius. — Over the past 80 years, changes in wood density in the tree samples Porter has analyzed have tracked closely to changes in temperature. Data: Porter et al (2013, Quarternary Research)

Dead trees help extend the chronology

Researchers can determine the date of tree rings on dead tree samples with help from living trees, where the dates are known. By lining up rings on a dead tree with samples from living trees, researchers can compare the pattern of growth on the outermost layers of the dead tree. Once they find a match in the sequence, they can extend the tree ring chronology.