Are New England sugar maples in trouble? A citizen scientist goes in search of answers.
It was the color that had her worried. Martha Carlson ’09G had never seen syrup like this before—and she’d been tapping maple trees on her Sandwich, N.H., mountainside property for more than 30 years. In 2009, though, the syrup was dark and glistening, more like molasses than the clear amber liquid she and her husband, Rudy, always produced in their tiny sugar shack. It smelled odd, too, and needed extra filtering, leaving behind a sticky residue. As Carlson tended the evaporator in the shack that March, steam curling around the window frames and out through the roof vent, the air heavy with sweetness, she couldn’t stop worrying.
Carlson, who grew up on a Maryland farm, had always paid close attention to the natural world. As an adult, she founded a school with an environmental focus, so she could share her passion with children. And back in the early 1990s, her school was one of the first to participate in the Forest Watch program founded by UNH’s Barry Rock, a botanist and forester known for tracking the effects of acid rain and climate change on forest health. Showing her students how to collect and record tree health data for Forest Watch, Carlson became a pioneer in an outreach program that has enlisted thousands of citizen scientists in a regional educational research effort.
Even when she retired from teaching, Carlson never stopped observing her trees. And now, along with the syrup mystery, she began noticing sugar maples with sparse leaf coverage and misshapen leaves. Others were dropping leaves early. She decided she had to do something. So Carlson returned to the classroom—as a graduate student. “Martha was interested in figuring out what kind of ‘thermometers’ we could use to see if maples are running a fever,” says Rock, who is her doctoral advisor. Carlson, who earned a master’s degree from UNH in natural resources in 2009, wanted to know if her beloved sugar maples—the ones that flame yellow and orange each autumn, the ones that help to define the character of New England itself—were in danger of disappearing.
One of five species of maple found in New England, the sugar maple (Acer saccharum) can grow to more than 100 feet and live to be 300 to 400 years old. Also prized for its beautiful hardwood, the maple is best known for the sweet sap it produces every spring. While there are no authenticated accounts of how maple syrup was invented, in 1609 French author Marc Lescarbot noted in his Histoire de la Nouvelle-France that the Indians “get juice from trees and distil a sweet and very agreeable liquid, which I have tasted several times.” Popular legend holds that it all began when a Native American chief hurled his tomahawk at a tree, which turned out to be a maple. The clear liquid that dripped from the gash was collected by the chief’s wife and used to cook venison—and the sweet results were a hit. So, perhaps, began a great New England tradition.
As Carlson pored over old agricultural records and syrup-making journals and interviewed maple syrup producers, her findings confirmed her fear that maple-syrup-making was changing. The old rule of thumb recorded throughout the early 20th century—boil 25 or 30 gallons of sap to make one gallon of syrup—was no longer true. Today that ratio has increased to 40 or 50 to one. And in the lab, where she boiled sap samples in test tubes, Carlson found that the average sugar content has decreased—from 3 percent to 2.2 percent. Noting that the trend in decreasing sugar content parallels a global trend in warming temperatures since the 1970s, she began to wonder about a correlation between the two. Maples need just the right mix of cold springtime nights and warm days for the sap to run. What if warming temperatures were disrupting the cold-thaw cycle, or other stages of tree development, and causing stress to her trees?
Carlson has good reason to worry, according to Rock. “Climate change is still being debated in the media, but the huge majority of scientists agree that the problem is starting to run away from us,” Rock says, citing NOAA data that shows average winter temperatures in the Northeast have increased by 2.8 degrees since 1971. The 2001 New England Regional Assessment, a National Science Foundation study led by Rock, indicates that temperatures are warming faster than anticipated, and that at the rate things are going, by 2100 the climate in New England could be more like North Carolina’s or Georgia’s. In the 1950s and ’60s, 80 percent of the world’s maple syrup came from the United States and 20 percent came from Canada. Today it’s just the opposite. “Our New England climate is moving north,” says Rock. “We need to get people’s attention. If we don’t, we’re going to be in deep yogurt.”
Carlson is worried about more than the economic impact of climate change for maple syrup producers and the fall foliage tourism industry. “We’re not just talking about losing a few sugar maples,” says Carlson. “We’re really talking about losing a dominant species in our forest. That’s a scary idea.”
The sugar maple, Rock says, is sort of like the polar bear of the New England forest, the proverbial canary in the coal mine. While other species, like red maples, can adapt and proliferate quickly, sugar maples are so long-lived and such slow growers that they are not as adaptable. If you’re looking for a tree to mark climate change, he suggests, the sugar maple is a good candidate.
- The average maple tree will produce between 9.2 and 13 gallons of sap per season, which is roughly equal to 7 percent of its total sap.
- Sugar maple trees can live to be 400 years old; after about 140 to 150 years, they stop growing in height (110 feet maximum) and grow more slowly in diameter.
- A tree should be 10 to 12 inches in diameter (40-50 years old) when measured 4.5 feet above ground level before it is tapped for sap.
- Maple syrup can be made from the sap of red, black or bigleaf maples as well as sugar maple trees.
As she launched her research, all Carlson had to start with was a lot of questions—and a sense of urgency. “If in the spring the trees are producing less sugar, which is their food, what is the impact on their ability to grow new leaves and buds, to make wood and roots? Is the lack of sugar and the shorter sap season changing the health of the trees?”
Carlson wanted to know if her trees would be able to respond to climate-change stress if they have less sugar to make protective phenolic compounds (some of which are antioxidants, known for their protective powers against cancer-causing free radicals). “I don’t want to wait until my trees die to find out,” says Carlson. What she needed were some tools to help her track what was going on.
On a day in late May, Carlson grabbed the pole pruner from the back of her truck and headed out across the fields. Afternoon sunshine slanted low across the gardens and the grape arbor, through the bending grass, all the way to the stone wall that marked the north field where trees #811 and #812 stood, their new leaves catching the last of the light. The sugar shack sat in shadow, not far from the 1930s farmhouse where Carlson’s husband grew up.
She had spent the day gathering leaves from trees on more than a dozen properties across the state. The specimens here at Range View Farm would be her last. It takes a steady hand and a good dose of patience to reach 24 feet high with a pole pruner. Pausing to get her balance, Carlson tugged at the cord, snapping the blade overhead. A small branch fluttered to the ground; she popped it into a large freezer bag and placed it in a cooler for testing later.
At UNH the next day, Carlson worked in a small, darkened room, opening her plastic bags. One by one, she placed leaf samples in the field of view on the Visible Infrared Intelligent Spectrometer, calibrated the instrument and took three scans of each set of leaves. The scans, which show how the leaves reflect shortwave infrared and visible light, help determine the amount of water, chlorophyll and biomass. Later, Carlson measured the area of each leaf on a piece of graph paper. “This is so simple, a grade-school student can do it,” she notes, explaining that she hopes someday there will be a Maple Watch version of Forest Watch for K-12 students. The idea is that on-the-ground observations will at some point be correlated with images captured by NASA’s Landsat 5, which orbits the earth 500 miles overhead and is used by Rock for his research. Landsat data, he notes, has been collected every 16 days since 1972. “Learning to read leaves in the lab,” says Carlson, “along with the Landsat data, might help us draw conclusions about large groves of trees all over the sugar maple’s range.”
In the months that follow, Carlson will study a number of other biological indicators—buds, flowers and, of course, sap, thanks to the help of about a dozen maple syrup producers around the state who have agreed to provide her with samples. “It’s like taking blood samples from people,” she says, describing the clues sap might hold to deciphering tree health. With the aroma of boiling sap wafting through the halls, it’s easy for Carlson’s colleagues to know whenever she’s in the lab. She works over a small Bunsen burner, creating a baseline set of data about sugar content, color and density. And she sometimes wonders aloud with colleagues and visitors about her hypothesis: Perhaps the darker syrup is somehow related to one or more stress factors associated with climate.
Carlson’s hypothesis made sense to Walter Shortle ’68, ’70G, a senior research plant pathologist for the U.S. Forest Service and an affiliate professor of plant biology at UNH. Shortle has been studying sugar maple health for nearly half a century, looking at the effects of roadside salt, acid rain, ice storm injury, insect damage and more. Maple trees, he explains, draw on phenolic compounds to heal tap wounds, wall off infection and inhibit fungal diseases. Usually darker syrup comes from sap gathered at the end of the sugaring season, as the tree prepares to seal up its tap wound. So it seemed reasonable to conclude that the darker syrup Carlson and other producers noticed early in the 2009 season might have been an indication of stress, and that this darker color might correspond to increased phenols found in trees trying to protect themselves. “If we could find a simple marker like this for maple health, it would be like going to the doctor’s office and testing for cholesterol,” says Shortle.
Early on, things looked promising. In one batch Shortle tested, a lighter syrup color corresponded to a lower phenolic content, while a darker color correlated to a higher content. “It was a good hypothesis, based on our first samples,” says Shortle. But syrup color in the early part of the 2010 sugar season was not dark, leading to new questions about what made 2009 so unusual. “We didn’t find the same correlation in 2010—which doesn’t mean increased phenolic compounds aren’t connected to stress—they just may not be connected to color, which would have been a nice simple indicator. Turns out it’s a lot more complicated than that.”
It was time to enlist more help. Carlson turned to the separation science experts in UNH’s chemistry department. The goal this time for professor of chemistry Sterling Tomellini and his colleagues was to separate specific types of phenolic compounds from the dozens that exist in sap. Carlson’s ultimate question was the same: Is there a biochemical signal of stress from the tree that corresponds to the physical changes she’d noticed in syrup color, sugar content and leaf coverage?
But before they could even begin to take measurements, they had to develop a suitable method, explains grad student Elizabeth Brady, who works in Tomellini’s lab. “Our goal right now is just to create a method for measuring,” she says, pulling a sap sample from the green freezer at the back of the lab. Brady began by identifying 10 prominent phenolic compounds that could possibly be affecting the colors and grades of syrup, including vanillin (which contributes to flavor) and catechins (antioxidants found in green tea and wine) and gallic acid (responsible for inhibiting fungus growth). Then, using high-performance liquid chromatography, she started testing every sample in triplicate, keeping her eye on the peaks and valleys printing out on the chromatograph.
Studying maple sap to understand tree health is new territory. While some researchers are studying phenolic compounds in syrup in order to explore the potential health benefits to humans, few have concentrated on the sap itself and its implications for the health of the tree. “We’re looking at phenolic compounds in terms of the plant’s defense system,” says Brady. “But we’re a long way from having any data to report.” The process is labor intensive—Brady sometimes spends nine hours a day running samples. “Just developing a dependable method, and trying to make sure you can reproduce the results, is not easy.” Even small variations in experimental conditions have the potential to skew the data. “And the concentrations we’re dealing with are so low,” she says, “it requires an extra-sensitive method.”
Once the analysis is complete, there’s no telling what it will show. “We’re just one piece of the puzzle,” says Tomellini, who points out that the trees could be exhibiting stress caused by insects or disease, rather than climate change. “We aren’t drawing any conclusions—we’re the data-producers,” he says. “And when we’re through, the data will go back to the plant pathologists to determine what the findings might indicate.” Tomellini points out that whatever the findings, the value of the research lies partly in the process itself, which offers a great teaching tool for potential use by high school students. “We’re not just in the business of testing samples,” says Tomellini. “We’re in the business of training students at all levels.”
Carlson is philosophical about her ongoing search for explanations; she realizes that she’s on a journey without a clear destination. But she’s undaunted. When she was a teacher, she used to tell her students, “If you want to study something with all the answers, become a historian. Science, on the other hand, is all about questions.” As she heads into the final stretch of her research, she finds herself with far more questions than answers: “We know we’re onto something—we just don’t know what yet.”
Meanwhile, Carlson delights in the research journey itself. “The tools I get to use at UNH are high tech,” she wrote, describing her discoveries with a scanning electron microscope in a letter to the Providence Journal. “At 342x, maple xylem cells look like the sacred niches where Buddhas once stood in Afghanistan.” She rattles off a slew of courses she’s enjoyed and she revels in the interdisciplinary makeup of her thesis committee: a professor of natural resources, two chemists, a plant physiologist, a forester and a philosopher. She sees this mix as proof that the problem she is tackling is widely relevant, as evidence of the “thousand invisible cords” described by one of her favorite naturalists, John Muir, who wrote, “When we try to pick out anything by itself, we find that it is bound fast by a thousand invisible cords that cannot be broken, to everything in the universe.”
In fact, the circuitous route of her journey may be one of the most relevant aspects of Carlson’s research. Donning her teacher hat, she speaks often to groups of students. “I want to find simple measures of stress that can be used to figure out if a tree is in trouble,” she says. But the scientific path, she points out, is seldom a direct one, which is why forest researchers need help. “I want to train a whole cadre of syrup producers, children, students and teachers to observe these things,” says Carlson. “We need ordinary naturalists, people who go out and look at nature and observe changes.” Already, Carlson is enlisting students to help, developing a Maple Watch pilot program with several schools in New Hampshire. “Kids want to help—they want to be part of the solution to environmental challenges,” she says. “We need to give them hope.”
On a brilliant blue day in March, a group of fourth-graders tramps through the woods outside Moharimet Elementary School in Madbury, N.H., metal buckets in hand. They follow their teacher along a short trail, heading out to check on “their” trees. Peering into the buckets hanging against the trunks, the students exclaim about the overflowing sap and swap full buckets for empty ones. They clamor to take a turn with the hand drill, creating holes in the trunk for new taps. When they are finished, the students will bottle sap samples for Carlson to take back to her lab. They are keenly aware that they are participating in real science, supporting the work of a researcher.
And, while they’re at it, they are cultivating the very same habit that drove Carlson to her maple research in the first place: they are learning to pay attention to the natural world, to ask questions about it and perhaps, someday, to discover answers.