The biggest causes of deforestation in tropical countries are population growth in cities and agricultural exports, a finding that should shape decisions on preventing forest loss, experts say.

Under December's Copenhagen Accord, rich countries are pledging about $US10 billion ($A11.6 billion) over the next three years to help poor countries tackle climate change.

A big but so far unspecified chunk of the cash will go on programs to prevent loss of tropical forests, which is a major source of greenhouse gases.

Beyond 2012, tens of billions of dollars per year could be primed if a planned UN pact on curbing climate change comes to fruition.

But environmental scientists publishing on Sunday in the journal Nature Geoscience cautioned against a rush to favour schemes that are unlikely to work.