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Another change taking place at Walden Pond

Warming trend gives nonnative plants an edge

By Carolyn Y. Johnson
Globe Staff / February 4, 2010

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The Concord woods have changed since naturalist Henry David Thoreau strolled among the trees around Walden Pond, jotting down careful observations of the plants there.

Drawing on Thoreau’s detailed notes from the mid-19th century, a team of local researchers has been looking for the imprint of climate change in New England. Now the team reports that invasive and nonnative plants in Concord are more adept than native species at responding to earlier spring thaws and warmer temperatures by changing when they flower. That means, the scientists conclude, that global warming may be advantageous to invasive and nonnative plants, with their ability to flower early possibly giving them an edge they need to thrive and spread.

“What we imagine going forward is as climate change continues to be a problem, so will invasive species be’’ a problem, said Charles Davis, an assistant professor of organismic and evolutionary biology at Harvard University and an author of the study.

To Jeff Dukes, an assistant professor of forestry and natural resources at Purdue University, the new study furnishes evidence for what scientists have long suspected, that climate change could be a boon to invasive species that are already disruptive, crowding out native plants and changing landscapes.

The reason to care about the changing landscape is more than nostalgia, Dukes said.

“There’s the perspective of personal pain, that the vista is changing, or that some people will have great pain when they can’t see what Thoreau saw,’’ he said. But local insects and animals depend on the plants, and the repercussions could ultimately be transformative.

“There are real implications of some of these invasive species when they completely change the regeneration of the forest,’’ Dukes said. “It could be a much shrubbier and vinier environment.’’

The new work, published last month in the journal PLoS ONE, builds on a 2008 study that found that as the climate has warmed, the number of buttercups, orchids, roses, violets, dogwoods, and lilies has decreased in Thoreau’s woods. Plants are flowering earlier, and those that do not shift their flowering time have decreased greatly. The researchers found that 27 percent of the species Thoreau documented have been lost locally, and that 36 percent are in danger of disappearing from the woods.

The new paper found that nonnative plants, especially invasive species that become established and spread rapidly and widely in minimally managed habitats, have successfully adapted to the 4.3 degree increase in the region’s average annual temperature over the past 150 years. Those plants are better at shifting their flowering times to respond to warmer temperatures. For example, purple loosestrife, a European plant that is known to take over wetlands, has shifted its flowering time to three weeks earlier than it was in Thoreau’s time.

Flowering and putting out leaves earlier could give plants a longer growing season and a chance to outcompete other plants, said Richard Primack, a biology professor at Boston University and a coauthor of the study.

By looking for plants whose flowering times are flexible, Primack said, it could be possible to identify species with the potential to become invasive.

In the study, for example, researchers found that the mayweed chamomile now flowers 23 days earlier than it did in 1900. The mayweed chamomile, a member of the sunflower family, is not considered invasive now. But the fact that its flowering range is so flexible tells the researchers that people should keep an eye on the plant.

Bethany Bradley, a fellow in Amherst College’s biology department who has studied the impact of climate change on invasive plants, said that the message from studies like the one done in Concord is not solely disappointing.

“When I read this type of article, I find it fascinating, because it tells us what type of plants we should be looking for if we’re going to promote adaptation to climate change,’’ Bradley said.

But to many people, including Davis, it can be disheartening research.

“The fact that roughly 30 percent of species Thoreau saw I can no longer see in the Concord area, nor can I show my son these plants . . . some of them are just incredibly beautiful,’’ Davis said. “It’s a real shame to have seen that, and it’s also similarly sad that several of these are likely getting displaced by nonnative and invasive species.’’