Science Spin March 2009

Ash: The dilution of a native son

By Tom Kennedy

The native Ash, Fraxinus excelsior, grows rapidly and produces strong, pliable timber, as perfect for hurleys now as it was in the past for spears. For forestry, the native Fraxinus excelsior is one of our most productive trees, and on the assumption that we could never have too much of a good thing, farmers and foresters were encouraged to plant more ash.

However, when many of the trees failed to thrive it became apparent that there is ash, and there is ash. Unfortunately, the non-native, Fraxinus augustifolia, had slipped into the supply, and no one had noticed the difference until the seedling trees began to grow.

In fact, explained Mary Scannell, one of Ireland's leading botanists and co-author of the Census Catalogue of Irish Flora, there are about 25 species of Fraxinus growing in Irish heritage gardens. Of course most of these are ornamental, or cultivars, such as Raywood, grown for its display of autumnal colours, but Fraxinus augustifolia, which grows widely in southern Europe, North Africa, and western Asia, would be regarded as a fairly normal forest tree in countries such as France.

Most people, said Mary Scannell, would not see much of a difference between F excelsior and F augustifolia, and because of this the distinction between the two has become blurred. That distinction, although not obvious at first is actually quite important. F excelsior thrives in damp Irish conditions, but F augustifolia, while it does grow quite well here, would really prefer to be in a more southerly climate.

Wholesale planting brought the issue to a head, but as she pointed out, the spread of F augustifolia is nothing new. For many years Mary Scannell had noticed that two forms of ash occur in Ireland, one sparingly branched with lighter green foliage, and in the other, darker-leaved tree, margin teeth were less pronounced.

As she reported in the Irish Botanic News of March 2007, Mary Scannell identified the sparingly branched tree as the non-native Fraxinus augustifolia, and she noted how the tops makes a different "fretted pattern" against the sky.

We might think of F augustifolia as non-native, yet, as Mary argues, it has probably been with us at least as long as rabbits. Unlike rabbits, the difference in ash was not obvious enough to be noted in what became the standard reference works on British or Irish trees, yet the invasion may well have begun at Hastings.

Invaders

The famous Bayeux tapestry, she points out, is a great record of a major event, during which the heavily armed Norman invaders crossed the Channel before engaging in a bloody hand-to-hand battle. In preparation for that crossing, thousands of trees would have been felled, particularly ash, because this is weapon grade timber. Would carpenters have made any distinction between one kind of ash and other? Absolutely not, said Mary, and when Norman soldiers fell, their splintered ash handles must surely have taken root on fertile ground. Arrows of relatively fresh wood could well have planted themselves.

The arrival in Ireland could have taken a similar course, and even if we discount discarded arms, F augustifolia is an excellent traveller by air. The winged seeds, produced in abundance every Autumn, are designed for flight. The Normans were planters, and what could be more natural than to bring some ash seedlings along with the southerly herbs. "Do you think those ashes stayed behind stone walls?" asked Mary.

They went where the wind blew, but the escaping ashes do not seem to have completely succeeded in becoming, like the Normans, more Irish than the Irish themselves. It could be that some have "married in" to the natives, while others have kept to their own. Both species occur in northern France, and as Mary mentions in her Irish Botanical News paper, a study by Myriam Heuertz from the Free University of Brussels revealed that hybridisation between the two there is common.