About a decade ago, Pederson and David Orwig, a senior forest ecologist and co-manager of the Tree Ring Lab, showed that this is not always the case. They presented evidence that droughts and severe spring frosts affected various forests over hundreds of kilometers to the southeast 250 years ago. The disturbances abruptly killed some trees but accelerated the growth of others.
“Hopefully, this study will give us more insight into this relationship between climate events and forest disturbances, so that we can better predict forest responses under different climate scenarios,” said Gayle Smith.
For this, the laboratory is looking for guidance from tree rings. The rings, which look a bit like the concentric circles on a dartboard, indicate a tree’s age and provide clues as to what it has endured. The more annual rings, the older the tree. Bright ones stand for years of extreme cold. Wafer-thin wood indicates drought and slowed tree growth.
“We use tree cores to extract what I’ve called the tree’s memory,” Pederson said. “When a drought, hurricane, fire, ice storm, or insects hit, trees cannot run and hide like we can or like other animals can. They take these events, this abuse of time, and they’re recorded in their rings and we can extract that information and learn about anything.”
The first step to this science, called dendrochronology, is to obtain samples of the rings. For researchers, this often means driving on dirt roads, walking under miles of leaves, and walking up and down hills.
To core the trees, the researchers mainly 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 their core. As it twists out, it extracts pencil-sized slivers of tree ring.
“The auger is a lot harder to turn when you’re drilling into the tree, and sometimes it gets stuck on the way out, so you have to use your body weight to pull backwards as you turn,” Gayle Smith said. “I always tell people that between hiking into the sites and gutting trees, it’s a full-body workout. It can really tear your hands.”
Coring leaves a wound in the tree, but does not fatally injure it. The entire process takes between two and three hours. In addition to taking samples, the scientists note visual markers on the trees and demarcate a plot with a 20 meter radius.
The researchers plan to gut between 2,500 and 3,500 trees in 35 forests. The fieldwork is part of a four-year project funded by the National Science Foundation that began this summer with visits to 15 forests. So far the group has toured forests in Pennsylvania, New York, Maine and New Jersey. The group will eventually accumulate 600 years of tree growth data.
The explorers venture out in full hiking gear and backpacks full of supplies. The coring equipment adds about 40 pounds.
“Once I came back from a trip on a Wednesday, but I felt like my body didn’t arrive until Sunday,” Pederson said.
Lab members, which include two summer research assistants, describe the project as a nature lover’s dream and honour.
In the field, the scientists get used to wide views of landscapes, rock formations and sunsets. They see wildlife and sometimes do strange things. For example, in June, the team saw a black rat snake climbing a thick sugar maple tree in Ricketts Glen, Pennsylvania.
The forest is also a humbling place to work, especially for experts who know what it should look like.
“Every forest we visited had lost or was losing an overgrown tree species due to an introduced disease or insect,” Orwig said. “A couple of forests lost two species and one was about to lose a third. Even these forests, which have been the least directly disturbed by humans in recent centuries, are indirectly severely affected.”
Harvard’s Tree Ring Lab is located in a converted garage on the edge of Harvard Forest in Petersham, Massachusetts. Collections of the wood splinters obtained fill stacks of crates, while larger tree stumps showing the full rings are stacked in corners. The lab has a pleasant woody, earthy smell.
The samples are carefully smoothed with a belt sander, then sanded by hand. “It’s sometimes a long process, especially in species where the ring boundaries aren’t very clear,” Gayle Smith said. “We need to grind them down enough that we can see the cellular structure.”
The samples then come under a high power microscope, which digitizes the data.
Ancient eastern hemlocks, which took them hours to find, for example, showed that the two oldest trees could be traced back to 1490. They all lived in the 15th and 16th centuries – two periods of severe, prolonged drought. The trees survived what is known as the 16th-century mega-drought, which affected an area stretching from parts of Mexico to much of the continental United States, including Boston. It probably slowed the growth of these trees, which were smaller than most. The trees also showed signs of the Year Without a Summer in 1816, when a massive volcanic eruption in Indonesia triggered unusually cold, wet conditions across Europe and North America.
“I feel very fortunate to be involved in this project as we can visit, taste and learn from centuries-old trees,” said Orwig. “These forests are truly magical places. … I am awestruck to know that the forests we are studying have been there for hundreds of years and are now revealing clues about past climates, disturbances and the environment.”