Tapping into a tree’s memory… ecologist Neil Pederson in the ‘tree lab’ alongside core samples.

US forest ecologists hunting for climate clues have taken on an ambitious project to find and core the oldest trees in the state’s northeast.

The project aims to core the oldest trees and track ring data for a glimpse into the past

Four teams of researchers, led by Harvard University forest ecologists, searched for a patch of ancient trees deep in the woods of western Pennsylvania as part of a project to study how climate changes has affected trees over the centuries.

One of the scientists had come across them 40 years earlier, but they appeared to have vanished. Just as the group was about to give up and move on they came across someone who gave them a valuable clue.

“When he jumped out of his Jeep to greet us, we were about to plunge into another forest that was at least three-quarters or a mile away,” said Neil Pederson, a senior ecologist and co-manager of the ‘tree ring lab’ at Harvard Forest, an ecological research area of 1215 ha owned and managed by Harvard University and located in Petersham, Massachusetts.

The Jeep driver took them to a clump of scraggly-looking eastern hemlocks. “Several hours later, we’d finally found them,” Pederson said.

That day’s search was part of the lab’s ambitious project to find and core the oldest trees in the state’s northeast. Studying the colour and size of their rings offers scientists a glimpse into the past, allowing them to see how trees and forests responded to extreme climate events, like droughts or late-spring frosts.

They then use that data to map the long-term development of these forests and model the future impact on their health from climate-related weather events, which are growing harsher as the planet warms.

“Large-scale forest disturbances may represent the kind of extreme climate events that we expect to see increase with climate change, so understanding more about their frequency in the past could help to inform how far things are moving from baseline,” said Laura Gayle Smith, a research assistant at Harvard Forest, who works as a member of the ‘tree ring lab’.

“The common framework for temperate forests is that they are basically in equilibrium over large scales and somewhat agnostic to climate,” she said.

“Small disturbances happen at the individual-tree-to-stand level, but overall, the composition remains very stable over long periods of time – centuries to millennia.

“The ecologists used tree cores to extract what they have been leaning toward calling the memory of the tree,” said Neil Pederson.

About a decade ago, Pederson and David Orwig, a senior forest ecologist and the co-manager of the three-ring lab’, showed this isn’t always the case. They presented evidence that droughts and harsh spring frosts from 250 years ago affected different forests across hundreds of kilometres in the south east. The disturbances abruptly killed some trees but accelerated the growth of others.

“This study will hopefully give us more insight about the relationship between climate events and forest disturbance so that we can better predict forest response under different climate scenarios,” Gayle Smith said.

For that, the lab seeks guidance from tree rings. The rings, which look a bit like the concentric circles on a dart board, indicate the age of a tree and give hints of what it’s endured. The greater the number of annual rings, the older the tree. Light-coloured ones represent years of extreme cold. Wafer-thin wood indicates dry conditions and stymied tree growth.

“When a drought arrives, when a hurricane arrives, when fire arrives, or an ice storm or insects, trees can’t run and hide like we can,” Smith said.

“Take these events, these abuses of time, and they get recorded in their rings, and we can extract that information and learn about anything.”

The first step in this science, called dendrochronology, is getting samples of the rings. To core the trees, the researchers primarily use increment borers, tools that look like a cross between a drill and a screw. They must be manually twisted through the bark and into its core. As it twists out it extracts pencil-size slivers of the tree ring.

“The borer gets much harder to turn as you drill into the tree, and sometimes will get stuck on the way out so you have to put your body weight into pulling backwards while turning,” Smith said.

Coring leaves a wound in the tree but doesn’t fatally injure it. The entire process takes between two and three hours. Besides taking samples, scientists jot down visual markings on the trees and demarcate a plot that has a 20-m radius.

The researchers plan to core between 2500 and 3500 trees in 35 forests.

The fieldwork is part of a four-year project funded by the National Science Foundation and kicked off in the northern summer with visits to 15 forests. So far, the group has travelled to forests in Pennsylvania, New York, Maine, and New Jersey. The group will eventually amass 600 years of tree growth data.