Trees Breathe in Brooklyn

Using new technology, researchers can watch as trees grow, shrink, drink, and breathe. Illustration by Christelle Enault

When we start reading about using transducers to create precision dendrometers to see how a tree grows in Brooklyn, we know we are out of our league. But surprisingly readable, this story tells why it is important to be able to measure tree growth in real time:

A Day in the Life of a Tree

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 amphitheater 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.

Once we had read to this point the following paragraph led to an image search. What does this thing look like? The story did not show it, only described it, so our image search led here:

DendroCollection_2014-1024x237

Figure M1 Collection of point dendrometers (natkon.ch). The carbon frames are either T-shaped (for large stems) or O-shaped (for small stems or branches) and are anchored in the stem with stainless steel rods. Up to three sensors are attached to the different type of frames in order to measure different expositions at the stem or to measure stem radius fluctuations over bark and on the xylem separately.

And those images helped the following make a bit more sense:

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…

Read the entire piece here.

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