The Secret Life of Trees

Kevin Griffin uses remote sensing to track the daily rhythms of trees. His work is helping to bring tree biology to life for students and the general public.

Earth Institute
October 16, 2019
kevin griffing with dendrometer in tree
Kevin Griffin, a plant physiologist at Columbia’s Lamont-Doherty Earth Observatory, uses remote sensing to track the daily rhythms of trees. His work is helping to bring tree biology to life for students and the general public. Photo: Kevin Krajick/Earth Institute

By Lisa W. Foderaro

Kevin Griffin has studied trees his entire professional life. He has researched the ways the environment controls tree growth from New Zealand to Chile to Alaska, looking at forest ecology, plant respiration and global carbon cycles. He is just as passionate about investigating trees closer to home, like the chestnut oaks and red maples near his laboratory at Columbia University’s Lamont-Doherty Earth Observatory.

But Griffin admits to occasional bouts of academic envy as he considers his co-workers who research the wonders of wildlife. “I have colleagues who work on big fuzzy animals and I think I’m a little bit jealous,” he said recently from his office on the Lamont-Doherty campus. “I just love trees so much. But people are like, ‘Yeah, whatever.’ My dream for a long time was, ‘Can we make tree biology come alive somehow?’”

Griffin thinks he has found the answer in a cool device with a decidedly uncool name: a point dendrometer. Dendrometers, which measure tree growth, have been around for decades. In the old days, you would strap a band around a tree trunk and physically record the growth as the band stretched. The measurements captured an increase of up to an inch during the growing season. But thanks to new electronic and wireless capabilities, Griffin is using the point dendrometer, along with a wireless transmitter, to detect how a tree shrinks and expands during the course of a single day, as well as how trees grow over time.

The point dendrometer looks like a little piston attached to the bark of the tree. A flexible rod — the sensor — measures minute changes in the tree’s girth, as little as a few microns. (A micron is one thousandth of a millimeter.) Those changes are recorded by a data logger, which is also attached to the tree trunk, and the information is sent to Griffin’s computer, and those of other users, every 20 minutes.

closeup of dendrometer in tree
A closer look at one of Griffin’s dendrometers. Photo: Kevin Griffin/LDEO

Although Griffin didn’t invent this equipment, he is among the first to use them for citizen science and public engagement. He has been working to make the dendrometer equipment more affordable, so that private citizens can track tree growth in their own backyards. These citizen scientists could add to the data from his own network; currently he has 5 trees outfitted with the devices on the Lamont-Doherty campus; 60 at Black Rock Forest, a conservation area in nearby Orange County; 1 on Columbia’s Morningside campus; a handful in both Westchester County and Tenafly, N.J., and 36 in Alaska.

Through a collaboration with Black Rock Forest, Griffin has also created a website, the Virtual Forest Initiative, to monitor and analyze the data. He started studying trees in this way three years ago and has made some startling discoveries. For one thing, trees are continually changing shape. During the day, as water is drawn up through the outer layer of the tree to the leaves, the trunk contracts. At night, as it fills with water, it expands.

“It’s like dragging a stick of gum and pulling it and the sides move in,” he explained. “As the sun comes up and the water starts moving through the tree, the sides will come in just a little bit. A few microns. You will never see this with your eyes. But you can measure it.”

The “wow” factor that he has long hoped to instill in students and even backyard naturalists derives from the graphs that show the changes in the trunk circumference (or stem, as Griffin calls it) on the computer screen. His students at Columbia — as well as high school students who do research at Black Rock Forest — are now tracking the progress of dozens of trees.

chart of tree diameter from dendrometer data
In the top graph, the dark line shows daily light patterns, and the red line represents the girth of a red oak tree in the study. It shows the expected pattern of shrinking during the day and then refilling with water and expanding overnight. The bottom graph includes rainfall data (blue), and shows how the tree’s diameter swells after it rains. Source: Kevin Griffin

”My hope was to make an online tool where you could stream the data to see changes in real time,” he said. “Then you could use that for research and ask hard-hitting questions, but also introduce it to students and let them watch the trees grow.”

With more and more citizen scientists engaged in everything from bird counts to water sampling, Griffin also wants the tool to be made available to the general public, with homeowners attaching point dendrometers to beloved trees. As he explained, the bigger the network, “the more valuable it becomes” as it yields more and more data.

To make the dendrometer more affordable and accessible, Griffin teamed up with a former ecology student, Jeremy Hise, who has a background in computer science. With input from Griffin, Hise designed the data logger using consumer electronics parts, including a tiny yellow board that serves as the actual computer and a small antenna that transmits data from the tree.  Hise’s company, Hise Scientific Instrumentation, sells a complete dendrometer set, consisting of the sensor and data logger, for a few hundred to several hundred dollars, depending on the configuration. There is also an online tool, the EcoSensor Network, that Hise created so that users could share and analyze their data.

The vision of an ever-expanding network of dendrometers received a boost in August when The New Yorker ran a piece on Griffin’s research with the remote devices. Entitled “A Day in the Life of a Tree,” the article discussed the science behind the dendrometers while relating the experience of the author, M.R. O’Connor. In reporting the piece, O’Connor attached a device to a London plane tree near her home in Prospect Park and followed its progress on the EcoSensor Network.

“One afternoon, a light rain became a torrential downpour,” O’Connor writes. “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. 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 millimeters, minute by minute. It is at least a hundred and forty-five years old.”

Since the New Yorker piece ran, Hise has fielded dozens of phone calls from academics, nonprofit groups and private citizens who are interested in purchasing a dendrometer.

With a primary appointment in the Department of Earth and Environmental Sciences, Griffin has taught at Columbia since 1997. Before that, he practically ricocheted between the East and West Coasts in pursuit of academic degrees.

Originally from southern California, he went to Whittier College before moving east to earn a masters at the Yale School of Forestry and Environmental Studies. The next stop was the Carnegie Institution for Science on Stanford University’s campus, followed by a Ph.D. at Duke University (on the effect of C02 on two forest tree species). Then it was back out west for a postdoctoral research position at the University of Nevada in Reno. “I’m a western boy at heart,” Griffin allows when asked which coast he prefers. “I like the topography.”

Given his field of study and his love of the outdoors — he and his wife visited 28 national parks during his sabbatical last year — Griffin thinks about climate change perhaps more than the average person. He hopes that the point dendrometers will yield new insights into the effect climate change has on trees and vice versa.

“If you look at the global carbon cycle and the biggest pools or sinks of carbon, the ocean is huge, the soils are huge, the atmosphere is big and the plants are nearly as big as the atmosphere,” he says. “There is something like 780 billion metric tons of carbon in the atmosphere. But every year, 120 billion metric tons move from the atmosphere to the plants through photosynthesis.”

For Griffin, one of the biggest surprises from the dendrometer data was the fact that, at least for two of the three seasons, the trees he was tracking started to grow in March, about two weeks before the leaves came out, and stopped growing by the end of July. He suspects that, instead of putting on more girth, the trees were making and storing sugar in August and September. They then used that energy to begin growing the next year even before the leaves emerged to start the process of photosynthesis.

Griffin is starting to combine the dendrometer with other sensors — for light, soil moisture, precipitation and temperature — so that he and his students in his course on forest ecology can formulate hypotheses.

“What I love about this as a teaching tool,” he said, “is that you can see the data and say, ‘Oh look, the soil moisture changed or the light changed or the temperature changed.’ At the end of class, I can say, ‘Okay, make a prediction about what will happen next.’ By the next class, we have new data to see if their predictions were right. It’s a way to learn the scientific method.”

Just as exciting for Griffin is the prospect of putting dendrometers in the hands of younger students and citizen scientists so they can make their own discoveries. “I just imagine a kid or maybe my neighbor as they walk by a tree,” he said. “They see it, but they don’t ever think about its being alive. You just don’t think it’s active — that anything is happening. But when you see data every 20 minutes, and these changes hour by hour in the diameter of the stem, all of a sudden, it’s alive.”