THE LIST OF THINGS keeping John Parker up at night is long. He's not plagued by nightmares — he has trees on his mind.

Two weeks earlier, the ecologist received a shipment of a series of large boxes to his lab. Inside them were trees. Close to 24,000 of them, bundled carefully. The plants are a year old now and no more than a twig and a root — more like something you'd use to roast s'mores than the big maples and sycamores you see when driving down the road. But Parker has only one more week, or maybe two, to finish putting all those baby trees in the ground before they dry out. Which is what he's doing now. Parker, an ecologist at the Smithsonian Environmental Research Center (SERC), is standing in the middle of a fallow cornfield near Edgewater, Maryland. The pale-yellow husks have been chopped away, and the scraps left behind are crunchy underfoot. Nearby, a handful of researchers and local volunteers crouch on the ground planting. It's easy enough work: You nestle one of the trees into a hole as wide as your fist. Then you slowly scoop loose dirt in around it.

The team is rushing because the young trees are about to bud. If they're not planted when that happens, the trees will lose water through their roots and could die. Parker thinks the team will make its deadline. But add in hungry deer and uncertain rainfall this spring, and he has a lot to fret over.

"Every day, I come here and say, 'I wish these things were 10 feet taller and I could relax,'" he says.

"Done," says Phil Bishop, hopping up after planting his latest tree, an eastern black oak. Bishop, like many of Parker's crew today, is volunteering his time to help build this native forest near Edgewater, Maryland. Photographs by Daniel Strain.

But that goal is what makes the work worth it. When Parker is done planting, this farm — which extends over six fields and 30 acres of old farmland near Maryland's Rhode River — will become one of the largest biodiversity experiments of its kind. He's not just interested in planting a new forest. He wants to figure out how you build a better forest. In other words, do forests grow best when they contain only one species of tree? Or many?

"It's a way of asking, 'Does biodiversity do anything?'" Parker says. And could local biodiversity contribute to the health and stability of the Chesapeake Bay watershed?

That's a relatively new area of study, says Emmett Duffy, a marine scientist at the Virginia Institute of Marine Science (VIMS). Before, he notes, scientists were more interested in how the planet's biodiversity arose rather than what it did. But over the last 15 years, they've begun to address what happens when biodiversity disappears. "What are the practical consequences?" Duffy says.

"We know that species do certain things in ecosystems, so what happens if you start losing a bunch of them?"

Parker is hoping he doesn't lose any of his trees — at least not so soon. He's digging holes for them now, lugging a motorized auger across the farm. With a whir, he punches the heavy machine into the ground, tossing flecks of loose soil into the air, then moves down to excavate the next hole. Parker may be the lead researcher on this project, but "I'm also the beef," he jokes.

Parker, who's in his early 40s, sports a layer of stubble on his jaw and has the laid back vibe of a California surfer. That's not too far off: he did a lot of scuba diving on the Chesapeake when he launched his scientific career in the mid-1990s. His new forays through forests mean "I don't get to wear flip-flops as much anymore," Parker says. But his research still has relevance to the estuary.

One question that he hopes to answer here is: "We're putting in this big new native forest, how does that change the water quality?"

That's a big issue around the Chesapeake. Nutrient pollution, from farm fertilizers and city streets, represents the biggest threat to the water quality of the Bay. Trees, however, can absorb those nutrients through their roots, stopping the pollutants from reaching waterways and becoming an important tool for cleaning up the Bay. But forests aren't as common as they used to be in Maryland. In fact, nearly 90 percent of the state may have once been covered with trees, compared to a mere 40 percent today.

Forests once blanketed Maryland, covering around 90 percent of the state's land area when early Europeans first settled along the coast. Today, trees cover only about 40 percent of the state and are especially patchy around urban centers like Baltimore and Washington, D.C. Trees still grow in abundance, however, in parts of the state, such as in Charles and Calvert counties.

Many watershed groups across Maryland have reenergized efforts to restore these local forests, lost over centuries of clear-cutting, not simply for the sake of a few more trees. But to help the estuary recover some of what it's lost.

Scientists do not yet know, however, what kinds of replanted forests will prosper and filter the most excess nutrients. Particular mixtures of tree species — say oaks mixed in with maples or maples with sycamores — could help to boost the natural, sponge-like powers of the region's trees.

Depending on the species, trees mature fully over decades or even a century. So it will take time to see this farmland, which has been managed by SERC for 35 years, transform into a forest capable of absorbing the most nutrients. But Parker hopes that scientists will continue to collect data here for 100 years or more, gathering information that will help others to plant the best forests for the Chesapeake.

The scientist's setup for studying forest biodiversity isn't complex. Just big. Parker has split this 30-acre farm into 75 different plots like a checkerboard. In the end, some cells will contain diverse groups of tree species and others only one, something scientists call a "monoculture." The plot we're standing in right now, for instance, will contain nothing but rows upon rows of black oaks. But other plots will have either 4 or 12 different species mixed in together. There will be ironwoods, dogwoods, red maples, sycamores, tulip poplars, and other common tree species.

Over the years Parker and, eventually, his successors will measure how the diverse plots compare to the monocultures. Do the diverse forests grow faster? Do they better survive harsh winters, long droughts, or damaging insect pests and diseases?

He's not sure what he's going to find. Because forest biodiversity studies require a lot of time and land, fewer than 15 experimental forests like Parker's have been planted worldwide. Most are less than a decade old. Still, there are reasons to think that biodiversity has an invaluable effect on forest health. Preliminary results from one decade-old experiment in Panama, for instance, suggest that diverse assemblages of trees have grown faster there, on average, than less diverse mixtures.

"The evidence is really accumulating," says Kris Verheyen, a professor of forest and water management at Ghent University in Belgium. Verheyen directs TreeDivNet, a network of experiments ranging from Panama to Borneo that explore the role that biodiversity plays in forest growth and restoration. Studies like these, he says, will eventually help forest managers around the world to plant healthier forests and better maintain existing ones. "Using mixed forests is a means to manage our forests in a more sustainable way," Verheyen says.

Dogwood (Cornus florida)
Tulip poplar (Liriodendron tulipifera)
Red maple (Acer rubrum)
Eastern black oak (Quercus velutina)
Maryland is home to dozens of species of native trees. Parker’s team is planting just 16 of the most commonly found species, including the four above. Photographs: red maple leaves, Jeff Dean; other photos, Wikimedia Commons.

That conclusion has played out across an array of ecosystems, including in grasslands, says Susan Cook-Patton, a Smithsonian post-doctoral fellow and Parker's lead collaborator on the project. "The experiments tend to show that [diverse] ecosystems are more productive," she says. "They're often better at taking up nutrients from the soil and the water column. They usually support more abundant and rich animal communities." She could add many more benefits to the list.

That's because, according to theory, biodiversity provides ecosystems with an important commodity — what scientists call "redundancy."

Here's how it works: Say a fungal disease spread throughout Maryland, killing off dogwood trees right and left, as happened in the 1980s. If you had planted a forest that was filled only with dogwood trees, then you would be in trouble. But if you had mixed tulip poplars in with the dogwoods, the outlook would be sunnier. Even though your dogwood trees would die, some trees would still be left alive in your forest. Diverse forests should survive longer because, as with the fungal blight, when one species of tree runs into trouble, another can take its place.

Parker offers a widely used analogy to describe biodiversity: it's like the rivets holding together an airplane. The more rivets, or species, you have, the smoother your plane, or ecosystem, will fly. Or, to borrow another of his phrases, biodiversity seems to be the "beef" that keeps ecosystems healthy in an unpredictable environment. That stability, in turn, should translate into a better forest: trees that grow taller, can better fight off insect pests, and maybe even draw more nutrients from water in the soil.

But no one's sure just how beefy biodiversity will make forests — in other words, what kind of benefit and how much of one will you gain by planting a diverse forest instead of a monoculture of dogwoods or oaks? And, looking across Maryland, how much of what makes forests so good for the Bay are we losing every year through deforestation and the loss of trees?

Because tree biodiversity studies like Parker's are still young, "Everything we know about diversity and trees comes from a tree going from this big," Parker says, holding his palm down by his knees, "to maybe four or five meters," which is the height those experimental trees have reached. He, for one, can't wait to find out how this forest will look in 30 years when the trees have risen far above his head.

Skip forward a few weeks, and Parker's trees still only come up to his knees. But they're leafing out now. Small, crinkled leaves have emerged from the maples and sycamores, making them look less and less like anonymous twigs every day. The team, it turns out, was able to plant their entire delivery in time. Even the deer haven't been too bad so far, Parker says.

But the work's just beginning for him. Parker may be looking after these plants, and monitoring their progress, for much of his career. Asked how he feels about starting an experiment that may outlive him, Parker responds in his typically laid-back manner. "Yeah, that's weird, huh?" he says. "Eh, you know, when do you ever get the chance to leave something behind that's not just words on paper? ...Other people will be learning from this for a long time."

That, at least, is something he can sleep soundly about.