Getting to the root of our oak trees

THERE are many species of oak tree in the world and quite a few of them can be found growing in Ireland — I have half a dozen in my own small collection.

However, only two species are regarded as native. They are the sessile oak, Quercus petraea, which is officially our national tree, and the pedunculate oak, Quercus robur.

The pedunculate oak is also known as the English oak and some authorities have suggested it’s not an Irish native species.

It’s hard to be sure because fossil pollen grains of the two species are virtually indistinguishable. But the probability is that both species are native, although some large pedunculate oaks in parklands or estate woodlands originated as acorns imported from Britain.

The two species have slightly different habitats: sessile oaks preferring rockier places with acid soils while pedunculate oaks like a bit more lime. For this reason wild stands of sessile oak are commoner in the west, in places like the Killarney National Park, and pedunculate oaks are commoner in the midlands, in places like Charleville outside Tullamore in Co Offaly.

Sessile means “sitting” and pedunculate means “dangling”. This refers to the acorns, which dangle on long stalks from pedunculate oaks and sit on the twigs of sessile oaks (above, left). This is the easiest way to tell them apart provided, of course, that the tree has acorns on it. Unfortunately oaks don’t normally produce acorns until they’re 70 or 80 years old and even mature trees only have a crop every three or four years.

Sessile oaks tend to have longer leaf stalks and there are slight differences in leaf shape. They also tend to be larger trees with a wider crown. But none of these differences is reliable enough for positive identification and the whole business is further complicated by the fact that the two species hybridise and the hybrids themselves are fertile.

There is one other possible clue. There’s a tiny wasp called the knopper gall wasp that lays its eggs in female oak flowers which results in the acorns turning into weird, contorted galls that look a bit like miniature volcanoes. The galls eventually fall off the tree and can be collected on the ground. The knopper gall wasp has only been in Ireland since the 1980s, though it’s spreading fast, and it’s a much better botanist than I am because it can invariably tell one species of oak from another. It only lays its eggs in pedunculate oak flowers, so if you find these strange galls in the leaf litter at the base of an oak tree you can be certain that it’s not a sessile.

The designation of the sessile oak as Ireland’s national tree only dates from 1990 and it is a slightly dubious choice. In Iron Age and Early Christian Ireland other species such as yew and ash were much more revered. The choice probably dates from the widespread misconception that Ireland was once covered by a great oak forest and that this was cut down and exported by the English invaders to build the roofs of great cathedrals and fleets of wooden warships. The facts are, that even when Ireland was completely forested, oaks were not the dominant tree species. Ash and elm were far commoner. And the bulk of the deforestation happened before the Norman invasion —- in fact it started over 6,000 years ago at the beginning of the Neolithic when farmers began to clear land for agriculture. There is also no record of Irish oak timber being used in any large building in England (though there is one record from France) and the Royal Navy only ever used insignificant amounts of it.

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