On a map on ecologist Liu Wenjie's computer, the subtropical southern tip of China's Yunnan province is slowly turning from green to red.

Rubber plantations - shown in red on Liu's computer screen - have supplanted nearly all the low-lying forest in the prefecture of Xishuangbanna and are now starting to encroach on the highlands.

Liu and other scientists are worried that the expansion of rubber plantations to feed China's voracious tyre industry, the world's largest, will destroy the ecosystem of Xishuangbanna. The province is home to China's richest variety of flora and fauna.

Three decades ago, jungles and high mountain forests covered about 70 per cent of Xishuangbanna, tucked between China's borders with Laos and Myanmar. By 2003, that proportion had shrunk to less than 50 per cent.

"With rubber prices rising like crazy, any tree that can be cut down has been cut down to make way for rubber," said Liu, a professor at the Xishuangbanna Tropical Botanical Garden, run by the Chinese Academy of Science.

Rainforests have been reduced to patches of protected zones in Xishuangbanna, one of the top rubber producing regions in China, as double-digit economic growth has caused increasing encroachment on China's last remnants of uncultivated land.

The official figure for Yunnan Province's rubber acreage is 334,000 hectares, about 43 per cent of the country's total acreage, but the real figure may be much higher.

China consumed 2.35 million tonnes of natural rubber last year. About 70 per cent of that was imported from abroad, mainly from Thailand, Malaysia and Indonesia, Southeast Asian countries that are also facing deforestation from expanding rubber plantations.

China has seen natural rubber imports nearly double since 2000 thanks to fast-growing domestic auto sales and as it becomes the top tyre maker and exporter in the world. China produced 330 million units of tyres last year and shipped nearly half abroad. Global tyre heavyweights, such as Goodyear, Continental AG Michelin and Bridgestone, are either setting up or expanding their plants in China.

To feed the demand, China is seeking to expand its natural rubber output which is expected to grow by 30 per cent to 780,000 tonnes by 2010, the China Rubber Industry Association forecasts.

But available land suitable for growing rubber is very limited as the trees need to be planted in sub-tropical or tropical climates, weather conditions found only in small parts of southern China.

Scientists say that both state-owned and private plantations are responsible for shrinking forests.

State-owned Yunnan Natural Rubber Industrial Co, the province's top producer, denies the allegation. The company says its rubber plantations are only expanding into previously deserted farmland, or re-grown forests.

In Xishuangbanna, rows of graceful, grey-green rubber trees are visible everywhere.

Rubber saplings grow next to banana trees in fields. They even appear on patches of land around farmers' houses, displacing bamboo, once the distinguishing feature of the local landscape.

Little by little, local farmers are encroaching on protected forest zones, researchers said.

"The shrinking rain forests pose an obstacle for the reproduction of elephants, as well as other animals and plants," said Liu.

Besides endangered Asian elephants, tigers, green peacocks and monkeys live in the rainforest.

"Bird species richness has also decreased with the conversion of the tropical rain forest into rubber plantations," the 2006 research paper said.

Even Yunnan Rubber acknowledges the impact on biodiversity of expanding rubber acreage.

"Rubber plantations are man-made forests, and it's true there is not as much biodiversity here," Mr Yin said.

Scientists said it was not too late to save the forests. But an unchecked expansion of rubber plantations remains possible as long as rubber prices keep rising and the government does not intervene.

Rubber prices have almost tripled over the last decade and are now around 20,000 yuan ($2,815) a tonne.

"Natural products will continue to get more expensive, there's no doubt about it," said Prof. Liu.