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Environment

Fungal disease threatens Europe's ash trees

A dangerous fungal disease is spreading across the continent, affecting ash trees. But there is hope that the species won't disappear altogether, says Danish biologist Ditte Olrik.

Deutsche Welle: Trees infected by this fungus were first reported to be dying in large numbers in Poland in the early 1990s - by the 2000s, the disease had spread to other European countries. 90 percent of the ash tree population in Denmark is already infected. What is Denmark's experience with the fungal disease affecting ash trees?

Ditte Olrik: The experience was that we saw the first signs of this disease in 2003, but what was actually going on was not recognized until 2005 and that it was this disease which was affecting Danish ash trees. We saw the first signs of this disease in young trees, in young cultures. But now also middle-aged and older cultures or plantations are attacked.

Now Britain's 80 million ash trees are also under threat. You've shown representatives from the British Forestry Commission how the disease has affected your trees. What do the forests look like from your point of view?

All our stems are pretty heavily attacked and all trees seem to be affected. The situation is pretty bad. But there are a few trees that may have some resistance. It seems as if a few trees have some kind of a genetic mechanism which makes them able to withstand this disease. We can see that in stems which are obviously affected by the disease. Even in areas where many trees are affected, single trees seem to stay healthy. In our clonal field trials we see that a few clones or graftings are not attacked by the disease, while others are heavily attacked. And in our field trials we see that this kind of resistance is originating in genetics: healthy parents seem to get healthy offspring. And so, this kind of resistance is probably something in the genetic backbone of these trees.

Other European countries are probably very interested in the work you're doing in Copenhagen. Do you think all hope is lost for these ash trees, or is there some hope?

The glimmer of hope are probably those trees that we've seen to be healthy and which seem to be able to withstand the disease. They are not very common. Our expectation is that maybe one or two percent of the ash trees have this genetic resistance against the disease. But this autumn, together with the University of Copenhagen, we searched for these trees in the forests –-both in state-owned and in privately owned forest. Hopefully they can be the hope for the future.

What's the wider effect on wildlife?

It's always a loss when you lose a species. You lose some of the value. But ash trees also cover a very special niche in the forests, as they grow in these pretty wet areas. We don't have any other species that would be able to take the place of ash trees. What effect the disappearance of the ash tree will have on ash bogs - as we call the wet areas where ash trees grow - we don't know yet. But you can expect to have some effect on the ecosystem in general - on plants and on animals connected to these ash bogs. But we don't really know yet.

Is there anything you can do to prevent the spread of the disease to other countries, since it's spread by airborne spores?

It seems to be very difficult as the spores are very small and able to travel long distances. It's fairly difficult to prevent the spread of the disease.

What makes these trees so special, as far as you're concerned?

Well, first of all, it's a lovely tree. It has this very bright wood and the wood is very strong. Many will know the trees from wooden floor or from their furniture, where the wood of ash trees is commonly used. And people from the North of Europe will know the ash tree from Northern mythology, where the ash tree is the tree of life.

Ditte Olrik is a biologist with the Danish Nature Agency.

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