Strawberry Trees Forever

Flowers of the Strawberry Tree, Arbutus unedo

As an island, Ireland has less plant species than Britain, which in turn has less than continental Europe. It would stand to reason then to expect all the botanical species present here to be present in Britain, and then some. Yet this is not the case, with Ireland being home to a number of plants not present in the wild in Britain. There are three main classes of such plants, based on the regions that they are characteristic of, namely the Atlantic flora, the American flora and the Mediterranean-Atlantic flora (1). Of the last group, the most intriguing member has to be the Strawberry Tree, Arbutus unedo. As a member of the Ericacea family, it has wonderful bell shaped, heather-like flowers. The fruit give the plant its common name, being shaped like strawberries. These take a year to reach their full ripeness, and so the tree often has both flowers and fruit present, as was the case in the specimen pictured. It is of common occurrence throughout the Mediterranean (to the extent that it features on the coat of arms of the city of Madrid) where it tends to grow as a low shrub. Its range in Ireland extends along the west coast, with extant sites in Kerry, West Cork and Sligo (1). Here, it is more likely to be found as a tree and can grow up to 10 meters in height (2). A. unedo seemingly had a much wider distribution in Ireland in the past as it was a protected species under Brehon Laws here in the eight century. It was then known as "Caithne" and this is still present in many Irish place names (1).
 

Strawberry Tree, Arbutus unedo

So why is this plant present in Ireland but not Britain? The theory behind Ireland's floral distribution is that the communites we have with us now began arriving after the last major glaciation event c. 15,000 years ago from mainland Europe. Since sea levels were much lower at that time, the coast lines of Ireland and Britain must have been very different from what they are now and possible landbridges between these two islands and Europe may have existed (3). The exact nature of these is considered to be a possible reason for the presence of A. undeo in Ireland but not Britain. Post-glacial movement by human populations may similarly have brought the tree to Ireland, bypassing Britain (4).

Fruits of the Strawberry Tree, Arbutus unedo

However, the idea of Ireland being a clean slate for plant species after the last glaciation event has been challenged by recent genetic evidence from the Fringed Sandowort (Arenaria ciliata). This low growing plant, found in mountainous habitats is found in only one location in Ireland, on top of the mountain Ben Bulben in Co. Sligo. Analysis of a number of European populations has shown that the Irish plants in this location survived the last glaciation event and have been present here for at least 100,000 years (5). The presence of such a refuge indicates that other areas in the country may have similarly avoided the ice sheet, providing the possibility that A. unedo may have been in the country for a similar length of time. There is also the possibility that A. unedo was present in Britain prior to the last major glaciation event, but the next either left no refuges as were left in Ireland or, more likely, left refuges that were in areas where no populations of A. unedo existed.
 

Strawberry Tree, Arbutus unedo

And yet all of this is supposition and the real origins of A. unedo in Ireland remains a mystery. This may become lessened in the near future, however, as a research project being spearheaded by Colin Kelleher of the Irish National Botanic Gardens is sampling A. unedo plants from Ireland and from areas within its European to try to elucidate what the relationships are between populations. Results of this work will certainly help to bridge some of the gaps in our knowledge of this wonderful plant.

References:

1. O'Mahnoy, 2009. Wildflowers of Cork City and County pp. 118-138
2. Sterry, 2004. Collins Complete Guide to Irish Wildlife p. 188
3. Pilcher and Hall, 2001. Flora Hibernica pp. 1-17
4. Teacher et al., 2009, Heredity 102 pp. 490–496
5. Dang et al., 2012. Molecular Ecology Resources 12 pp. 894–908