One morning earlier this summer, the sun rose over Brooklyn’s Prospect Park Lake. It was 5:28 A.M., and a black-crowned night heron hunched into its pale-gray wings. Three minutes later, the trunk of a nearby London plane tree expanded, growing in circumference by five-eighths of a millimetre. Not long afterward, a fish splashed in the lake, and the tree shrunk by a quarter of a millimetre. Two bullfrogs erupted in baritone harmony; the tree expanded. The Earth turned imperceptibly, the sky took on a violet hue, and a soft rain fell. Then the rain stopped, and the sun emerged to touch the uppermost canopy of the tree. Its trunk contracted by a millimetre. Then it rested, neither expanding or contracting, content, it seemed, to be an amphitheatre for the birds.
“I wonder about the trees,” Robert Frost wrote. Monumental in size, alive but inert, they inhabit a different temporality than ours. Some species’ life spans can be measured in human generations. We wake to find that a tree’s leaves have turned, or register, come spring, its sturdier trunk. But such changes are always perceived after the fact. We’ll never see them unfold, with our own eyes, in human time.
To understand how trees transform, dendrochronologists, researchers who study change in trees, have developed a few techniques. They cut trees down to analyze their rings, which have been created by the seasonal formation of new cells, but this terminal strategy can provide only a static overview of the past. They “core” living trees, using bores to extract trunk tissue; this technique, however, can stress trees and sometimes, though rarely, wound them fatally. They measure tree girth with calipers and tape—a less invasive means of studying growth that is also frustratingly intermittent.
In the early two-thousands, a new technique emerged that changed the field. It relies on low-cost transducers: equipped with a tiny spring, a transducer—which converts, or “transduces,” physical motion into an electrical signal—can rest on the bark of a tree, sensing and logging tiny changes in pressure. Instruments that use this approach, known as precision dendrometers, allow scientists to do something entirely new: watch how trees change and respond to their environments on an instantaneous scale.
This spring, I walked along the eastern edge of Prospect Park Lake with Jeremy Hise, the founder of Hise Scientific Instrumentation, a company that sells affordable precision dendrometers to scientists, students, and members of what Hise called the “D.I.Y. makerspace.” Bearded and affable in jeans and a blue sweatshirt, Hise explained that his dendrometers could now deliver their measurements wirelessly to a cloud-based platform called the EcoSensor Network. Users of the network can monitor a tree’s growth, generate graphs, and correlate them with meteorological data. Together with Kevin Griffin, a professor of earth and environmental sciences at Columbia University, Hise is planning to build the largest network of dendrometers in the world, generating millions of data points each year. “We’re looking to be the Weather Underground of trees,” Hise said.
When the landscape architect Frederick Law Olmsted planned Prospect Park, in the eighteen-sixties, he wrote that he prized trees “which possessed either dignity or picturesqueness.” Hise and I were looking for an especially dignified or picturesque tree to study. Along the eastern edge of the park’s carriage concourse, we spotted a London plane tree—the last of five trees that had been moved there, around 1874, from the esplanade opposite Music Island. The tree had a giant canopy of pea-green buds that had not yet bloomed. Its lower branches had twisted and, in some places, grafted onto one another, creating a uniquely broad trunk.
Hise drilled a tiny hole, five centimetres deep, into the trunk, then screwed a threaded rod into the hole. (He had been granted a research permit from the Prospect Park Alliance, the nonprofit group that maintains the park and its resources.) The transducer was attached to the rod on a metal plate; to the plate, we connected a long black strap, which we wrapped around the tree—by our reckoning, it was around eleven feet in circumference—and cinched tight. To the strap, we attached a plastic box that held batteries and a circuit board; the box, Hise said, would wirelessly communicate the dendrometer readings to a nearby receiver. Installation took twenty minutes. All there was left to do was to wait for the data to come in.
The development of precision dendrometers has been well timed. Each year, the world’s forests extract billions of tons of carbon dioxide from the atmosphere—an estimated twenty-eight per cent of all emissions. For this reason, many scientists believe that our planet’s future climate is tied inextricably to the future of its forests. But trees, too, are vulnerable to climactic disruptions. Researchers are still trying to understand how different tree species will respond to environmental change, such as extreme temperatures, droughts, pests, and wildfires, and what their thresholds for adaptation might be. When it comes to trees, it turns out that almost every variable imaginable—from the temperature of the air and soil to wind speed, humidity, precipitation, cloud cover, and pollution levels—can influence their growth. “Climate affects the timing, rate and dynamics of tree growth, over time scales ranging from seconds to centuries,” the ecologist Gregory King has written. The data gathered by precision dendrometers may help researchers build predictive models.
In the last decade, precision dendrometers in Canada, Mexico, Ecuador, French Guiana, and elsewhere have generated vast quantities of new data about the complex relationships between trees and their environments. In the Tianshan Mountains, of northwestern China, researchers have used them to monitor the Schrenk spruce. Although it can live for up to eight hundred years, and covers much of the Central Asian region responsible for China’s watershed, virtually nothing was known about its seasonal growth cycle. The scientists discovered that the number of days of rain between mid-May and late July has an extraordinary effect on tree growth—suggesting that future shifts in precipitation could have dramatic effects on the trees’ survival, and in turn on the forest’s ability to absorb and conserve water. In the Great Lakes and St. Lawrence forest region of Ontario, dendrometers on sugar maple trees have revealed that a single, three-day heat wave in the spring is enough to cause lower growth rates. After one such recent event, the trees stopped growing earlier in the season, and, the next year, they grew less than average.
In the Black Rock Forest, a nearly four-thousand acre conservation area in Orange County, New York, west of the Hudson River, Griffin and Hise have attached wireless dendrometers to around sixty trees: red maples, sugar maples, birches, hemlocks, pines, spruces, and oaks. The data is already changing Griffin’s understanding of the growth cycles of trees. “We didn’t know what we were going to learn,” Griffin told me. “We set them up in early September, and I was making wild interpretations about the data—but the real story was still coming.” That April, Griffin started to see radical changes in the oak trees’ data, indicating that growth had started. But, confusingly, the trees had no leaves. “How in the world have they started growing two weeks prior to leaf out?” he wondered. It turned out that cell division and radial stem growth started two weeks before leaf development—something that Griffin hadn’t known even “after twenty years of being a professor and working with trees.” Similarly, Griffin predicted that the trees would continue to grow until their leaves began to fall, in autumn. Instead, growth came to a halt in mid-July. He now believes that trees suspend growth for the second half of the summer in order to store carbon, which they use to grow new wood before the leaves come out the following spring.
In the northernmost boreal forests of Alaska, where trees and tundra meet, Griffin and his students have installed thirty-six dendrometers on white spruce trees. There, warming is occurring two to three times faster than the global average, and the tree line is predicted to drift north as temperatures rise. This may or may not be a boon for spruces; nobody knows much about what controls tree growth at the northern tree line. It could be that—even if forest fires or new pests don’t harm them—warmer weather will weaken the integrity of the trees’ structure. “The complexity of biological systems makes prediction difficult, and in ecology that complexity just radiates,” Griffin said. “Everything is connected to everything else.” Detailed data can reveal connections that would otherwise stay hidden.
Trees, it turns out, grow mostly at night. In May, after weeks of cool spring weather, the temperature in Brooklyn spiked above eighty degrees. Logging into the EcoSensor Network, I could see that, within a few days, the cells of the London plane tree in Prospect Park had begun to divide. Every night, the wood and bark of the cambium—the cellular tissue between a tree’s bark and wood—enlarged until the sun came up. Then, during the day, as the temperature increased, the tree went through small micron-level contractions—shrinkage caused by tension in its molecules as they pulled water up from the roots and delivered it to the leaves. In the sunlight, the pores of the leaves would open, letting the water out and the carbon dioxide in—a whole tree, from roots to branches, breathing.
“You walk by a tree every day and it looks the same as it did yesterday, as it does tomorrow and the next day, and you hardly realize it’s alive,” Griffin told me. “I have this pipe dream that people will appreciate trees as living, growing, changing, responding organisms instead of seeing them as static on the day-to-day time scale.” I sometimes checked on my tree half a dozen times a day, seeing in the data a vitality I hadn’t recognized before. I made frequent visits to the park, checking on some nest-building sparrows and running my hand over the tree’s mottled bark.
One afternoon, a light rain became a torrential downpour. The wind blew one of two sparrow nests on a branch to the ground. As the thunderstorm arrived, the plane tree’s roots seemed to take a long drink: in a single minute, from 2:42 to 2:43 P.M., the tree’s trunk grew nearly five millimetres. At 6:13 P.M., thunder boomed, drowning out the sound of an ambulance on Ocean Avenue; at 6:40 P.M., the rain seemed to fall horizontally, and the tree grew another three millimetres. I was sitting under it, on a rock, watching as the raindrops faded. A mute swan swam past on the way back to its nest, and two catbirds scavenged the ground, pecking in the gathering darkness. At 8:19 P.M., the sun set and the birds disappeared into the canopy. I went home. The data shows that, long after I left, the tree continued to contract and expand by fractions of millimetres, minute by minute. It is at least a hundred and forty-five years old.
