The Planet Needs Prosperous Forests. These Scientists Are Planting More Than 33,000 Trees to Find the Perfect Species Blends

Ecologist Jamie Pullen opens the earth with an orange dibble bar, a shovel-like device with a slim, cleaver-shaped tip. Then, she plucks a roughly one-foot-tall sapling from a white sack full of baby witch hazel trees. She dunks its scraggly roots into a bucket of clear gel—which will help the youngster suck moisture from the soil—and plops its base into the slit she dug.

Only about 15,000 more trees to go.

Pullen and her colleagues at the Smithsonian Environmental Research Center (SERC) in Edgewater, Maryland, are building a 22-acre forest. It’s unlike most you’ll find elsewhere in the United States—or even the world. This one will be a carefully planned patchwork of plots with tightly controlled variables, such as certain species combinations, to test how they might affect the trees’ growth and environmental benefits.

The planting of 33,518 saplings from 20 species marks the beginning of a decades-long experiment called the Functional Forests project. SERC scientists aim to explore how tree biodiversity merges multiple reforestation goals, such as building fire-resistant and climate resilient forests; recruiting new animals, like vital pollinators; producing food and increasing timber.

The researchers, alongside volunteers, have been toiling away on 200 sections, many of which will house a five-species blend of trees. Other plots will contain a single species or no trees, for comparison. Select mixtures lean toward specific objectives, such as the timber-centric blend of sycamore, bald cypress, black cherry, loblolly pine and poplar, as well as the wildlife recruitment-centric combination of Chickasaw plum, black chokeberry, persimmon, hazelnut and witch hazel.

The main question is, “when you plant these mixtures, can you get more than one benefit out of a forest?” says SERC ecologist John Parker, who co-leads the project with conservation biologist Justin Nowakowski. Thirty-eight unique species combinations will help the team figure out the co-benefits of each. They’ll also investigate how tree spacing (and fencing off some of the trees to protect them from deer) might affect short- and long-term reforestation costs and goals.

The team has spent many of their workdays outside since last August, transforming a former soybean farm into their test bed. Even after a heavy snowfall in January, the researchers and volunteers dug holes in the hardened “snowcrete” and stuck thin bamboo stakes into the ground to map where each tree should go.

They began planting the trees—purchased from the Maryland Department of Natural Resources—in March, and they hope to finish in the next few weeks, depending on weather. A drought in Maryland this month has delayed planting, since the saplings wouldn’t fare well in dry conditions. But on a good day, a team of up to about six staff and eight volunteers can plant nearly 2,000 saplings.

The experimental forest’s status will be tracked with sensors that measure air and soil temperature, along with soil moisture, every 15 minutes. The team will also regularly fly a drone over the trees to measure their heights and shapes and record how they grow. And they’ll eventually add camera and pitfall traps and audio recorders to see which animal and insect visitors stop by.

Our long, complex relationship with forests

Humans have relied on resources provided by forests throughout history. As our population grew and we turned to large-scale agriculture and industrialization, humans cleared an estimated five billion acres of global forests—an area twice the size of the United States. People removed trees for crops, livestock, timber and mining.

Today, humans deforest about 25 million acres each year. But people also recognize a need to restore these landscapes, as they provide myriad benefits to us and the organisms we live alongside. Our hulking plant companions help mitigate climate change caused by the heat-trapping carbon dioxide that humans produce at an unprecedented rate.

“Growing trees is a really wonderful method for soaking carbon dioxide out of the atmosphere,” says Susan Cook-Patton, lead reforestation scientist on the Nature Conservancy’s natural climate solutions team, who is not involved in Functional Forests.

Woodlands can also help prevent soil erosion, protect water quality and cool the surrounding environment. Some research even suggests that spending time with trees can boost mental and physical health.

In pursuit of those benefits, global organizations have made commitments to plant huge numbers of trees. These plans were largely catalyzed by the Bonn Challenge, an effort launched in 2011 by the German government and the International Union for Conservation of Nature to restore about 865 million acres of the world’s degraded and deforested landscapes by 2030.

However, some ambitious planting projects are poorly designed and managed. Many aim to create a monoculture, planting a single tree species because they’re working to get timber or some other commercial benefit, says Parker. But monocultures can easily be felled—pine trees are highly flammable, for instance, and bark beetles destroy spruce trees, he adds. “Biodiversity is about increasing productivity while mitigating risk,” he says.

Parker leads the similar SERC forest initiative, BiodiversiTree, in which researchers planted nearly 18,000 trees from 12 native species. Since that experiment began in 2013, the scientists have found that trees in a monoculture end up “kind of racing with each other,” growing tall and skinny to reach the top, says Parker.

Combining species, however, allows trees to spread out more, because they grow at different rates, he adds. “So, they don’t get as tall, but they get a lot wider—and that’s where all the biomass is, that’s where all the merchantable timber is.”

He and his colleagues discovered that trees in the 12-species plots produce up to 73 percent more biomass per year than those in monocultures, and that the diverse woodlands recruit 50 percent more spiders, which regulate pests in ecosystems. Additionally, the varied-species mini forests seem to have better microclimates, which have helped draw more birds.

Current reforestation guidance often relies on anecdotes and outdated numbers, says Matt Fagan, an ecologist at the University of Maryland, Baltimore County who is not involved in the project. The work at SERC, particularly Functional Forests, is “trying to inject science into the practice of tree planting,” says Fagan.

Functional forests beyond big-scale, regional research

Although the Functional Forests undertaking features just 20 species that can grow in Maryland, the insights gained can help restorations in other parts of the United States and the world. Tree traits, such as producing animal attractants like acorns or fruit, that work well together can be mapped onto other species relevant to different locations, Pullen says.

The research will add to a few dozen similar projects around the globe that together include hundreds of species, Parker says. Scientists can examine all their data to detect broader trends, such as whether tree species grow in similar ways worldwide and how different regional climates affect forests.

Large-scale tree diversity experiments are relatively recent developments: The oldest in a worldwide research consortium is a Finland-based project that began in 1999. Only a handful currently exist in North America, including another Smithsonian-founded forest, part of BiodiversiTree, at the Conservation Biology Institute in Front Royal, Virginia. SERC was made to “do big, long-term, whole-ecosystem things at scale,” says director Monty Graham.

“A lot of these trees that were planted can last at least 250 years,” Parker says. “So, as long as someone takes care of it, it’ll be here.”

Beyond those grand visions, the researchers want the knowledge gained from Functional Forests to be used in people’s everyday lives. For example, shoppers can often find seed packets with wildflower mixes at their local hardware stores and garden centers, Pullen says. If someone wants to start a forest, even if it’s just a few trees, the team aims to help them figure out the best possible species combination for their goals.

“Reforestation is something many of us can do, if we’ve got the room,” the Nature Conservancy’s Cook-Patton says. “I’ve got my own tiny little reforestation project in my little postage stamp of a backyard, and it’s really a gift to the future.”