Last week, on the leafy Icraf campus in Gigiri, Nairobi, probably the biggest assembly of African forest experts ever gathered to discuss Africa's forests as the earth heats up. The world is relying on trees to help fight climate change. For forester Mercy Derkyi, one of the 400 experts who attended the conference of the International Union of Forest Research Organisations, it all hinges on who owns them. "Since colonial times, in Ghana you do not own trees if you did not plant them," she explains. "So there is little incentive to protect mature trees even if you want to. A timber dealer can get a licence to cut it. All you are paid is compensation for damage to your crops."

Large trees provide a multitude of ecosystem services. Among others, moisture from the canopy of East African forests provides 35 per cent of the rain in West Africa. The classic water cycle of evaporation from oceans later descending as precipitation turns out to be overly simple. Much rain is terrestrial in origin. In the 1990s, to strengthen tree tenure, Ghana legislated that a farmer owns at least the trees she or he plants. However, Mercy's example illustrates one of the hurdles to maintaining trees in Africa, which Tony Simon, the director general of Icraf - the World Agroforestry Centre - described as the world's "least forested, poorest, and most rural continent".

In countries such as Uganda, women culturally do not own trees. In West Africa, raffia palm -- an exceptional multipurpose tree that provides wine and kola nuts among its 52 products -- passes from father to son. Females do not inherit them as "women are part of men", explained Cameroonian scientist Paul Donfack ruefully.

Few farmers hold land titles in sub-Saharan Africa, and lack of secure land tenure also impedes tree growing. Miombo woodland stretches across 10 southern and central African countries. In Zambia, if farmers leave it to regenerate, the state may allocate what appears to be idle land to companies, like foreign tobacco investors.

Tougher still, Africa may not have enough foot soldiers to play its part in restoring 150 million hectares of deforested and degraded land by 2020, the second goal to come out of the just concluded Rio dialogue. The countries of the Congo basin, with forests 30 per cent of the size of the Amazon, have the least capacity, and the Ethiopian government has only one forester tucked into the Ministry of Agriculture, according to Dr Victor Temu of the African Forest Forum.

Those foresters who are being trained need a revamped curriculum, said the Tanzanian professor, who wrote Africa's classic forestry text. "We used to assume two forest systems -- the natural one, which needed conservation, and plantations for production. Now we recognise the role of trees on farms. So we are adding areas like agroforestry. As we add, what comes out of the curriculum is depth of science. Our scientists are aging, and the new graduates are not good enough."

There was little sign of loss of shine, however, as younger scientists held forth. Using three mathematical models, forester Achille Assogbadjo from Benin demonstrated that climate change will shrink the habitat of tamarind. Kenya's Shem Kuyah talked on why "allometric biomass equations differ between trees". Some findings were paradigm changing. Old miombo woodland in Zambia is "moribund", slow growing and dominated by one tree species, said foresters Stephen Sympungani and Coert Geldernhuys. In contrast, cut patches grow faster, capture more carbon and can contain up to 75 species of fruit tree. "By protecting everything, we lose biodiversity" was the counterintuitive conclusion -- at least in miombo, which evolved to be disrupted by elephants and fire.

Even more counterintuitive, the much-maligned Eucalyptus has been found to be a nurse tree for indigenous African conifers, helping to regenerate natural forest, said Ethiopian forester Demissew Serte. So are re-greening and safeguarding forests possible in countries where wood provides up to 95 per cent of energy, and forestry is desperately underfunded? Payments for ecosystem services (PES) and for stopping deforestation and conserving forest carbon stocks (REDD+) should provide some hope.

Twenty-five per cent of Nairobi's water comes from a mere 5,000 farms on the Aberdares. City dwellers pay just 20 US cents per 1,000 litres of piped water, and by rights those farmers should receive some of that cash. Unfortunately, "nature does not send an invoice", said Icraf's Simons.

Furthermore, even if such payments materialise, they may evaporate as they trickle down down from UN to government to NGOs, with little reaching the small person. So African forestry should not hold its breath waiting for the money. "Not everyone who got rich had a business plan," intoned Zambian professor Emmanuel Chidumayo, a coded message to the foresters to just get going.