Thoreau started his journal in 1837, taking notes about the flora and fauna he saw during his daily rambles. In 1851, he began an ambitious project to analyze the changing phenomenon of the seasons. For the next 10 years until his death, he took copious notes about his observations. He pressed flowers in a music book, saved specimens in his hat and used his walking stick as a measuring tape. He took the temperature of ponds and streams and watched migratory birds through his spyglass.
It is a clear day with a cold westerly wind, the snow of yesterday being melted. When the sun shines unobstructedly the landscape is full of light, for it is reflected from the withered fawn-colored grass, as it cannot be from the green grass of summer. (On the back of the hill behind Gourgas's.)
The bluebird carries the sky on his back.
On May 14, 1852, Thoreau made a list of the dates about 35 species of spring flowers appeared, from the male Acer rubrum on April 28 to a Cerasus Pennsylvanica ? on May 13. He chided himself for not observing closely enough the first common elm and the first red maple. Over a decade of such observation he recorded the exact flowering dates of 500 species of plant, flower and shrubs.
Richard Primack, biology professor at Boston University, pored over the journals and compared Thoreau’s observations a century and a half ago with his own. Primack and his colleagues found one-quarter of the wildflowers Thoreau observed can no longer be found around Concord, and one-third of them are rare. That’s because of urban development, pollution and the increased population of people and deer.
But the wildflowers still around Concord are blooming earlier than they did in Thoreau’s day. Plants in Concord are generally flowering seven days earlier than they did in 1861. Highbush blueberry blooms appear three weeks earlier than they did in the mid-19th century. The shadbush and the marsh marigold do too. Some, like birdfoot violet, rhodora and flowering dogwood, changed by one or two weeks. Some didn't shift at all.
According to the Blue Hill Meteorological Observatory, the average April temperature in Concord is five degrees warmer than it was in Thoreau’s day. Primack blames global warming and the urban heat island that metropolitan Boston has become.
Primack is continuing to study how some plants are more affected by climate change than others. The journals of Henry David Thoreau make that possible.
In 2012, Primack wrote in the New York Times,
Despite their dramatic cumulative effects over the last 160 years, these changes would be largely imperceptible without the biological yardstick Thoreau's records provide.
Thoreau, of course, provided more than biological yardsticks. He offered poetic, even humorous, observations about the interrelatedness of all nature.
On April 13, 1854, he noted in his journal,
The golden-brown tassels of the alder are very rich now. The poplar (tremuloides) by Miles's Swamp has been out -- the earliest catkins -- maybe two or three days. On the evening of the 5th the body of a man was found in the river between Fair Haven Pond and Lee's, much wasted. How these events disturb our associations and tarnish the landscape! It is a serious injury done to a stream. One or two crowfoots on Lee's Cliff, fully out, surprise me like a flame bursting from the russet ground.