Thoreau’s Journals For more than a decade, starting in 1845, the writer and philosopher Henry David Thoreau tracked leaf-out dates and flowering dates for as many as 300 plant species in the woods of Concord, Mass. In 2003, Richard Primack, a College of Arts & Sciences biology professor, learned of Thoreau’s records, and used them as a baseline for comparison with contemporary data.

Warmer Temperatures, Earlier Leaves Primack warns that changes in leaf-out dates matter, because they have the potential to upset the delicate timing of nature’s food web. Leaf-out is the first major event in the seasonal food chain. It coincides with the emergence of insects, which eat young leaves, and with the arrival of birds, which feed on the abundance of insects. So far, leaf-out and insect emergence remain in sync, because both are influenced primarily by temperature. The arrival of birds, however, is influenced by other things, such as light and the temperature of their winter home. It’s possible, says Primack, that birds may someday arrive too late to dine on the abundance of insects that accompanies leaf-out.

Food for Insects A well-fed insect is a healthy insect, one that is able to perform its important role in the ecosystem of the Concord woods. Insects pollinate flowers, which yield berries, which are eaten by birds, which pass seeds along in droppings, which in turn become the next generation of trees and shrubs, which each spring sprout new leaves, which feed young insects.

Competitive (Dis)advantage Trees and shrubs that leaf out early may benefit from a longer growing season, but that longer season may also present some new threats. The hemlock tree, abundant in cold temperatures, is now plagued by the woolly adelgid insect, which thrives as temperatures rise. Primack believes that in Massachusetts, apple trees and birch trees may thrive in warmer weather, while sugar maple and beech may languish. Invasive shrubs, such as nonnative barberries and honeysuckle, are likely to be the big winners.

Disappearing Plant Species Since the 1850s, the woodlands around Concord have gained about 83 species of plants and lost about 243. This represents a net loss of 160 species, or about one species a year. Primack says there are probably many contributing factors—new roads, shopping centers, and office parks—but he is convinced that climate change has been a prominent factor.

Concord Seems to Attract Scientists For nearly 200 years, the small town of Concord has attracted an extraordinary number of first-rate botanists, biologists, and ornithologists, many of whom kept detailed records of local plant and animal life. Even today, Concord is home to distinguished professors of ornithology at Harvard and Tufts, as well to David Allen Sibley, author of The Sibley Guide to Birds, and Peter Alden, one of America’s leading naturalists.

Extreme Changes Predicted Extrapolating from historic temperature and leaf-out dates, Primack’s team predicts that in 2061 lowbush blueberries will leaf out on April 13. And the projected leaf-out date for the end of this century is April 7, about 20 days earlier than this year, and a full month earlier than in Thoreau’s time.

What the Primack Lab Does The Primack Lab, led by Richard Primack, a College of Arts & Sciences biology professor, studies the impact of climate change on the flowering and leaf-out times of plants, the spring arrival of birds, and the flight times of insects. Its research is focused mainly in Concord, Mass., where modern data can be compared with the 19th-century records of Henry David Thoreau and later naturalists. At Bostonia’s request, Primack’s team extrapolated from historic records to predict the leaf-out dates of lowbush blueberries through the end of the 21st century.

The Hottest Year on Record The National Oceanic and Atmospheric Administration reports that 2010 was the hottest year on record. It also reports that 9 of the 10 warmest years in the 134-year period of record have occurred in the 21st century. Only one year during the 20th century—1998—was warmer than 2013.

1.7 Days Earlier Per Degree Plants in Massachusetts are responding to climate change by flowering about 1.7 days earlier for each degree Fahrenheit increase in temperature. A comparison of historical leaf-out times noted by Thoreau and others with contemporary leaf-out times recorded by BU researchers shows that plants have been responding to warming temperatures for quite some time. Primack says the temperature rise in Concord has been caused by such things as the heat island (a metropolitan area whose temperature is significantly warmer than surrounding rural areas) effect of Boston and global warming caused by the production of greenhouse gases.

Insects and Birds Respond Differently Some butterflies (elfins) fly 1.7 days earlier for each degree Fahrenheit, and others (hairstreaks) fly 1.6 days earlier. Bees fly 2.0 days earlier for each degree Fahrenheit. In contrast, Massachusetts birds arrive only 0.4 days earlier for each degree warmer. The disconnect could spell trouble if birds and their primary food source get too far out of sync.

New England, Now More Like Virginia Some scientists believe that by the end of the century New England will have a climate like that of Virginia today. Others say it could be more like North Carolina or South Carolina. A warmer climate will result in increasing invasions of certain insects, like mosquitoes and ticks, possibly bringing Lyme disease from the ticks and eastern equine encephalitis and West Nile virus from the mosquitoes. New England’s wetland areas, home to many endangered species, are themselves endangered, as rising temperatures create a drier habitat.

Temperature Data Temperatures from 1851–2013 are mean spring temperatures from Blue Hill Observatory. Temperatures from 2014–2100 are predicted.