Thoreau's Walden Pond Trees Leafing Out 18 Days Earlier

First Posted: Jan 14, 2014 05:41 AM EST
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Scientists have found further evidence of climate change in the works of Henry David Thoreau. They've discovered that the leaf-out times of trees and shrubs at Walden Pond are an average of 18 days earlier than they were in the 1850s, a time when Thoreau first recorded his observations.

In order to see how environmental conditions might have shifted over the years, the researchers examined Thoreau's unpublished observations of leaf-out times for common trees and shrubs in Concord in the 1850s.

"We started to wonder if all trees and shrubs in Concord are equally responsive to warming temperatures in the spring," said Caroline Polgar, one of the researchers, in a news release. In the end, they found some surprising results. "All species--no exceptions--are leafing out earlier now than they did in Thoreau's time. On average, woody plants in Concord leaf out 18 days earlier now.

When it comes to leafing out, plants have to be cautious. Too early and their young leaves could suffer late-season frosts.  Yet the fact that the plant species are leafing out earlier isn't what has the scientists worried. What is worrying is the particular types of species of plants that are leafing out and when they're doing it.

"We found compelling evidence that invasive shrubs, such as Japanese barberry, are ready to leaf out quickly once they are exposed to warm temperatures in the lab even in the middle of winter, whereas native shrubs, like highbush bluberry, and native trees, like red maple, need to go through a logner winter chilling period before they can leaf out-and even then their response is slow," said Amanda Gallinat, one of the researchers, in a news release. "Our current observations show that plants in Concord today are leafing out earlier than in Thoreau's time in response to warm temperatures. However, the experiments show that as spring weather continues to warm, it will be the invasive shrubs that will be best able to take advantage of the changing conditions."

The findings are published in the journal New Phytologist.

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