How climate is changing Walden Pond

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There are places around the Concord woods near Walden Pond that the naturalist Henry David Thoreau probably would not recognize.  As climate has warmed in the northeastern U.S., many of the native plants that flourished 150 years ago have been giving way to the invasion of weeds.

To a team of Harvard University biologists, Thoreau's famously meticulous observations provide a unique database to track changes that have been taking place in the region's environment.

Thriving in place of many of the buttercups, orchids, roses, violets and other flora studied by Thoreau are invasive, non-native species such as mayweed chamomile and purple loosestrife.  As this photo by Harvard researcher Abraham Miller-Rushing illustrates, purple loosestrife, one of the most troublesome invasive species, crowds out many native species and competes for local pollinators like this monarch butterfly.

Aside from a kind of wistful nostalgia such studies provoke, biologists are interested in the adaptations to changing climate that have been underway and their broader, practical implications for the northeast.

"Our research suggests quite decisively that non-native and invasive species have been the climate change winners," said Charles C. Davis, author of a new study in the public online journal PLos ONE.  Since Thoreau's day, the mean annual temperature around Concord, Mass., has climbed 4.3 degrees Fahrenheit.

These warming temperatures bring on ever-earlier springs, prompting some plants, such as the purple loosestrife, to shift their flowering time as much as three weeks earlier.  Davis said his findings point to the importance of the ability to adapt "flowering time" as the key trait for the success of non-native species.

In a Harvard Gazette description of the work, Davis said invasive species "can be intensely destructive to biodiversity, ecosystem function, agriculture, and human health," problems with an estimated annual cost of $120 billion in the U.S. alone.