Surveys slash away at forest estimates.

Surveys slash away at forest estimates

Two new studies indicate Earth's forests may hold far less vegetation than commonly believed -- and therefore may be much less able to store the carbon dioxide emitted by human activities, primarily fossil-fuel burning. These findings, reported last week at the American Institute of Biological Sciences' annual meeting in Toronto, could complicate scientific and political efforts to balance the planet's carbon budget and to slow a climate-altering buildup of atmospheric carbon dioxide.

Green plants inhale carbon dioxide, exhale oxygen and harness carbon for growth. To gauge how much of the world's carbon dioxide emissions such plants can absorb, scientists need an accurate tally of vegetative mass, or biomass. However, maintains Daniel B. Botkin of the University of California, Santa Barbara, until now "there have been no statistically valid estimates of biomass for any large area of the Earth."

Two years ago, Botkin began surveying North America's boreal forests -- the largely coniferous woodlands running from the Arctic tree line down through Canada and dipping into the northern United States. His statistically representative sampling of 760 circular plots, each 10 meters in diameter, indicates they contain a mean biomass of 4.2 kilograms per square meter (kg/m.sup.2). Botkin says that adds up to 1.9 billion metric tons of stored carbon within the roughly 5 million km.sup.2 boreal forest he surveyed -- only one-third the total indicated by most previous assays of that forest.

He attributes most overestimates to ecologists' practice of trekking into known mature forests, measuring what's there and then multiplying those values by the presumed area of the forest. He thinks the "same casual [survey] techniques" have probably exaggerated biomass estimates for all other ecosystems. In contrast, Botkin randomly surveyed all regions able to support forests. This enabled him to identify not only existing forest but also areas cleared for agriculture, burned, logged or covered by bedrock or water.

Forest ecologist Sandra Brown of the University of Illinois at Urbana-Champaign reports that accepted biomass figures for tropical woodlands may be similarly exaggerated. As recently as 1980, she says, "we thought we knew what the biomass of the tropical forest was" -- generally about 35 kg/m.sup.2. But she says she and other researchers had obtained those estimates by averaging values from about a dozen places worldwide, using a total sampling of less than 30 hectares (0.3 km.sup.2). Having recently tapped into "statistically sound" forestry inventories conducted for economic reasons and stored at the United Nations Food and Agricultural Organization, she says she's now finding that tropical biomass values vary regionally -- from about 5 to 55 kg/m.sup.2. And because of "rampant" cutting and degradation of tropical woodlands, she says the average of 35 kg/m.sup.2 is no longer valid in many places.

For years, policymakers in industrial nations have pressured developing nations in the tropics to preserve their trees from overexploitation because forests play such an invaluable role in the global environment, notes Charles A.S. Hall at the State University of New York at Syracuse. Hall thinks Botkin's data hint at some hypocrisy in that stance by intimating that "we in the United States and Canada are also chopping down our forests as fast as we can."

Together, Botkin's and Brown's studies suggest the world's forests "have been degraded more than we had thought," says Sandra Postel, a natural resource analyst with the Worldwatch Institute in Washington, D.C. If so, she says, "there's even more reason to step up reforestation rates -- not just to recapture the carbon we've lost, but also to regain the ecosystem services [such as erosion protection and water-holding capacity] that we're obviously losing."