Naturalist Henry David Thoreau might well be surprised that while much of the land around Walden Pond remains undeveloped, many of the plants he knew so well are gone, probably a result of climate change.Some 27 percent of the species documented by Thoreau in the mid-1800s have disappeared, researchers report in Tuesday's edition of Proceedings of the National Academy of Science.

And as many as 36 percent exist there in such small numbers that their disappearance may be imminent, report researchers led by Charles C. Davis of Harvard University.

Warming temperatures have led to changes in the timing of seasonal activities in some plant species, but not others, the team reports. The mean temperature in the area has risen 4.3 degrees Fahrenheit (2.4 Celsius) over the past century.

They found that plants that move to an earlier flowering time in warmer weather do well.But species that did not respond to temperature changes have decreased greatly, including anemones, buttercups, asters, campanulas, bluets, bladderworts, dogwoods, lilies, mints, orchids, roses, saxifrages and violets.