Finding A Climate Signature in Tree Cores

This year’s Climate and Society class is out in the field (or lab or office) completing a summer internship or thesis. They’ll be documenting their experiences one blog post at a time. Read on to see what they’re up to.

A cedar stand in Westminster Swamp. Credit: Kinley Tshering

By Kinley Tshering, C+S ’15

It was a clear and beautiful June morning when I joined a team led by Neil Pederson, Senior Ecologist at Harvard Forest to venture out into a swamp forest in Westminster, Massachusetts. Our mission was to core old-growth Atlantic white cedar that grows in these forests. According to the U.S. Forest Service, the Atlantic white cedar is one of only six species in this genus. Only three of the six are native to the continent, and two of them are West Coast species. This leaves Atlantic white cedar as the only representative in the East, where it occurs in a narrow band along the Atlantic coast from Maine to Florida.

Although the idea of a swamp forest conjured up images of moss-covered swamp creatures and the likes to me, I must admit that I was quite excited at the prospect of being in a swamp forest for the first time. I quickly discovered that I was grossly unprepared for the hike into the swamp woods in my summer sneakers. Mosquito swarms were not as bad as in some of the hemlock stands that I had visited two days earlier, but they seemed to have developed a sudden attraction to my high-elevation veins and I had to pull on the hood of my jacket over my head on this otherwise warm, sunny day.

After a brief walk from the road in thick undergrowth dotted with slushy pockets, we arrived at the coring site in the forest. The cedar stand was moderately thick, interspersed with spruce and larch as associates and covered with a rich carpet of sphagnum moss. The white cedar bears an uncanny resemblance to Himalayan Cypress, the national tree of Bhutan, my country, where it only naturally occurs at elevations of 6,500 feet above sea level. A sniff of a strip of the bark and a core piece confirmed that it has the same aromatic fragrance, too. In less than three hours, we finished coring about 30 cedar trees.

Flower of Northern Pitcher Plant. Credit: Kinley Tshering

For six weeks this June and July, I am working with Pederson and Daniel Bishop, a researcher at Harvard Forest, learning how to reconstruct forest growth and decipher climatic information contained in tree-rings. This work is part of a larger Paleo-Ecological Observatory Network Project that aims to reconstruct forest composition and forest history across the Northeast U.S. over the past 200 years. These data will eventually be used to drive and validate terrestrial ecosystem models.

Back home in Bhutan, the Department of Forest and Park Services has embarked on the huge task of conducting the National Forest Inventory (NFI) and Carbon Stock Assessment Program. This effort comes after more than three decades when traditional forest growth information was collected in late 1970s as a part of Pre-Investment Survey (PIS) of forests conducted by the Indian Government.

The current NFI — besides collecting of an array of parameters missing in the PIS — has also gone ahead to collect tree cores from all of the 2,424 permanent NFI plots. The exposure and skills learned during the internship at Harvard will be instrumental in building up the existing dendrology lab in Bhutan and then in analysis of the huge collection of core samples collected from the NFI. The idea then will be to replicate a process similar to PalEON to understand growth dynamics and climate information contained in our trees for conservation and management of our forests, which cover 81 percent of the country.