Sharp increase in dying alder trees

A FEW days ago I was conducting river walks along the banks of the Barrow as part of the Carlow Floral Festival.

I have been doing this every August for some years now and this means I can monitor any changes that are happening to the wildlife.

One thing I noticed is a sharp increase in the number of dead and dying alder trees. This is sad because, although alders are not of any huge economic importance, I like them and they do have environmental value. They mostly grow along the banks of rivers and canals or on lake shores and their roots stabilise the banks and prevent erosion damage.

What’s happening to alders is similar to what happened to elms 40 or so years ago. They are being infected by a disease. This was first reported in southern England in 1993. It has since been discovered in Ireland and eight other European countries, and seems to be spreading quite rapidly.

At first the disease was suspected to be a virus. But we now know tit’s a new hybrid between two fungi. One of the parent species was already known to attack some broadleaf trees, but not alders. The other parent, rather surprisingly, attacks strawberries. They belong to a fungal family with the un-pronounceable name of Phytophthora.

The first signs of the disease appear at this time of year when some of the leaves start looking abnormally small, sparse and yellow and often fall early, leaving bare branches. Next dark, tarry spots appear on the bark on the lower part of the trunk. Over the following few years the tree will die and start to break up, although it’s common for a few living shoots to appear in the spring from strips of bark that are still viable.

There’s no cure and removing the dead or dying trees tends to distribute the spores of the fungus which can travel both in soil and in water. But there is one study that suggests that some degree of control over the disease can be achieved if trees are coppiced when they first catch it. This involves cutting the trunk in winter about 20 or 30 centimetres above ground level and allowing new shoots to grow from the stump the following spring. There are also a few examples of trees that have recovered spontaneously from the disease without any intervention.

Alders are the only native Irish tree that can fix nitrogen in the soil in the same way that peas and beans or furze bushes do. This means they flourish on difficult soils that are water-logged or deficient in nitrogen where other tree species won’t grow.

They are also the only Irish tree species that uses water as the principal means to distribute its seeds. The little cones that ripen in the autumn have bubbles of gas in them and the seeds inside are coated in oil, which means they are able to germinate even if they’ve spent several weeks floating around.

Partly because of this oil coating, the seeds are very nutritious and provide an important source of winter food for a number of species of small bird. The tangle of roots in the bank is a favourite site for an otter’s holt or a kingfisher’s nesting burrow. And the large, dark green leaves of healthy alder trees are one of the hallmarks of our waterways in summer. Let’s hope they develop some immunity to the fungus.

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