20,000 trees stretch their limbs toward a century of research

A solar-powered sensor station
SERC researchers use solar-powered sensor stations like this one to collect data from different BiodiversiTREE plots. The station is powerful enough to send information straight to the Terrestrial Ecology Lab over one mile away. (Marisa Sloan/Medill Reports)

By Marisa Sloan
Medill Reports

A forest unlike any other hides among the Chesapeake Bay’s winding back roads. Viewed from above, it resembles a patchwork quilt the size of 25 football fields. Look closer, though, and you’ll discover that each square is home to a unique mixture of native tree species — from sweetgums and sycamores to tulip poplars and tupelos.

Scientists from the Smithsonian Environmental Research Center planted the nearly 20,000 saplings in 2013 as part of an experiment dubbed BiodiversiTREE. Spearheaded by senior scientist John Parker, BiodiversiTREE manipulates tree species to investigate the importance of biodiversity for key ecosystem functions. Each of its 70 square plots contains either a single species or a randomly generated combination of four or 12 species.

“The old saying is that the best time to plant a tree is 20 years ago, and the second best time is today,” Parker said. “I feel the same way about long-term data.”

Forests cover a third of the Earth’s land surface and provide important ecosystem functions such as carbon sequestration in the face of climate change, soil nutrient stabilization and watershed protection. Increased biodiversity delivers added impact on these functions.

Parker and his colleagues are discovering that these relationships are often more complicated than meets the eye, but they and generations of scientists to come will have plenty of time to piece together the mysteries — because the project is planned to continue for more than a century.

“One thing we could find is that forest diversity buffers the effects of climate change by creating habitable microclimates better than a monoculture might,” he said. “The beauty of this experiment is that once we plant it, anything that happens with respect to climate change will be reflected in forest health. All we have to do is monitor and then interpret what we’re seeing.”

Saplings in an equidistant grid
Within each of the 70 plots, researchers and volunteers planted trees in an equidistant grid. The plots are heavily maintained throughout the year to ensure there is no cross-contamination. (Marisa Sloan/Medill Reports)

With almost 20,000 trees to keep track of, however, data collection can be an arduous task. Tree diameter and height, timing of leaf production, the culinary habits of deer and even numbers of birds and insects are all fair game for the researchers. Jamie Pullen, head technician at SERC’s Terrestrial Ecology Lab, recently found herself using mustard to hunt for and catalog earthworms. Yes, mustard.

“Basically, if you’ve bought mustard powder to cook with, that’s what it is,” Pullen said. “We’ve got a big water tank, and you fill a five-gallon bucket with this mustard powder and pour it onto the soil, and up they come!”

An oil in the mustard, allyl isothiocyanate, irritates the worms’ skin to the point where they crawl out of the soil for relief. It’s the same compound that makes wasabi and horseradish taste hot to humans, and it isn’t harmful to the worms beyond a slight discomfort that keeps some human spice-seekers coming back for more.

Pullen admitted she’s not an earthworm expert — or even an expert at hunting them. But BiodiversiTREE lends itself to a range of research topics for dozens of scientists with various expertise. For example, with the Brood X cicada invasion right around the corner, visiting researchers plan to investigate the impact of decaying cicada bodies on soil nutrients and tree growth.

“We continually have people coming in thinking about these things and adding new dimensions to the project, which I think is really cool,” Pullen said.

A monoculture plot of fast-growing sycamore trees
This monoculture plot of fast-growing sycamore trees (left) is easy to distinguish from the scrubby, slow-growing polyculture plot to the right and the natural forest behind it. SERC researchers believe the sycamores reach greater heights when grown together because there is increased intraspecies competition for light. (Marisa Sloan/Medill Reports)

Although the project is still in its infancy, researchers have begun to notice some trends. Slow-growing, shade-tolerant species on average have grown taller when sharing space with other species, likely because they benefited from a canopy of trees above them. Conversely, faster-growing, light-demanding species sprouted taller when planted among only their own kind due to increased competition for light — proving that the impacts of biodiversity on a forest ecosystem can be complicated.

The trees require a lot of maintenance now and will continue to need tender, loving care as they grow across the timespan of the project and beyond. For Pullen and hundreds of volunteers, that means braving ticks and extreme heat to mow between trees and ensure there is no cross-contamination between plots. Anyone who has witnessed the hordes of spiky balls dropped by a single sweetgum tree will be able to appreciate the task of cleaning up after thousands of them.

“We would no longer have our monocultures and our different diversity levels,” she said. “They would just be sweetgums and tulip poplars.”

Pullen said she has become more attached to the trees than she ever imagined, adding that all the toil is worth the reward. When she first joined the project six years ago, many of the scrubby saplings barely reached her knees. They have since shot upward, with some now towering 30 feet above her — nearly the length of a bus.

“I got really excited today because I saw some fruits and flowers on some of the trees that hadn’t yet matured enough to set fruits,” she said. “It’s amazing to think about how much they’re going to change in another five years and another 20 years. There aren’t a lot of projects like this, and I just feel lucky to be a part of it.”

Marisa Sloan is a health, environment and science reporter at Medill. You can follow her on Twitter at @sloan_marisa.