Trees record everything about their environment—including what the climate was like in the past.
In the nineteenth and twentieth centuries, the Eastern Seaboard’s old-growth forests were cut down almost in their entirety. Today, trying to find a tree in this area that is more than two hundred years old is like looking for a button that you lost a few years back. But New York City—unlike the surrounding forests—is host to a great crowd of old wood. It’s just that it exists in the form of beams and joists within buildings.
Douglass had obsessions, too. He wanted to study solar cycles and sunspots. He thought that they might explain cycles of drought in arid regions like those around Flagstaff. Although Douglass had excellent techniques for making solar observations, climate data for the relevant bits of the Earth near where he was staying went back not more than a dozen years or so.
His theory about sunspots turned out, after much examination, not to be true. But it has had an unexpected afterlife. It gets cited by climate-science skeptics, who falsely claim that sunspots play a significant role in climate change. In order to try to assess what the climate was like at various points in the past, dendrochronologists look at the width and density of rings, at variations between early and late wood bands, and also at the chemical composition of rings. Leland explained how the ratio of heavy to light stable carbon isotopes can serve as a particularly good indicator of how efficiently a tree is using water: “When a tree opens its pores to take in carbon dioxide, it also loses water through those same pores.
The Tree Ring Lab’s dendroarcheological work helped to assign a date and provenance to a sunken ship discovered in 2010 some twenty feet below street level during the excavation for the World Trade Center. “We always like to tell this story when schoolkids come to the lab,” she said. The ship, made mostly of oak, was identified as having been built with timbers cut down a few years before the Revolutionary War.
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