Yet many scientists are worried that these changes will impact badly on environmental factors such as biodiversity and the hydrological cycle.

Alan Ziegler of the National University of Singapore and colleagues from the East-West Center, Hawaii, and Kunming Institute of Botany, China, have been studying the hydrological impacts of rubber planting. However, their project in China's Yunnan province from 2004 to 2006 was shut down by the Chinese government, which cited security reasons as the researchers were near the Myanmar border. "A clear explanation for the abrupt termination (they removed our equipment when we were away) was never provided to us directly," said Ziegler.

The researchers are now conducting new experiments on water use by rubber in Thailand and Cambodia and hope to set up a project in Laos, together with local institutions, the National University of Singapore, University of Hawaii, East-West Center Hawaii, and University of Arizona.

"We are behind in the game in terms of quantifying catchment water balances at various spatial and temporal scales needed to truly understand the hydrological effects of the conversion to rubber," Ziegler told environmentalresearchweb. "However we are not as far behind as we would be if we were looking at similar impacts of oil palm in Malaysia and Indonesia where millions of hectares are already affected."

According to Ziegler, the conversion of large tracts of forest – regardless of whether it's primary or secondary – to monoculture plantations almost certainly results in a loss in biodiversity. "The negative effect on biodiversity is perhaps a little more intuitive for the layman than the hydrology issue, especially when pristine forest is converted permanently in biodiversity hotspots," he added. "Knowing the impacts on hydrology involves quantifying the water use of the original land-cover versus that of the rubber plantations. Also one must assess the land degradation caused by planting the rubber, e.g. is infiltration and groundwater recharge affected by the planting activities? In some areas, stream desiccation could be explained by increased dry season water use, not simply land-cover change."

Ironically, in some cases governments have encouraged rubber plantations as a replacement for traditional swidden agriculture systems because they believed that they were more environmentally friendly. In such traditional farming systems, local people slash and burn an area of forest to clear it for crop growth before leaving the land fallow for a period to recover after harvesting.

"The [Chinese] government favoured rubber plantations over swiddening because it involved planting trees in a near-permanent setting," said Ziegler. "It was therefore perceived as an afforestation process that would restore ecosystem functions in degraded watersheds. A common misconception is that tree planting and reforestation are always the same thing, which they are not. Another misconception is that fallow lands are always degraded, which they are often not."

That said, the Chinese government is now starting to be concerned about the associated biodiversity loss.

Ziegler believes that the use of swiddening to grow cash crops such as opium has given traditional subsistence-level swiddening a bad name. In traditional swiddening the forest conversion is neither widespread nor long term so the landscape as a whole recovers quickly.

"One of the primary drivers in the demise in swiddening has been negative attitudes of governments," he said. "These views were often founded erroneously on cases where 'traditional' swiddening had evolved into other systems, including reduced fallow systems or cash cropping."

According to Ziegler, traditional swiddening (i.e. landscapes with ample forest, long fallow recover times, and short cropping periods) almost certainly had an imperceptible imprint on the environment, in terms of hydrology, erosion, and landslides – except maybe at hillslope or very small catchment scales. "In contrast, the intensive agriculture practices that have been replacing swiddening over the last several decades have much greater impacts," he said.

It's generally believed that the environmental footprint is less if other types of permanent agriculture land, rather than forest, are converted to rubber production. But that's not true for traditional swidden agriculture, where the landscape often includes fallows and forests at various stages of regeneration. That said, it's unlikely that we could ever go back to swiddening at an appreciable scale, says Ziegler, as there just isn't enough land any more to support it. In addition, most people now need cash on a daily basis.

Ziegler and colleagues reported their work in a perspective in Science and will also be publishing in Human Ecology.