Oct. 28, 2008 -- "I want to go away soon and live away by the pond," wrote Henry David Thoreau, "But my friends ask what I will do when I get there? Will it not be employment enough to watch the progress of the seasons?"
Indeed, it has proved useful employment for modern climate researchers. From 1852 to 1858, Thoreau kept meticulous records of the plants that bloomed in the Concord, Mass., woods near Walden Pond. Researchers who recently repeated those measurements have compared the findings, old and new, revealing the signature of climate change in Thoreau's woods.
Richard Primack and graduate student Abraham Miller-Rushing of Boston University surveyed the plants in woods near Concord from 2003 to 2007, recording the abundance of various species and what day they flowered each season. They combined these results with data collected by Thoreau in the 1850s and another naturalist in the late 1800s.
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The pair found that 27 percent of the species appearing in Thoreau's surveys have disappeared from the area, and another 36 percent are now so rare they may be gone soon. Over the last 100 years, the average annual temperature in Concord has risen by 4.3 degrees Fahrenheit.
Charles Davis and colleagues at Harvard University in Cambridge, Mass. then projected these findings onto a "tree of life"-- a phylogenetic tree that groups closely related species together, to see which groups of plants have won and lost over time as temperatures warmed.
"Plants in Concord aren't really responding to climate change in the same way," Davis told Discovery News. "Some are able to adjust their flowering time by upwards of three weeks, and others are not."
"The species that have very flexible flowering times with respect to temperature have tended to flourish," Primack said. "Those that have a very rigid flowering time are the ones which have tended to decline or go extinct on the landscape."
"For the first time, it shows that climate change is not impacting these plants in a uniform or random way," Davis added. "It is major branches in the tree of life that are being lost. It happens to be the most charismatic plants -- groups that we all know and love: the dogwoods, the orchids, members of the lily family, members of the rose family."
Although other studies have shown that certain groups of animals, particularly, are more susceptible to extinction than others, Davis said, this is the first time that such an effect has been shown related to climate change. The results were published today in the Proceedings of the National Academy of Sciences.
Thoreau provides part of the allure of studying the woods in Concord, but the location also happens to be a good test site because about 60 percent of natural areas there remain undeveloped and 35 percent of the landscape is protected, reducing the effect of habitat loss or urban development on the plants. This points the finger to climate change, Davis said.
The researchers do not yet know why some flowers can adjust their flowering times while others can't, nor why changing the time of flowering improves survival. Perhaps insect pollinators are also shifting their life cycles in response to temperature, leaving behind plants that flower on a rigid schedule. Differences in the plants' root systems or the season of pollination may contribute.
"I think Thoreau could have a lot to say to us today about how we could keep this problem from getting worse," said Thoreau scholar Phil Cafaro of Colorado State University in Fort Collins. "He's got a whole philosophy of how we could lead lives that are materially simpler and a lot more enjoyable and valuable to us. That gets to why we're seeing these problems...I think we could go back to Walden and learn about how we could live better lives."
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