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CLEARING hillside forests can trigger landslides, with disastrous
consequences. Now geologists have found a way to predict the location of
landslide hotspots, information that could be vital for safe logging.

“The total area at high risk from clear-cutting is fairly small, but the risk
of clear-cutting in those areas is very high,” says Dave Montgomery of the
University of Washington in Seattle. He says loggers should cut safe zones and
avoid dangerous areas, where landslides are 10 times as likely after
clearing.

Montgomery’s group spent a decade mapping every small landslide on a stretch
of mountain near Coos Bay, Oregon, after it was cleared in 1987. Of the 35
slides they mapped, 30 occurred in hollows into which sediment had been washed
from the hillsides. Montgomery says the most vulnerable areas tended to be steep
wet spots, and landslides were also common along roads cut into the
hillside.

Water weakens the soil as well as adding to its mass. In mature forests, tree
roots increase the cohesion of soil in risky zones. But after logging the roots
decay, and it takes years for young trees to develop their own root networks to
replace them. Montgomery found that more than half the landslides in his study
area occurred between three and five years after logging. Many of the slides
occurred in insignificant storms likely to occur every year or two, rather than
once-in-a-century events, he found.

He argues that there should be no logging at all in the highest-risk
zones—because they are so small, selective logging would not be
economically worthwhile and might cause problems.