Researchers at Boston University and Harvard are using Henry David Thoreau's notes from the 1850s to discern patterns of plant abundance and decline in Concord - and by extension, New England - and to link those patterns to changing climate. Their conclusions show that on average, common species are flowering seven days earlier than they did in Thoreau's day and that 27 percent of the species documented by Thoreau have vanished from Concord and 36 percent are present in such small numbers that they probably will not survive for long.