Global warming could cause a dramatic decline in plant species diversity on the rangelands of the Qinghai-Tibet Plateau in southwest China, suggested a joint study made by the CAS Northwest Institute of Plateau Biology (NIPB) and Colorado State University (CSU).

Research into climate change and grazing conducted in the northeastern Qinghai-Tibet Plateau from 1998 to 2001 showed a decrease of plant species between 26 % to 36 %, said Julia Klein from CSU who led the research.

Global warming specifically had led to losses of 21% of medicinal plants and 25% of pasture plants, said experts at a recent joint meeting of the International Rangeland Congress and the International Grassland Congress held in Hohhot, capital of Inner Mongolia Autonomous Region.

The research was carried out at four sites of the Haibei Alpine Research Station, a facility in Qinghai Province run by NIPB, where the annual temperature is minus two degrees Celsius and the elevation is 3,200 meters.

Two sites were in grasslands and two in the shrub land habitats. The two types represent around 35% of the area of the plateau. Researchers fenced each 900-square-meter site and laid out 16 plots, where they simulated warming by using open-top greenhouses and grazing through selective clipping.

The greenhouses, each 1.5 meters in diameter and 40 centimeters high, were left on the plots year-round, elevating the average daily temperature by 0.6 to 2 degree Centigrade in the growing season. There were around 30 plant species in each.

The study showed medicinal plants had an average annual loss of4.9 species from 1999 to 2001, while edible plants had an annual average decline of 5.3 species, according to the researchers, who included John Harte of CSU and ZHAO Xinquan from NIPB.

The researchers said the plants' individual characteristics, such as their history and root depths, influenced their reactions to the warming.

For example, they found deep-rooted species, which lost an average of 20%, were less affected than shallow-rooted ones, which had an average loss of 39%.

The warming caused soil to dry, which was harmful to plants with shallow rooting systems.

The researchers also found warming lowered rangeland quality by decreasing the plants' productivity while grazing could maintain or improve it, by extending the plants growing season, for example.

Our findings suggest the rangelands on the Qinghai-Tibet Plateau, and the pastoralists who depend on them, may be vulnerable to future climate changes, Klein was quoted as saying.

Grazing could mitigate the negative effects of warming on the rangelands. For example, grazing management may be an important tool to keep warming-induced shrub expansion in check, she said.

Global warming, caused by greenhouse gas emissions, is the increase in the average measured temperature of the earth's near-surface air and oceans since the mid-20th century and its projected continuation. The average global air temperature near the earth's surface increased about 0.66 to 0.92 degree centigrade during the 100 years ending in 2005, according to studies.

More extreme weather-related disasters such as flood and drought, the melting of glaciers and the expansion of desert and rangeland degradation, are believed to be related to the warming trend.