Harlyn Ordoñez Cruz measuring photosynthesis. Credit: Beth Perkins
Mornings here at La Selva Biological Station, Terry McGlynn counts bugs. He heads into the forest, maps out two one-square-meter grids and sticks a wire into the damp coating of dead leaves that line the ground of this jungle at the northern end of a volcanic national park in Costa Rica. He pokes at the earth with the wire in several spots, measuring the depth of the dead leaves, then crams them into a plastic bag and weighs the bag on a portable digital scale. On his knees and sweating in the sticky jungle heat, McGlynn empties the bag back onto the ground and hacks the contents into bits with a machete, 20 or 30 whacks, to chop up the twigs and other big pieces. This extracts the bugs that live inside.
McGlynn, an assistant professor at the University of San Diego, and his lab technician, Eduardo Lopez, take measurements from dozens upon dozens of sites throughout the forest and bring carefully chosen collections of leaf litter—everything that falls from the trees—back to the lab. It is a closet of a room, lined with beetle carcasses, feathers and seed pods; the walls feature intricate pencil drawings of bugs seen under a microscope. Here, the two sift the litter through a curious device made of shiny black fabric, which looks like a cross between a butterfly net and a magician's hat. Next, the litter goes into another shiny-fabric contraption, this time white, that's known as a Winkler and is handmade by a woman in Brazil. Sifted litter goes into the Winkler, things happen, and unlucky bugs—mostly springtails, ants and bark beetles—fall out the bottom and into a cup containing a 70% ethanol solution. McGlynn sorts these insects, then weighs and analyzes them.
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McGlynn's routine is part of an attempt to understand how animals affect the decomposition of dead plant matter in a tropical rainforest, which, in turn, is part of an effort to understand the way carbon moves through the forest. The project is spearheaded by a husband-and-wife team of erstwhile botanists named David and Deborah Clark, who, much to their dismay, may have come up with some really bad news for life on Earth.
Even small changes in the way this type of forest functions can have big impacts on global climate. Tropical forests cover 17% of the Earth's land mass, but account for more than a third of the world's plant growth and store roughly 40% of all the carbon in terrestrial life, plus a third or more of all the carbon stored in soils. "Tropical forests move more carbon in and out of the atmosphere than any other ecosystem," says Alan Townsend, an ecologist at the University of Colorado who studies carbon and nutrient cycling at another site in Costa Rica. "They're the United States of carbon dioxide emissions and uptake."
For years, scientists assumed these ecosystems would save us from ourselves: that they'd soak up some of the extra CO2 we're adding to the atmosphere by driving our cars and powering our houses and cutting down other parts of the forest for wood and crops. It makes sense: If plants use CO2 to grow, and there's more CO2 available, then the plants will grow more. This has been demonstrated in short-term experiments. The concept is called CO2 fertilization, and it was built into many early models that tried to predict the future of the Earth's climate.