Surveys Slash Away at Forest Estimates

Article excerpt

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. …