Heavy monsoon rains across Southeast Asia in recent weeks have turned Sumatra, Indonesia’s westernmost island, into one of the hardest-hit areas, in a recurring natural disaster that appears to be worsening by the season.

The death toll on the island of 60 million people from a rare cyclone, which triggered floods and landslides, rose to nearly 850, with hundreds still missing, authorities said. Environmental groups say rampant deforestation has exacerbated the devastation in Sumatra.

Logging and the clearing of forests for plantations and mining have stripped the earth of its protective cover. Photographs taken in several disaster areas showed huge piles of logs washed downhill by flash floods pouring off denuded slopes.

In the Sorkam subdistrict of North Sumatra, the widespread de

See Full Page