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Perry Smith/UNH Photographic Services FOREST WATCHER: Professor and botanist Barry Rock founded the research project Forest Watch 20 years ago, enlisting teachers and students in grades K-12 across the state in collecting white pine needles to monitor tree health.

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.

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.

Erin Gleason/UNH Photographic Services

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."

Erin Gleason/UNH Photographic Services

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.

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