LONDON - Plans to pay tropical countries to protect forests under a U.N. pact to fight climate change are flawed and risk alienating voters in rich nations, Britain's top adviser on forests told Reuters.

Clearance of forests to create farmland in developing nations emits about a fifth of the greenhouse gases blamed for climate change. Trees store heat-trapping carbon dioxide as they grow and release it when they rot or are burnt.

"I believe there are real flaws in the whole model of avoided deforestation which the U.N. (climate body) says it wants to use," said Barry Gardiner, British Prime Minister Gordon Brown's special envoy for forests.

Major tropical countries felling their forests fastest, especially Indonesia, argue their trees provide a global service to slow climate change and have lobbied for compensation not to chop them down, as part of a new U.N. deal for combating global warming.

But problems include estimating the threat of future deforestation, and therefore the level of any payments for "avoided deforestation".

"To say I'm going to take your taxes and I'm going to pay other countries around the world for not doing what we think they may have done otherwise under some hypothetical model, that doesn't strike me as a very good argument," Gardiner said.

Almost all nations at a round of fractious U.N. climate treaty talks in Ghana last month expressed support for including ways to avoid deforestation in a new U.N. pact meant to be agreed in Copenhagen by the end of 2009.

Gardiner proposed instead that rich countries should simply make payments to tropical nations based on the size of existing forests. If countries continued to log or burn they could be expelled from the scheme.

"There is a very simple, very positive, direct relationship between the payment and the forest, between the product and the payment for the product," he said.

"Trees are performing a function for the world, sequestering carbon, and you pay for that function."

The U.N. talks on forestry focus on slowing deforestation and have made little progress in finding parallel incentives to reward countries which are not clearing their forests. They may otherwise earn nothing and even have an incentive to start logging again to take up slack in timber supply.

"I'm not seeking to blame Brazil and Indonesia for pursuing their national interest, that's inevitable. What I'm trying is to look at a model which functions equally for Gabon, Ecuador," Gardiner said.