Global warming could reduce the range of plant biodiversity by more than nine percent by century's end, and poor countries least to blame for the problem would be worst hit, a study published yesterday says.

German biologists used the UN climate panel's computer models for possible temperature rise, and crunched through data on "capacity for species richness", or CSR, meaning the likely count of plant species per area.

In 13 out of 18 scenarios, global CSR "declined significantly" by 2100, by an average of 4.9 per cent. Under "B1", the most optimistic scenario used by the UN's Intergovernmental Panel on Climate Change (IPCC), the global average temperature would rise by 1.8°C by 2100. As a result, there would be a tiny gain of 0.3 per cent in global CSR, as flora benefited from rising carbon dioxide (CO2) levels.

But under the A1F1 scenario - viewed by many experts as a grim but realistic prospect - temperatures would rise by 4°C. In such a case, CSR would fall by an average 9.4 per cent.

Change, though, would be unequal. In far northerly latitudes, land locked in permafrost would open up to vegetation through warming, which implies uninhabited tracts of Canada or Siberia could be opened up to agriculture.

But deserts, savannahs, moist tropical forests and other habitats where humidity holds the key to species survival would be damaged by water stress. The Amazonian rainforest would be the most vulnerable of all.

One consequence is that "generalist" species that can adapt to change could expand at the expense of less versatile native plants that can only survive in a narrow temperature range, says the study. These could become rarer and even become extinct. "While in most temperate and Arctic regions a CSR increase is expected, the projections indicate a strong decline in most tropical and subtropical regions," say the researchers, led by Jan Henning Sommer of the University of Bonn.

"Countries least responsible for past and present greenhouse-gas emissions are likely to incur disproportionately large future losses in CSR, whereas industrialised countries have projected moderate increases."

The paper is published by a British journal, Proceedings of the Royal Society B.