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SURPRISINGLY, satellite pictures have not been used widely to monitor how fast rainforest is vanishing around the world. For this reason, estimates of the rate of destruction vary widely. Now, a team of researchers in the US has used satellite images to gauge the extent of the rainforest in Madagascar and the rate of its destruction.

Glen Green and Robert Sussman of Washington University in St Louis, Missouri, have monitored the rate of deforestation in Madagascar over a period of 35 years using satellite images and aerial photographs.

‘Remote sensing’ of the Earth from satellites is a powerful tool for monitoring deforestation and the diversity of living things, say the researchers. But they say that it has not been applied systematically, either in Madagascar or anywhere else in the world. They say that the images of the world’s rainforests gathered by satellites in the past two decades provide ‘a remarkable but essentially unused database’ (Science, vol 248, p 212).

Green and Sussman examined satellite photographs of Madagascar from 1985 and from 1973, and vegetation maps based on aerial photographs from 1950. They found that in 1985, there were 38,000 square kilometres of rainforest. This was half the amount that existed in 1950 and only 34 per cent of the extent of the forest when humans first arrived on the island between 1500 and 2000 years ago.

Biologically, Madagascar is one of the richest areas on Earth, and its plants and animals, most of whom live in the rainforests of the east of the country, are among the most endangered. Because the island has been isolated from the mainland of Africa for about 20 million years, many species unique to the island have evolved. Madagascar contains nearly 8000 endemic species of flowering plant alone.

Estimates of the extent of the remaining rainforests in the east of Madagascar have varied widely – from 25,000 square kilometres to 69,000 square kilometres. And estimates of how much forest is being cleared have varied by an even greater amount: they range from 100 to 1650 square kilometres per year.

None of these estimates was based on a reliable survey from the air or the ground, say Green and Sussman. In each case, researchers extrapolated from earlier estimates. The two Americans say that without using satellite images, it is impossible to tell whether the deforestation is increasing.

The threat to Madagascar’s rainforest comes mainly from people who cut down the trees for subsistence farming and for fuel. Madagascar’s population of about 11.6 million is growing at a rate of 3.1 per cent each year. The per capita income in Madagascar is only $230 a year.

Since 1972, satellite images of the Earth at optical wavelengths have been available from the Landsat series of satellites. In such images, Madagascar’s eastern rainforest appears quite distinct from the surrounding savannah and other vegetation. Chlorophyll, the green pigment in plants is a strong absorber of red light, so at red wavelengths the forest has a ‘dark homogeneous’ tone.

According to Green and Sussman, the deforestation of Madagascar’s eastern rainforest has created a ‘mosaic of small plots’ at various stages of clearing. These show up on Landsat images as an uneven tone. ‘Active deforestation fronts can be recognised by bright patches along the forest boundary,’ say the researchers.

The only areas where continuous forest remains, they say, are on the steeper slopes of mountains (see Figure). Where forest survives on moderate slopes, it is because the population density is low – in the northeast of the island, for instance. According to the researchers, forest boundaries near densely populated areas are distinct on the images. They tend to occur where the slope becomes abruptly more steep. In areas of low population density, however, the forest boundary is ‘diffuse’, made up of ‘a patchwork of isolated strands cut by a series of deforestated corridors’.