Uncovering the Secrets of Forest Canopies

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Deep in a Maryland forest, George Rasberry sits on an upturned pail and peers through a camera with a calibrated lens at the trees above him. He is measuring the leaf coverage present at each level of the forest. Here and there in the wilderness, buckets are placed to catch samples of the leaves. These reveal what species are in the canopy and how much it grows annually.

Nearby stands a tower so tall I nearly break my neck to see the top, 165 feet up, a good 30 feet above the canopy. Aluminum arms stick out at intervals, each bearing instruments that measure forest conditions such as light, wind speed and carbon dioxide concentration. The anemometer at the bottom is still, but the ones higher up are turning slowly.

This is the world of George Rasberry, a biological science technician at the Smithsonian Environmental Research Center (SERC) in Edgewater, and of his colleague Geoffrey "Jess" Parker, a forest ecologist there. Parker specializes in the study of the canopy, the importance of which in forest science has only recently been recognized.

In the early 1980s pioneer researcher Terry Erwin collected great showers of insects from the canopy of a tropical forest. "The samples occupied sorters for years. We discovered lots of new species, many of them specific to particular tree species," Parker tells me. For instance, in one tree species in Panama, they found ten times the number of tropical beetle species they expected. When their findings were applied to the whole world, one estimate of the total number of all species went from 1.5 million to 30 million.

"The canopy," Parker says, "is a part of all vegetation. Even your lawn has a canopy. So do vineyards, shrubs, orchards. Most people think it's just the top, but you can't tell where the top ends, or its influence. The canopy is all the leaves, twigs, fine branches — all of the surfaces. It's the surfaces of a forest that drag the wind and make it calm, that take noise out and control rainfall. The forest canopy is home to a majority of earth's species. It combs pollutants out of the air, takes energy from the sun and in general controls the exchanges of energy or heat, and material, such as carbon dioxide and water vapor."

What use is all this information? Well, for instance, I wanted to know why the streams often go dry in the summer in this part of the world.

"It's a lot of invisible things," Parker says, "like taking up carbon dioxide. Everything in the forest, the wood and leaves, was once carbon dioxide. Fallen leaves represent some of the carbon dioxide that was taken up this year. And as carbon dioxide goes into the leaves and wood, water vapor is released. As it is released, there is less in the soil to supply the streams.

"Also, tree roots compete for moisture. Trees are just pipes that connect groundwater to the atmosphere: they're valved pipes controlled by leaves. This area has a precipitation of about three or four inches a month. The amount that plants take up and release increases during the growing season, so in summer, streams can go dry."

At SERC the objective is to study one specific area's canopies and from that learn the rules for all canopies. "It's fun to spend a decade looking at one particular forest, but we want to say powerful things about forests in general," says Parker. "One way to do this is to study the changes that a forest goes through as it ages."

SERC scientists are studying some 50 different forest plots. "We know their ages from taking tree-ring cores, and by now we know pretty much what forests in this region do, from the youngest stands, 5 years old, to one that might be 350 years old. This means we can put the canopy structure into context in the general scheme of forest development."

On SERC's 2,800 acres, besides its fine range of forests young and old, are salt marshes, a freshwater marsh, cornfields, winter cover crops and newly abandoned fields.

"The problem with studying a forest canopy," Parker observes, "is that it's a giant pain to get to. We sometimes climb trees."

The trouble with having to climb, though, is that researchers can only get, say, 80 percent of the way to the top, and the action is at the top 20 percent of the canopy. That's where the light is, where photosynthesis and production take place, where leaves have the highest nutrient content, where carbon dioxide is taken up and water vapor is released, and where the wind decelerates the most dramatically.

"There are other ways to get into the canopy. We can rent a crane, which allows us to come in from above, from the point of view of the atmosphere," he tells me. "When you go up on a tower crane with a video camera, the world is so different from life on the forest floor. It changes your way of thinking."

But a crane is expensive. Another solution is to build a tower. This only gives you a view of one place, and besides, it makes a hole of its own in the forest, skewing some observations. There are various optical techniques for measuring canopies, including a laser that provides a sample of light reflection. And then there are the balloons. These refrigerator-size helium bags were invented by George Rasberry. They can take light sensors and measuring devices up into the trees and reach all sorts of odd places. They also can be raised gradually to take measurements from the ground up to the top of the canopy. And they are cheap.

"Another thing we study is the bumpiness of a forest," Parker tells me. "Not roughness but rugosity, corrugation. A young forest can be very dark, compact, because its growth has been efficient. As the canopy gets taller, the leaves spread out and more light comes in. Some trees die, leaving gaps. New trees grow in, mostly species that thrive in dark places, and gradually the forest becomes more complex, bumpier, with more variations. Assessing a forest's bumpiness is a quick, cheap way to learn what's going on inside," he says.

A laser technique for measuring the bumpiness of canopies also came out of SERC's studies here in Edgewater, one of many SERC contributions to the science of canopies. These studies are becoming steadily more important as people try to manage such things as atmospheric gases and species diversity.

"If a city manager's concern is how to take up greenhouse gases like carbon dioxide, for example, I can advise him to conserve younger forests, which have a higher uptake of CO2. If his concern is species diversity, however, I would recommend conserving older forests. This whole thing feeds quickly into the question of land management," explains Parker.

The main forest that Parker studies was a pasture 110 years ago. (The SERC buildings stand close by on a former dairy farm.) It is 130 feet tall, with 31 species of trees up to eight inches in diameter. "At 110 years old," he says, "this forest is just postadolescent. It's still vigorous, still putting on more carbon than it gives off. This is my lab."

It is here that his staff measures the trees and calculates their ages, studies the structure of the canopy and analyzes the effects of pollution and other variables.

He also has a staff of volunteers who help measure water flow and leaf nutrients, and who keep an eye on the tree species that come and go in the forest.

"We study the seeding of plants," he says, "and you find that some seeds are eaten by birds, who then sit on a fence and excrete them, so you get a line of cherry trees, or persimmons, or sassafras along the line of an old fence." He added drily, "Poison ivy berries, too. I don't eat them myself, but the birds do."


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