Weighing the world's trees: researchers are racing to determine whether forests will continue to act as a brake on climate change by soaking up more carbon

[ILLUSTRATION OMITTED]

In a forest just west of Chesapeake Bay, Geoffrey Parker wraps a tape measure around a young tulip tree. He jots the reading down in a field notebook, marks the tree with blue chalk and moves on to the next trunk. Parker spends about 10 seconds on each tree. Wrap, measure, record. Since 1987, he and others have logged more than 300,000 tree measurements at their plots in the Smithsonian Environmental Research Center (SERC) near Edgewater, Maryland.

This 1,070-hectare site is filled with tulip trees, oaks, beeches and other mostly deciduous trees. Some stout specimens have stood here for centuries. Others are just a decade old, sprouting from land that was recently logged. To keep tabs on the growth, the researchers measure their trees every three to five years.

All that patient record-keeping can help to answer two major questions about climate change: how much carbon dioxide pollution are forests mopping up, and will their capacity shrink over time? Studies from Parker's group and others reveal that trees around the globe are going through a growth spurt and are absorbing billions of tonnes of the greenhouse gas, meaning that forests are putting a brake on global warming. But there is no guarantee that forests will keep that up, Parker says. "I think of it like these performance enhancers that some stellar athletes use: it bumps up performance, but not for ever."

In fact, studies of some regions suggest that forest growth may already be slowing down. And humans are adding to the problem by cutting down trees, especially in tropical forests. Getting an accurate reading on the status of Earth's forests is hard because scientists cannot wrap measuring tapes around the roughly 400 billion trees scattered across the planet. So researchers are exploring ways to track forest growth more efficiently, using planes and satellites. And they are feeding all of their data into sophisticated computer models that are designed to forecast how trees will respond in the future.

Such forest measurements are sorely needed as nations wrestle with how to slow climate change. Some plans call for wealthy governments or private companies I to pay poorer nations in return for safeguarding the carbon in their forests. With a major international climate negotiation approaching later this year, and billions of dollars in forest payments potentially on the table, scientists are racing to advise countries and other stakeholders about just how much carbon trees are storing, and how long that carbon will stay locked up.

"The critical thing that matters is to what extent the biosphere remains a brake on the rate of global climate change," says Yadvinder Malhi, a forest ecologist at the University of Oxford, UK. That brake will weaken or disappear if forests take up carbon more slowly. Worse, if forests start emitting more carbon than they absorb each year, they could become an accelerator. If that were to happen, says Malhi, "it makes it all the more challenging for us to bring C[O.sub.2] down to avoid...