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."