Germany's once magnificent forests are feeling the effects of climate change, with one in every two oak trees officially sick, researchers said yesterday. A report says the state of the nation's forests has improved marginally since last year but that oaks are dying off at an alarming rate.

One in two of all Germany's oak trees is thinning. In Baden-Württemberg three-quarters of the oaks are showing damage. The phenomenon of Waldsterben, or forest death, is nothing new, but the latest annual report by Germany's agriculture ministry is likely to raise fears that pollution and climate change are taking a heavy toll. "The forest has recovered a bit [since 2004]. But there is still no clear recognisable trend," said Peter Paziorek, the junior agriculture minister.

Yesterday environmental campaigners urged the German government to do more to cut CO2 emissions. German trees were in bad shape, they said, because of freak weather in 2002 and 2003, and because of air pollution; over the past 30 years they had suffered badly because of climate change. "It isn't getting any better," said Rüdiger Rosenthal, of Germany's Friends of the Earth. "The worst affected trees are oaks. They can live for up to 1,000 years. This makes them vulnerable."

The trees are not equally affected: beech has recovered from the scorching summer of two years ago, and most pine trees are still healthy, the report says. But overall, nearly a third of all German forests are damaged. Mr Rosenthal said: "Over the next two decades the oak could die out completely."