Walden Warming

Following in Thoreau's footsteps, Massachusetts researchers are creating the most complete picture yet of global warming's impact on the biology of a U.S. region

  • T. Edward Nickens
  • Oct 01, 2007

MAY 10, 1853, was a warm day outside Concord, Massachusetts--an early spring day when a New Englander outdoors would "begin to think of thin coats," noted Henry David Thoreau. Walking from Concord towards Saw Mill Brook, Thoreau jotted down what he saw. "The deciduous woods were in their hoary youth," he wrote, "every expanding bud swaddled with downy webs." Nodding trillium had flower buds, and hornbeam was about to bloom. Pear trees had blossomed, and the butternut buds were the most pronounced of all the woods' hickories. He heard the spring's first veery. "It is remarkable," wrote Thoreau, "that I saw this morning for the first time the bobolink, gold robin [most likely a northern oriole], and kingbird."

Remarkable, too, that he kept such meticulous records. In fact, on almost every spring morning between 1851 and 1858, long after his private tenure at Walden Pond, Thoreau explored the ponds and shady woods around Concord, observing nature. For day after day, year after year, he searched for the first blooms of more than 300 plant species and watched for the first arrivals of migrating birds.

Today, nearly 160 years later, Thoreau's detailed observations form the basis of a long-term study of how climate change is altering the timing of seasonal biological events--or phenology--and how such shifts may in turn impact the wildlife and wild places of an entire region. Researchers from Boston University have assembled a vast array of biological data--arboretum specimens, old photographs and the observations of local citizens, in addition to Thoreau's journals--to produce a baseline of springtime events for the Concord area. Comparing these data to the results of their own exhaustive, five-year effort to walk, literally, in Thoreau's footsteps, the scientists can now tell a story that New England's favorite naturalist-philosopher might never have imagined: As Massachusetts warms, flowers are blooming, trees are leafing out, and birds are arriving as many as three weeks earlier than they did in the mid-nineteenth century. "If Thoreau were alive today, he would be very concerned about this," says Richard Primack, a biology professor at Boston University and lead researcher on the project.

Thoreau, famous for his prodigious note-keeping, recorded his seasonal observations in tables sketched on large sheets of surveyor's paper. "I take infinite pains to know all of the phenomena of the spring," he explained in one journal entry. Thoreau intended to publish a book about the unfolding of spring in the woods around Concord, but his death in 1862 derailed the project, and his notes were scattered among library collections across the country.

Four years ago, however, Primack learned that an independent New Hampshire scholar named Brad Dean had spent 10 years tracking down these original sheets, making copies and reassembling the data. By then, Primack, author of A Primer on Conservation Biology, was looking for studies demonstrating physical evidence of global warming. He and graduate student Abraham Miller-Rushing couldn't believe their good fortune. Still, it took Primack's team nearly nine months to decipher Thoreau's famously poor handwriting and archaic species names and plug the information into a usable spreadsheet.


At the same time, the scientists' sleuthing uncovered a trove of other regional records to augment Thoreau's notes. At Harvard University's Arnold Arboretum, one of the oldest public botanical gardens in the United States, they were able to compare the flowering times of 229 plants in 2003 with records of flowering times of the same individual plants going back as far as 1885. In Concord, they found a collection of images from a photographer, Herbert Wendell Gleason, who between 1900 and 1921 took and dated photographs of many of the plants and places mentioned in Thoreau's journals. From these, the scientists gleaned flowering data on 17 species of wild plants, including pink lady's slipper, which flowered six weeks earlier in 2005 than in 1917.

Some of the richest sources of data turned out to be citizen-scientists in the mold of Thoreau himself. From 1888 to 1902, a Concord shopkeeper named Alfred Hosmer, inspired by Thoreau's writings, recorded the first flowering dates of more than 700 plant species in the area. A passionate nature aficionado named Pennie Logemann provided flowering records between 1963 and 1993. And for more than half a century, Middleborough, Massachusetts, resident Kathleen Anderson has kept meticulous track of the timing of bird arrivals, plant flowerings and spring choruses of frogs and toads on her 100-acre farm. "I keep a stack of those desk calendars with one full page for each day of the week," she explains, "and I was pretty intense about it. I noted weather conditions, temperature, rainfall, and whatever I happened to notice. Were the Canada mayflowers blooming? Were the juncos around? It was for my own enjoyment. It never occurred to me that these records would be of any use or interest to anyone whatsoever"--until she was contacted by Primack and Miller-Rushing, who crunched her observations into their expanding database.

The researchers, meanwhile, were making their own detailed observations. For the past five years, Primack and Miller-Rushing have traveled to Concord three times a week in spring and summer, walking the woods to ask the same questions that Thoreau asked: When do the flowers bloom? When do the birds return? So far, they have amassed another 100,000 data entries about the phenology of springtime plants and birds.

Pooling their data, the researchers have discovered that many plants in the Concord region are flowering more than a week earlier today than when Thoreau made his observations. Highbush blueberry--one of Thoreau's favorite wild edibles--is blooming some two weeks earlier than it did 150 years ago. Yellow wood sorrel can be found in bloom about a month earlier. During this same period, Primack says, long-term weather data show that the average temperature of a Concord spring has increased by approximately 4.5 degrees F.

Much of the temperature rise in the intensely developed Northeast is due to what's known as the urban heat island effect--parking lots, streets and buildings absorb heat while vegetation loss lessens the release of cooling water from trees and other plants. But at least some of it can be attributed to global warming, says Primack. And on Anderson's farm, many of the wild creatures that appear regularly each spring seem to be responding. Wood ducks are arriving about a month earlier than they did 30 years ago, for example, while ruby-throated hummingbirds show up more than 18 days sooner.

Scientists say such changes have the potential to wreak ecological havoc if interdependent species do not shift in concert. Many birds, for example, have evolved to time their spring migrations to take advantage of a flush of food sources. In New England, warbler species such as the black-throated blue warbler and American redstart feed heavily on leaf-eating caterpillars, which peak in abundance after leaf out and before leaves mature and grow tough.

In northern Europe, biologists already have found troubling evidence that one migratory bird, the pied flycatcher, has suffered from getting out of sync with its springtime food source. In the past, flycatchers arrived from their West African wintering grounds just as winter moth caterpillars were hatching. But warmer springs have pushed the caterpillar's emergence date two weeks earlier--unbeknownst to flycatchers that are still 2,800 miles away. In regions where the timing of caterpillar abundance has shifted the most, researchers have documented a 90 percent decline in flycatcher numbers. In the United States, a similar "potential for mistimed relationships is very real," says Primack, "but it is understudied."

To increase much needed data on global warming's impact on U.S. species, some scientists propose identifying and training a network of modern-day Thoreaus. According to Primack, Miller-Rushing and other researchers, there is the potential for a rich interaction between scientists and members of the general public interested in gathering observations on natural phenomena such as plant flowering and the arrival of migratory birds. Countries such as England, Belgium and Canada have long embraced monitoring programs that rely, in part, on observations of nonscientists. Recently, a consortium of U.S. government agencies and academic institutions, with funding from the National Science Foundation, launched just such an effort, the National Phenological Network, to help researchers collect and disseminate information about seasonal changes.

"We desperately need a wall-to-wall, coast-to-coast network of phenological observation points--literally thousands of points on par with what is being done with meteorological observations," explains Julio Betancourt, a biologist with the U.S. Geological Survey's Desert Laboratory in Tucson, Arizona, and one of the network's founders. Volunteer observers are an important part of the process. The group's Project BudBurst, begun as a pilot program in spring 2007, will launch nationally in January 2008. Volunteers from across the country are asked to choose from a long list of plants to watch for signs of a particular phenophase, such as budburst, first leaf or first flower, and to report observations online.

"So much of the discussion about global climate change has centered on numbers--fractions and degrees of fractions," says biologist Mark D. Schwartz of the University of Wisconsin–Madison, who is helping to coordinate the network's startup. "But when you talk about how lilacs are blooming six days earlier than they were 30 years ago, people start relating to the issue. And tell them that they can involve themselves in the process of documenting these changes, and that makes it very real.

"That's something Kathleen Anderson understands well. "This kind of work should inspire more people to be more observant," she says. At the age of 84, she still keeps notebooks handy at home, in the car and in the kitchen. "And it really doesn't matter where you live. If you look closely, you'll find enough things to interest you in the little bit of land that is around you."

After all, as Thoreau told his friend and sometime walking companion, Ellery Channing, in 1859, "There is nothing but the seasons." By which he might have meant that the seasons will tell all, to those who wish to hear.

Writer T. Edward Nickens is based in North Carolina. To find out how to participate in the National Phenological Network, go to www.usanpn.org.

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Thoreau: In His Own Words

Although Henry David Thoreau earned an international reputation as a naturalist, social philosopher and literary artist more than a century ago, no scholarly edition of his writings had been undertaken until a few decades ago. The Writings of Henry D. Thoreau--a project established in 1966 by the National Endowment for the Humanities and the Modern Language Association--is providing, for the first time, accurate texts of Thoreau's complete works: his writings for publication, his journal, his correspondence and other uncollected papers. The project is based at the University of California–Santa Barbara's Davidson Library and directed by Elizabeth Witherell, who collaborates with editors working at institutions nationwide.

Among the project's products will be the contents of all 47 volumes of Thoreau's handwritten journal (based on line-for-line transcripts and not corrected for spelling, grammar or punctuation) that will appear in 16 printed volumes. The journal entries include detailed, daily records Thoreau kept of the changing seasons around Concord, Massachusetts, between 1851 and 1858. Reprinted below are two examples of the entries--October 18, 1857, and October 22, 1858--written during fall. To read other selections from Thoreau's journal, and to learn more about the project, go to www.library.ucsb.edu/thoreau.

Oct 18th 57

Pm to Conantum--

Clear & pleasant afternoon--but cooler than before-- At the brook beyond Hubbards grove I stand to watch the water bugs--(gyrous) The shallow water appears now more than usually clear there, as the weather is cooler--and the shadows of these bugs on the bottom--half a dozen times as big as themselves--are very distinct and interesting with a narrow & well defined halo about them-- But why are they composed as it were of 2 circles run together--the foremost largest--? Isn't owing to the manner in which the light falls on their backs--in 2 spots? You think that the insect must be amused with this pretty shadow. I also see plainly the shadows of ripples they make, which are scarcely perceptible on the surface--

Many alders & birches just bare

I should say that the autumnal change & brightness of foliage--began fairly with the red maples (not to speak or a very few premature trees in water) Sep. 25 & ends this year say generally Oct-- 22nd or may be 2 or 3 days earlier The fall of the leaf in like way began fairly with the fall of the red maple leaves Oct. 13th--& ended at least as early as when the Pitch pines had generally fallen Nov 5th (the larches are about a week later) The red maples are now fairly bare--though you may occasionally see one full of leaves--

So gradually the leaves fall--after all--though individuals will be completely stript in one short windy rain storm--that you scarcely miss them out of the landscape--but the earth grows more bare--& the fields more hoary & the heavy shadows that began in June take their departure--November being at hand.

I go along the sunny west side of the Holden wood. Snakes lie out now on sunny banks--amid the dry leaves now as in spring-- They are chiefly striped ones-- They crawl off a little into the bushes & rest there half concealed till I am gone--

The bass & the black ash are completely bare--how long? Red cedar is fallen & falling-- Looking across to the sproutland beneath the Cliff, I see that the pale-brown of withered oak leaves begins to be conspicuous, amid the red, in sproutlands.

In Lees wood--white pines leaves are now fairly fallen (not Pitch P--yet)--a pleasant soft but slippery carpet to walk on-- They sometimes spread leafy twigs on floors--would not these be better? Where the pines stand far apart on grassy pasture hill sides these tawny patches under each tree--contrast singularly with the green around

I see them under one such tree completely & evenly covering & concealing the grass--and more than an inch deep--as they lie lightly-- These leaves, like other broader ones, pass through various hues (or shades) from green to brown first yellow--giving the trees that particolored look--then pale brown when they fall--then reddish brown after lying on the ground--& then darker & darker brown when decaying.

I see many robins on barberry bushes--probably after berries-- The red-oaks I see today are full of leaves--a brownish yellow (with more or less green--but no red or scarlet) I find an abundance of those small densely clustered grapes--(not the smallest quite) still quite fresh & full on green stems--& leaves crisp but not all fallen--so much later than other grapes--which were further advanced Oct. 4th when it was too late to get many-- These are not yet ripe--and may fairly be called Frost Grapes-- Half way up Blackberry Steep--above the rock--

The huckleberries in Conantum--appear to have been softened & spoilt by the recent rain--for they are quite thick still on many bushes-- Their leaves have fallen. So many leaves have now fallen in the woods that a squirrel cannot run after a nut without being heard

As I was returning over Hubbards stump fence pasture-- I heard some of the common black field crickets--(3/4 inch long) 2 or 3 rods before me--make as I thought a peculiar shrilling, like a clear & sharp twittering of birds--that I looked up for some time to see a flock of small birds going over--but they did not arrive. These fellows were, one or two, at the mouth of their burrows--& as I stood over one I saw how he produced the sound--by very slightly lifting his wing cases (if that is the name of them) & shuffling them (transversely of course) over each other about 1/8 of an inch--perhaps 3 or 4 times & then stopping.

Thus they stand at the mouth of their burrows, in the warm pastures, near the close of the year--shuffling their wing cases over each other (the males only) & produce this sharp but pleasant creaking sound--helping to fetch the year about. Thus the sounds of human industry--and activity--the roar of cannon blasting of rocks--whistling of locomotives--rattling of carts--tinkering of artisans--& voices of men--may sound to some distant ear like an earth song & the creaking of crickets-- The crickets keep about the warmth of their burrows as if apprehending cold.

The fringed gentian closes every night & opens every morning in my pitcher.

Oct 22d

Pm to Cliffs & Walden

A thickly overcast yet thick & hazy day.

I see a lombardy poplar or 2 yellowing at last many leaves clear & handsome yellow-- They thus(like the balm of gilead--& aspens) share this relation to the willows-- Horsechestnuts are yellow & ap in prime--I see locusts are generally yellow--but thinly leaved & those at extremities

Going by Farrar's field bought of John Reynold's--I examined those singular barren spots produced by putting on too much meadow mud of a certain quality. In some places the sod was entirely gone--there was no grass & only a small sandy desert--with other yellowish fimbristylis capillaris & sorrel in it--

In most places this sand was quite thickly covered with sarothra (now withered)and making a dark show at a distance & sorrel--(which had not risen from the surface) These are both sour juiced plants.

It was surprising how completely the grass had been killed.

I see the small narrow leaves of the Aster dumosus & also the yet finer ones of the Dip. linifolius in hard paths--turned a clear light yellow--

The sagitate leaves of the v. ovata too now flat in the path--& the prettily divided leaves or fingers of the V. pedata with purple petioles (also fallen flatter than usual?)--are both turned a clear handsome light yellow Also the V. cucullata is turned yellow These are far more conspicuous now than ever before--contrasted with the green grass so that you do not recognise them at first on account of their very conspicuousness or brightness of color--

Many other small plants have changed now--whose color we do not notice in the midst of the general changing-- Even the Lycopodium complanatum (evergreen)is turned a light yellow (a part of it) in its season like the pines(or evergreen trees)

I go up the hill from the spring. Oaks(except the scarlet) esp the small oaks are generally withered or withering--yet most would not suspect it at a little distance They have so much color yet yet this year at least they must have been withered more by heat than frost--for we have had very hot weather & little if any frost since the oaks generally changed-- Many of The small scarlet ones are withered too--but the larger scarlet appear to be in their prime now Some large white--black--& red--are still pretty fresh--

It is very agreeable to observe now from an eminence the different tints of red & brown in an oak sproutland or young woodland--the chocolate is one-- the brownish predominated-- Some will tell you that they prefer these more sober colors which the landscape wears at present to the bright ones it exhibited a few days ago-- It is interesting to observe--how gradually but steadily the woods advance through deeper & deeper shades of brown to their fall.

You can tell the young white oak in the midst of the sproutland--by its light brown color--almost like that of the russet fields see beyond--also the scarlet by its brighter red--but the pines are now the brightest of them all.

Apple orchards--throughout the village or on lower & rich ground are quite green--but on that drier F. H. Hill all the apple trees are yellow--with a sprinkling of green--& occasionally a tinge of scarlet.

I can see the red of young oaks as far as the horizon on some sides. I think that the yellows--as birches--&c are the most distinct this very thick & cloudy day--in which there is no sun--but when the sun shines the reds are lit up more--& glow

The oaks stand browned & crisped (amid the pines) their bright color for the most part burnt out--like a loaf that is baked--& suggest an equal wholsomeness. The whole tree is now not only ripe but as it were, a fruit--perfectly cooked by the sun. That same sun which called forth its leaves in the spring--has now aided by the frost--sealed up their fountains for the year--& withered them. The order has gone forth for them to rest--As each tree casts its leaves it stands careless & free--like a horse freed from his harness--or like one who has done his years work--& now stands unnoticed but with concentrated strength & contentment--ready to brave the blasts of winter without a murmur--

You get very near wood ducks with a boat nowadays--

I see, from the cliffs, that color has run thro' the shrub oak plain like a fire or a wave (not omitting a single tree--) though I had not expected it-- Large oaks do not turn so completely & now is for the most part burnt out for want of fuel--i.e. excepting the scarlet ones. That birch swamp under the Cliff is very interesting-- The birches are now but thinly clad & that at top--its flame-shaped top more like flames than ever--now-- At this distance their basic slender stems are very distinct dense & parrallel--apparently in a somewhat smoky ground (caused by the large twigs) & this pretty thicket of dense parallel stems is crowned or surmounted by little cones or crescents of golden spangles.

Hear a cuckoo & grackles--

The birches have been steadily changing & falling for a long long time. The lower most leaves turn golden & fall first--so their autumn change is like a fire which has steadily burned up higher & higher--consuming the fuel below--till now it has nearly reached their tops These are quite distinct from the reddish misty maze below--(if they are young trees) or the fine & close parallel white stems if they are larger-- Nevertheless the topmost leaves at the extremities of the leaves are still green.

I am surprised to find on the top of the Cliff--near the dead white pine--some small Staghorn Sumacs-- (Mother says she found them on the hill behind Charles Davis!!) These are now at the height (?) of their changes--as is ours in the yard--turned an orange scarlet--not so dark as the smooth which is now ap. fallen It is generally--but I see some (one or 2 the 24th). But ours being in a shady & cool place is prob later than the average--for I see that one at Floods cottage has fallen. I guess that they may have been at height generally some 10 days ago-- Near by the Aralia hispida turned a very clear dark red.

I see Heavy Haynes fishing in his old gray boat--sinking the stern deep. It is remarkable that of the four fishermen who most frequent this river--Melvin--Goodwin & the 2 Hayneses--the last 3 have all been fishermen of the sea--and have visited the grand banks--& are well acquainted with Cape Cod. These fishermen who sit thus alone from morning till night--must be greater philosophers than the shoemakers--

You can still pluck a variegated & handsome nosegay on the top of the cliff-- I see a mullein freshly out--very handsome A. undulatas--& an abundance of the little blue snapdragon--& some P. persicaria &c &c--

The black shrub oak on the hill side below the Bearberry--fast falling & some quite bare-- Some chinquapin there not fallen Notice a chestnut quite bare--

The leaves of the hickory are a very rich yellow though they may be quite withered & fallen but they become brown

Looking to Conantum, the huckleberries are ap. fallen.

The fields are now perhaps truly & most generally russet--esp. where the blackberry & other small reddish plants--are seen through the fine bleached grass & stubble--Like a golden russet apple. (This occurs to me, going along the side of the Well Meadow Field.)

Ap. the scarlet oak. Large & small is in prime now after other oaks are generally withered or withering.

The clumps of Salix tristis--half yellow--spotted with dark brown or blackish--& half withered & turned dark ash colored--are rather interesting. The S. humilis has similar dark spots

Hornets' nests are now being exposed--deserted by the hornets While pines have for the most part fallen-- All the underwood is hung with thin brown fallen needles giving to the woods an untidy appearance

C. tells of hearing after dark the other night frequent raucous notes which were new to him on the Ammannia meadow--in the grass-- Were they not meadow hens? Rice says he saw one within a week-- Have they not lingered to feed in our meadows the late warm & pleasant nights?

The haze is still very thick though it is comparatively cool weather--& if there were no moon tonight--I think it would be very dark-- Do not the darkest nights occur about this time when there is a haze produced by the Ind-- summer days--succeeded by a moonless night??

These bright leaves are not the exception--but the rule--for I believe that all leaves even grasses &c &c--Pan. Clandestinum--& mosses as sphagnum under favorable circumstances acquire brighter colors just before their fall-- When you come to observe faithfully the changes of each humblest plant--you find it may be unexpectedly--that each has sooner or later its peculiar autumnal tint or tints--though it maybe rare & unobserved--as many a plant is at all seasons-- And if you undertake to make a complete list of the bright tints--your list will be as long as a catalogue of the plants in your vicinity

Think how much the eyes of painters--both artisans & artists--& of the manufacturers of cloth & paper--& the paper stainers &c are to be educated by these autumnal colors

The stationers envelopes may be of very various tints--yet not so various as those of the leaves of a single tree sometimes-- If you want a different shade or tint of a particular color you have only to look further within or without the tree--or the wood. The eye might thus be taught to distinguish color & appreciate a difference of shade or tint.

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