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Africa may contain twice as much wood as previously believed, according to studies just completed for the World Bank and the UN. The findings have important implications for both the greenhouse effect and the conventional view that there is a ‘fuel wood crisis’ in Africa. Similar underestimates could have been made in many other parts of the tropics.

A British study for the World Bank, published last month, found that Africa probably has almost 70 billion tonnes of carbon stored in trees. A similar calculation made recently by the UN Food and Agriculture Organization (FAO) reached a similar figure, almost twice the previous estimates. ‘It seems that we have greatly underestimated the amount of woody biomass in tropical countries,’ says Phil O’Keefe of the University of Northumbria at Newcastle, who coordinated the World Bank report.

‘The crisis of fuel wood on the continent may have been seriously exaggerated,’ says O’Keefe. ‘Our findings also mean that there are many more trees to absorb the carbon dioxide that the industrial nations pour into the air from burning fossil fuels.’

The discovery could also help to explain the ‘missing carbon sink’, which has perplexed atmospheric chemists for several years. Current estimates of how much carbon dioxide is absorbed by oceans and by vegetation on land leave unexplained the fate of some 1 billion tonnes of carbon from the atmosphere each year. Perhaps it has been in these ‘invisible trees’ all along, suggests O’Keefe.

Past calculations of how much wood there is in Africa have originated with foresters. But commercial foresters are interested only in tree trunks. They usually ignore the wood in branches and roots. Yet this wood, which typically makes up about 60 per cent of the total wood in a tree, is widely burnt on domestic fires in Africa.

Another drawback of past field surveys, according to Richard Critchley, one of the authors of the World Bank study, is that ‘many estimates were made from roads or other convenient points’. Vegetation may be stripped bare for a few kilometres to either side of a main road, when beyond there is dense bush.

The World Bank study attempts to remedy these failings. It was put together over four years by academics from the universities of Reading and Northumbria, who used data from satellites and local surveys. ‘We have produced a detailed land-use map of Africa and combined it with information on the standing crop and yield of timber,’ says Andrew Millington, one of the authors.

One important finding, says O’Keefe, is that tropical rainforests are not nearly so dominant as sources of wood as had previously been thought. ‘There is a surprising amount of wood growing on farms in the arid and semiarid parts of the continent, which is vital to life in these areas but which has been largely missed by past surveys.’ Comparisons of the new estimates with past estimates from the FAO for individual countries show, for example, that there is four times as much timber in Mozambique, and twice as much in Nigeria.

Gerald Leach of the Stockholm Environment Institute has recently completed a similar study on Pakistan for the World Bank. ‘The official estimate was that less than 3 per cent of Pakistan is covered by trees, and that there is a catastrophic fuel wood crisis. But this just isn’t true,’ he says. ‘When we did detailed groundwork, we found that half the country’s standing stock of timber, and two-thirds of the growth, was in trees on farms that were simply ignored in the official statistics.’

Leach’s studies in Africa found the same. ‘We found a third of the standing stock in Kenya was on farms, and a similar figure for Uganda. I call these ‘invisible’ trees, because commercial foresters ignore them and satellites can’t see them.’

The FAO has started to revise its own calculations. A recent assessment from the organisation took foresters’ estimates of standing timber and multiplied the figure by 1.8 to reach an estimate of woody biomass. But the results may still not be reliable. ‘The African database is extremely poor,’ says K. D. Singh of the FAO. ‘Most countries have not estimated their standing stock of trees for more than a decade.’

The new study, says Millington, should help the World Bank to decide which countries need help in creating alternatives to wood for fuel – by funding rural electrification programmes, for instance. It will also provide African countries with ammunition in international negotiations on measures to combat the greenhouse effect. ‘This could be their great bargaining chip,’ says Millington. ‘If woodland in African countries contains more carbon than previously assumed, then they may be able to attract international aid to help maintain that woodland as a global ‘carbon sink’.