One of the first signs of spring in New England comes when wildflowers sprout on the forest floor. Bluets, marsh marigolds ,and bird’s-foot violets usually flower and leaf out several weeks before trees.
There’s a reason for this. In order to photosynthesize and gain energy so they can grow, flower, and fruit, the wildflowers need the full sunlight of early spring before the trees grow leaves and shade the forest floor. Once the trees leaf out, forest wildflowers don’t get as much sunlight.
Comparing present-day observations with records that date back to Henry David Thoreau’s time in Concord in the mid-1800s, researchers have noticed a worrisome new trend.
Boston University biologist Richard Primack and colleagues from other institutions have discovered that because of climate change, common trees such as oaks, maples, and birches are leafing out a couple weeks earlier than they did in the 1850s. Yet some wildflowers are only leafing out a week earlier than they historically did.
The scientists said this may be having a negative effect on wildflowers, as their data shows that the period of light availability for wildflowers in the spring is shrinking.
“Our results show that there is a mismatch between the response of trees and wildflowers to a warming climate that could lead to the decline and loss of wildflower species from our forests,” Primack said in an e-mail.
“If trees are leafing out earlier in response to a warming climate, this means that the period of full sunlight needed by wildflowers in the early spring is getting shorter. As a result, wildflowers will have less energy to mature their fruits, to grow, and to produce flowers next year,” he explained. “As a consequence, many wildflowers are expected to decline in abundance in our forests.”
Primack and Caitlin McDonough MacKenzie, a post-doctoral researcher at the University of Maine who recently got her doctorate in Primack’s lab, said in a news release that they were very surprised by these results as they had assumed that wildflowers and trees responded in the same way to climate change.
They think it’s possible that trees respond faster because their branches are in the warm spring air, whereas it takes longer for the ground in which the wildflowers are lying dormant to warm up.
For the past 15 years, in order to determine what effects climate change may be having in the Concord area, Primack and his students have been making scientific observations of phenology — the timing of biological events in the life cycles of plants and animals — and comparing them to historical records of these events in the Concord area.
The observations that Thoreau recorded in the 1850s while the naturalist lived in Concord have been particularly valuable, Primack said.
“Of all of the records that we have found, the records kept by Thoreau have the most species and are the oldest,” Primack said. “So, they are a powerful tool for investigating climate change. Also, by connecting our research to Thoreau’s observations and his book ‘Walden,’ we can help reach a wider audience with the message of climate change.”
McDonough MacKenzie said she was drawn to Primack’s research because he was looking at the local impacts of climate change, and she had visited Walden Pond on school field trips and swam there in the summer as a child. She also understood that many people, including her family, wouldn’t see or care about the impact of climate change until it was in their backyards.
“Thoreau is the hook — he’s got the name recognition,” McDonough MacKenzie said in an e-mail. “When you talk about Thoreau, most people can imagine Walden Pond and have an image of the temperate deciduous forest in their minds. And it’s this interesting intersection of a ‘wild’ place that is fully embedded in suburban Massachusetts. It’s so close to Boston, and so many people have personal memories associated with the place, so it’s not this untouchable wilderness.”
Primarily as a result of human activities, temperatures in Concord and throughout the region have warmed by 4 degrees Fahrenheit over the past 150 years, Primack wrote in his news release. And for reasons which are not fully understood, the New England region has warmed up by twice as much as the global average of 2 degrees Fahrenheit.
Primack said that according to current predictions, the world’s climate will continue to warm up by 2 to 4 degrees Fahrenheit in the next few decades, leading to even earlier flowering and fruiting of wildflowers and trees in future forests.
“In practical terms, this may mean that in the future we may see fewer wildflowers when we walk through our forest,” Primack wrote, “and there may be fewer wild blueberries and raspberries for us to pick and eat in late summer, and less food for migratory birds to feed on before heading south for the winter.”