Harvard University researchers discovered that during the past 150 years, some plants in Thoreau's woods have shifted their flowering time by as much as three weeks as spring temperatures have risen, the researchers said. Other plants have been less flexible with nearly two-thirds of the plants Thoreau found in the 1850s near Walden Pond in Concord, Mass., already eliminated from the landscape.

"It had been thought that climate change would result in uniform shifts across plant species, but our work shows plant species do not respond to climate change uniformly or randomly," said Assistant Professor Charles Davis. "Some plants around Walden Pond have been quite resilient in the face of climate change, while others have fared far worse."

He said approximately 27 percent of all species Thoreau recorded in the mid-19th century are now locally extinct, and another 36 percent face imminent extinction.

The study that included Harvard graduate students Charles Willis and Brad Ruhfel and Boston University graduate students Richard Primack and Abraham Miller-Rushing appears in the Proceedings of the National Academy of Sciences.