In flowers, BU botanists see global warming
By Tim Stoddard
The burst of spring in Boston’s Arnold Arboretum draws swarms of visitors every year to the rolling 265-acre property in Jamaica Plain, where hundreds of species of flowering trees and shrubs open their petals to pollinators and humans. The floral spectacle has been a Boston attraction for more than 100 years, and to Richard Primack it tells a subtle story about global warming. Primack, a CAS professor of biology, and graduate student Abraham Miller-Rushing (GRS’08) have recently reported that the arboretum’s flowering denizens are blooming more than a week earlier on average than a century ago. The advancing bloom times correspond to Boston’s rising annual temperatures, which since 1885 have increased by nearly three degrees Fahrenheit. “These plants have been flowering about eight days earlier than they did at the beginning of the 20th century,” Primack says, “and the earlier flowering is in response to warming conditions.”
While scientists have seen similar trends worldwide in the past 20 years, this study is the first in North America to draw extensively upon historical data reaching back to the late 19th century. Primack found the data in the arboretum’s herbarium, a museum of 80,000 dried and flattened plant specimens mounted on cardboard, many of them in flower, and labeled to show when they were collected. During the summer of 2003, Primack and undergraduates Carolyn Imbres (CAS’06) and Daniel Primack (CAS’06) combed the herbarium for records of when plants flowered in the past. They focused on 229 plants, all still alive and blooming in the arboretum. The oldest specimen was cut from a flowering lilac in 1885.
Imbres, whose research was supported by BU’s Undergraduate Research Opportunities Program, and Daniel Primack also strolled the grounds of the arboretum every week during the spring and summer of 2003, keeping careful records of the flowering times of the 229 plants. Together with Miller-Rushing, they analyzed the data from the field and herbarium and found a distinct trend: with every one degree Celsius (1.8 degrees Fahrenheit) increase in Boston’s average temperature, the plants flowered 3.9 days earlier.
This is important scientifically, Primack says, because it offers a preview of what we can expect to see in rural areas of the United States in the future. Boston’s annual temperature has increased more than in most areas of the country because roads and buildings tend to radiate heat, creating an urban heat island. “The warming that we’ve experienced in Boston over the past 100 years is the same level of warming that rural areas of the United States are expected to experience in the coming decades,” Primack says. “This is going to have enormous implications for agriculture, horticulture, and also for the spread of disease.”
If the average temperature rises by one degree Celsius, Primack says, some crops, such as wheat and corn, will no longer grow in regions where they currently thrive. The impact could be even greater on fragile biological communities. “Certain rare plant species that are just surviving now will be eliminated by the warming conditions,” he says. “It’s quite likely that invasive species, which tend to be more flexible in their physiology, will become more common. Around Boston, hotter, drier summers may eliminate many species that are sensitive to this, and areas that are presently wetlands may dry out entirely.”
Primack hopes that these findings will encourage other scientists to locate historical data in herbaria, museums, and other institutions around the world to measure regional effects of climate change on plant communities. “This opens the door to a whole new way of studying climate change,” he says. “All over the world, there have been tens of millions of herbarium specimens collected.”
Primack’s work at the Arnold Arboretum is part of a larger effort to identify historical sources of data on how climate change affects the timing of biological events in New England, such as the return of migratory birds and the flowering of plants. Through word of mouth and by contacting natural history societies in Boston, Primack has found a number of amateur scientists whose journals contain a trove of data.
One such person is Betty Anderson, who’s been living on a farm in Middleborough, Mass., since the 1950s. Anderson started keeping a diary in the 1970s of the birds she’s seen around her farm. “She was very consistent about keeping this data,” Primack says, “and there were a lot of diaries.” With Miller-Rushing, Imbres, and Anna Ledneva (CAS’04), Primack sifted through 30 years of journal entries for data on migration times. “We’ve seen a very clear pattern in her diaries that birds have been arriving earlier in the spring,” he says. “The wood duck, in particular, has been arriving more than 30 days earlier over the course of 30 years.”
The researchers are now trying to locate similar data sets from other naturalists. “There are many hunters and fishermen and birders and botanists who are also collecting these kinds of data,” Miller-Rushing says. “We’re working to uncover as many data sets as we can, and to see whether we can use them scientifically.” One drawback to using data from amateur scientists, he admits, is that the observations are not always regular or systematic. “But in our analyses,” he says, “we’re still able to pull out a trend demonstrating a response to climate change. It’s in line with trends found using more scientifically collected data.”
A Thoreau record
With a grant from the National Science Foundation, Primack and Miller-Rushing are now analyzing the flowering times of wildflowers and trees in Concord, Mass., which has “an extraordinary botanical record,” says Miller-Rushing. “We think it’s the most thorough record for any place in North America.”
The historical data traces back to Henry David Thoreau, who gathered extensive records on the flowering times of more than 300 plants around Walden Pond between 1851 and 1858. “He intended to write a book about phenology, or the timing of biological events, but he never finished it,” says Primack.
In the summer of 2003, Primack and Miller-Rushing started visiting several locations in Concord two or three times a week, recording the flowering season of every plant they could see. Their perambulations took them around Walden Pond and through Fairhaven Woods, the Minuteman National Historical Site, Great Meadows National Wildlife Refuge, and Sleepy Hollow Cemetery, where Thoreau, Ralph Waldo Emerson, and Louisa May Alcott are buried.
They returned to Concord this summer, assisted by Jolie Olivetti (UNI’06), and will continue to botanize there over the next two years. These current observations will give them a better perspective on how flowering times have changed in Concord from the days when Thoreau wandered through the woods. “We’d like to use the past,” says Miller-Rushing, “to predict what’s going to happen in the future.”