The upright citizens of Concord, Massachusetts, didn't think much of young Henry David Thoreau. The cabin on Walden Pond, the night in jail for tax evasion, the constant scribbling in journals—it all seemed like a waste of a perfectly good Harvard education. Even more mysterious was his passion for flowers. "I soon found myself observing when plants first blossomed and leafed," Thoreau confided to his journal in 1856, "and I followed it up early and late, far and near, several years in succession, running to different sides of the town and into the neighboring towns, often between twenty and thirty miles in a day."

Watch a video of Concord's flora

Thoreau planned to turn his vast botanical records into a book, but he died of tuberculosis in his mid-40s, the project undone. Walden and his handful of other published writings languished in near obscurity, and even his close friend and mentor, Ralph Waldo Emerson, said that Thoreau had squandered his talents on the woods. "I cannot help counting it a fault in him that he had no ambition. ...Instead of engineering for all America, he was the captain of a huckleberry party," Emerson lamented in his eulogy of Thoreau.

Walden, of course, is now a classic of American literature, and Thoreau is considered a secular prophet. In Concord, tourists buy T-shirts printed with Thoreau's best-known sayings, including "beware of all enterprises that require new clothes." Much has changed in Concord. On the shore of Walden Pond in summer, warblers and blueberry bushes are still commonplace, but so are teenagers in shocking-pink bikinis.

Thoreau's unassuming gravestone, marked simply "HENRY," rests on a mossy ridge not far from the center of town and is decorated with pine boughs and pebbles left by admirers. On a sunny slope nearby, two botanists crouch in the grass, paying a different sort of tribute to Concord's famous son.

"We've got bluets. First time this year," Abe Miller-Rushing says.

"Are you sure you didn't see some yesterday?" teases his mentor, Richard Primack of Boston University.

"First time," Miller-Rushing says with a grin.

The late April afternoon is clear and warm, and the slope at Sleepy Hollow Cemetery is dotted with the pale, four-petal blooms of the native plant. Were Thoreau here to marvel at the changes in Concord, these delicate flowers might surprise him most of all.

"How sweet is the perception of a new natural fact!" Thoreau remarked in his journal in 1852. Throughout the 1850s, while his neighbors toiled in their fields and offices, Thoreau spent hours each day walking Concord's woods and meadows, contemplating nature. His outings, he insisted, were anything but leisurely: "I have the habit of attention to such excess," he wrote, "that my senses get no rest—but suffer from a constant strain."

He taught himself to recognize hundreds of local plants, placing specimens in his well-worn straw hat. "When some whom I visited were evidently surprised at its dilapidated look, as I deposited it on their front entry table," he wrote, "I assured them it was not so much my hat as my botany-box."

The earliest blossoms and other signs of spring especially fascinated Thoreau. "I often visited a particular plant four or five miles distant, half a dozen times within a fortnight, that I might know exactly when it opened," he wrote. The author Louisa May Alcott, a Concord resident, remembered that the writer "used to come smiling up to his neighbors, to announce that the bluebirds had arrived, with as much interest in the fact as other men take in messages by the Atlantic cable."

Thoreau organized his eight years of botanical notes into detailed monthly charts, listing the first flowering dates for several hundred species. After his death, the dozens of pages of charts were scattered to libraries and collectors, forgotten by all but his most ardent students. Thoreau's data finally found a champion in Bradley Dean, an independent scholar, who supported his research on Thoreau with a trickle of fellowships and grants. Dean, who died in 2006, tracked down every page of Thoreau's charts, collecting a full set of copies at his home in rural New Hampshire.

Primack, 57, lean and sharp featured, had spent decades researching tropical forests in Malaysia, Central America and elsewhere before turning to his own backyard in 2002. Like Thoreau, he was interested in springtime, but his motivations went beyond a simple love for the season: Primack wanted to study how the natural world was responding to global warming. "Over the coming decades, we're likely to see a lot of significant changes caused by global warming—more and more extinctions, for example—but we can't measure most of those things yet," he says. "Bird migrations and flowering times are the best indicators we have that natural communities are starting to change."

Primack began searching for natural-history records from Massachusetts, talking to bird-watchers and amateur botanists. Through a former student, he learned that Thoreau, of all people, had collected exactly the sort of data he was looking for. In 2003, Primack called Dean to ask about his collection of Thoreau's charts. Dean, not at all surprised, said he'd expected that scientists would one day come looking for Thoreau's data.

Dean wasn't the first person to take an interest in Thoreau's record keeping. Sixteen years after Thoreau's death, an enigmatic Concord shopkeeper named Alfred Hosmer decided to continue Thoreau's botanical project. In 1878, and then consistently from 1888 until 1902, he recorded the first flowering dates of more than 700 species in the Concord area. A bachelor, Hosmer spent his Sundays exploring meadows, swamps and even the town dump. "Fred is...better informed about Thoreau's haunts than any man living or dead," wrote his friend Samuel Jones. "I, poor miserable I, admire Thoreau; Fred lives him!" Like Thoreau, Hosmer turned his field notes into hand-lettered tables, sometimes pressing a leaf or flower between the pages. He died in 1903, leaving no explanation for his dedication.

Primack, joined by his doctoral student Miller-Rushing, now had detailed reports on Concord's flora from Thoreau and Hosmer, and it was time to compare the past with the present.

It's not easy to collaborate with dead botanists. Thoreau's penmanship was atrocious, and he used antiquated botanical names. Using the research of an amateur botanist and Thoreau admirer named Ray Angelo, Primack and Miller-Rushing deciphered Thoreau's and Hosmer's tables.

During their first year of fieldwork, in 2003, Primack and Miller-Rushing searched the sunniest, warmest corners of Concord, just as Thoreau had, looking for the first blooms. They found a place on the campus of the private Middlesex School where flowers turned up especially early. They talked a local farmer into allowing them to survey his fields. They walked the railroad tracks behind the site of Thoreau's cabin at Walden Pond.

When Primack found the season's first blue violet bloom on the gravel railroad bed, he was so absorbed that he failed to hear a construction truck approaching on the rails. The driver pulled up just 20 yards from the surprised researcher and angrily demanded that he explain himself. Primack quickly made clear he was no saboteur, but a botanist, and vowed to be more cautious. But as Thoreau himself surely would have, Primack and Miller-Rushing continued to inspect the tracks for flowers, paying for their persistence with a few run-ins with local police.

"We learned that if you're going to look at plants along the tracks, look at them briefly, always have a lookout and be ready to run into the woods," Miller-Rushing says.

In the spring of 2004, they began reprising Thoreau's work in earnest. With the help of several undergraduates, Primack and Miller-Rushing combed the warmest places in town. As they navigated crowds of tourists at Minute Man National Historical Park or stepped around the sunbathers at Walden Pond, they found they had a lot in common with their quirky collaborator. "We'd come out of the woods, sometimes covered with mud, and start asking people if they would move their towels so we could see the flowers," Miller-Rushing remembers. "That's when we realized that we weren't normal people."

What they discovered wasn't quite normal, either. Primack and Miller-Rushing compared three years of their results with those of Thoreau and Hosmer, focusing on the 43 plant species with the most complete records. They learned that some common plants, such as the highbush blueberry and a species of sorrel, were flowering at least three weeks earlier than in Thoreau's time. On average, they found, spring flowers in Concord were blooming a full seven days earlier than in the 1850s—and their statistics clearly showed a close relationship between flowering times and rising winter and spring temperatures.

Primack and Miller-Rushing also found other naturalists who had carried on Thoreau's tradition of obsessive observation. Robert Stymeist, a retired accountant and devoted birder, frequents the trails of Mount Auburn Cemetery in Cambridge, a shady, arboretum-like spot that attracts colorful waves of migrating birds each spring. Stymeist, 59, has been watching and recording them for almost as long as he can remember: when he was just 10 years old, too young to be trusted with a key to the cemetery gates, he began sneaking into the grounds, binoculars and bird guide in hand. "It's just always been my spot," he says.

The ecologists' quest also led them to Kathleen Anderson, a great-grandmother and lifelong birder, who has lived on a wooded property south of Boston for nearly six decades. Born in rural Montana, she remembers that her mother rewarded her and her siblings for spotting the first bluebird or daffodil, inspiring a record-keeping habit that Anderson, now 84, continues to this day. Her elaborate daily diaries, shelved in her low-ceilinged farmhouse, detail not only family weddings, births and the news of the day but also natural phenomena ranging from bird arrivals to frog choruses to the newest blooms in her yard. "I guess I'm an old-fashioned naturalist—I'm curious about everything," she says. "But I never in my wildest dreams thought that these records would be of any significance. I even wondered if my children would be interested in them."

Like Thoreau's data, the records of these naturalists were idiosyncratic and tricky to analyze. Amateurs don't usually record exactly how long they searched for an animal, or how many people were looking, or how certain they were about what they saw—and these gaps make professional scientists nervous. "Scientists are used to analyzing other scientists' data," says Miller-Rushing. "We're not so comfortable venturing into the world of personal journals."

But Primack and Miller-Rushing found that the bird records from Mount Auburn, Anderson's diaries and data collected by trained researchers at the Manomet Center for Conservation Sciences on the Massachusetts coast all told a similar story. On average, migratory birds are turning up earlier every year in eastern Massachusetts. And as with the precocious blooms in Concord, the shifts in schedule are best explained by warming temperatures.

Even in the mythic American landscape of Concord, global warming is disrupting the natural world. Since Thoreau's time, average temperatures have risen more than four degrees Fahrenheit because of local urban development as well as global climatic warming. Concord, once a farming community, is now a busy suburb—Boston is just a half-hour drive from Walden Pond—and expanses of warmth-absorbing concrete and blacktop have created a "heat island" of higher temperatures in the greater metropolitan area.

Seasonal routines such as migration, blooming and breeding are the pulse of the planet, and everything from agriculture to allergy outbreaks depend on their timing—and, often, their precise coordination. "Pollinators have to be around when plants are flowering, seed dispersers have to be around when seeds are available, leaves have to be around for herbivores to eat them," says Miller-Rushing. "There are endless numbers of these relationships, and we don't have a lot of good information about what happens when their timing gets jumbled up."

While some flowers in Concord, like the bluets in Sleepy Hollow Cemetery, are blooming weeks earlier than in Thoreau's time, others haven't changed their schedules. Observations from Thoreau and other naturalists reveal that plants are reacting to temperature changes more dramatically than short-distance migratory birds, suggesting that climate change could divide plants from their pollinators. Spring's acceleration is far from orderly.

That's disturbing news, because many plants and animals are already declining in eastern Massachusetts for other reasons. Though Concord has more parkland and natural spaces than many communities, thanks to strong local support for land conservation, human habits have changed over the past century and a half, and habitats have changed with them. River meadows, once mown for hay, have declined, along with local agriculture, and many have gradually turned to swamp forest. As hunting dwindled, white-tailed deer began devouring woodland plants. Invasive plants such as Oriental bittersweet and black swallowwort have infiltrated Concord, even covering the banks of Walden Pond. "The woods are being repopulated by things Thoreau never even knew about," says Peter Alden, a Concord native and veteran naturalist.

Of the nearly 600 plant species for which Thoreau recorded flowering times during the 1850s, Primack and Miller-Rushing found only about 400, even with the help of expert local botanists. Among the missing is the arethusa orchid, which Thoreau described with admiration in 1854: "It is all color, a little hook of purple flame projecting from the meadow into the air....A superb flower."

Walking the well-traveled path that circles Walden Pond, searching for the earliest flowers of the highbush blueberry, Primack says his results make him uneasy. "I don't think scientists should just be studying things until they go extinct," he says. "I think they should be doing something to make sure they don't go extinct." He supports "assisted migration," deliberately moving rare plants and animals to new, more promising habitats. The idea is controversial among biologists, many of whom fear that the transplants could interfere with native inhabitants. But Primack argues that the risks are low and the need is pressing. "In the past, some of these species might have been able to move on their own, but now there are barriers—highways, cities, fences," he says. "We have an obligation to move them."

Primack and Miller-Rushing argue good-naturedly about whether certain plants and animals can adapt to climate change, but they, and other ecologists, know such issues are far from resolved. "Now that we know what's changing, what are we going to do about it, and what are species going to do on their own about it?" asks Miller-Rushing. "Those are unanswered questions."

For now, Primack and Miller-Rushing are helping other scientists build a national network of observers—ranging from schoolchildren to amateur naturalists to professional ecologists—to collect data on flowering times, bird migrations and other signs of the seasons. The goals are not only to understand how plants and animals are responding to climate change but also to fine-tune future environmental restoration efforts and even allergy forecasts. It's a project that will require Thoreauvian stubbornness.

"These things are almost always heroic efforts by individuals," says Julio Betancourt, a hydrologist with the U.S. Geological Survey and a co-founder of the national observation network. "Thoreau, and those that came after him, made a decision to make these observations, and to make them routine. To continue that for decades takes a lot of commitment and stick-to-itiveness and vision."

Michelle Nijhuis lives off the electrical grid in Paonia, Colorado. She wrote about Winchester, Massachusetts.