POPULATION and poverty are putting enormous pressure on the forests that cover and help sustain much of the Third World. People in search of new farm land and fuel take down an estimated 27 million acres of tropical forest a year, an area greater in size than Austria. Up to 40 percent of all tropical forest land has now been affected, if not cleared and lost.

The dimensions of the problem, which many consider the most serious environmental problem in the world, are laid out in an international "call to action" just issued by the World Resources Institute here, the World Bank and the United Nations Development Program. Because population is growing faster than trees, the clearing of land in one generation compounds the problem of the next. Wood provides half the energy in the Third World, 70 percent of the people there use it for their fuel -- and they have to range farther and farther to find it. Where they burn dung instead, they deprive the soil of nutrients. Stripped of trees and strong vegetation, the earth loses its capacity to retain rain water. Runoff and erosion increase, and the water table falls. When uplands are cleared there is greater lowland flooding. Where dams are built in part for purposes of flood control, their reservoirs silt up. Some countries are also systematically selling off their forests for foreign exchange.

The call to action affirms that there are means to arrest this cycle. Some are familiar conservation devices of the sort that were adopted here during the energy shortages and price run-ups of the 1970s: better stoves, more efficient heating systems, recovery of logging debris that once would have gone to waste. Others involve better use of existing agricultural land, some grazing controls, and of course reforestation.

The problems have to do with more than plants and soil. The report notes, for example, that in some countries better use of the land may argue for land reform, and that in some settings it might make sense for governments to act to raise the price of wood, to encourage both increased production and conservation.

The study warns that the world's major aid agencies will have to adjust as well. Forestry in the past has been a minor item for them, barely a blip on their budgets. Their emphasis has been on agriculture instead, and there has been little recognition of the relationship between the two. The report envisions a program that would cost $8 billion over the next five years, a doubling of present forestry aid, half of which it suggests should come from international lending institutions and the aid programs of developed countries, half from private and Third World sources.

World Bank president A. W. Clausen endorsed the report when it was issued last month, and U. S. Agency for International Development administrator Peter McPherson and Sen. Robert Kasten, chairman of the Senate Appropriations subcommittee on foreign operations, both commended it. There will be a meeting in The Hague later this month on what to do next. It is a distant but steadily worsening fundamental problem that cannot be brushed off.