The First Irish Trees

The wettest winter on record has finally brought us a cold snap. Temperatures that were touching the mid-teens have finally reverted to the winter norm and started to approach zero, bringing with them the long missed pleasure of the crisp, dazzlingly bright morning. Blue skies frame naked trees, an architecture that is all too absent from the Irish landscape. Despite having a reputation for being a verdant land, Ireland has long had the lowest area of forestry in the EU. Only 10.5% of the country is covered in forest (1). And while this is estimated to be at its highest level in 350 years (in 1928 the percentage cover was a measly 1.2%), more than three quarters of this are non-native coniferous plantations. The methods of cultivation of these forests, namely close and intensive planting which discourages lateral branching to maintain the bulk of the timber in one solid piece, renders such forests as areas of monoculture (predominantly Sitka Spruce, Picea sitchensis (1)) with a very low level of biodiversity. What we would refer to as native woodlands account for only a small fraction of Irish forests, but even calling them native is rather misleading as many of these are also themselves deliberately planted, such as Powerscourt estate in Co. Wicklow which dates to the first half of the 18th century (2). In truth, even trees in Ireland unmolested by humankind can only date from the after the most recent global glaciation period, when the vast majority of the island of Ireland was covered in ice.

Yet the history of Irish trees does extend back beyond the ice age, back into the mists of the Devonian period, some 350 million years ago when Ireland, as we know it now, was to be found near the equator. The present Irish landmass which had been previously divided by an ocean, now began to develop small lakes (3). These were surrounded by what many people believed to be the first tree, Archaeopteris, specifically Archaeopteris hibernica. Paleobotanical evidence points to plant species prior to this as tending to be ground covering. This was also thought of A. hibernica when it was first discovered, as the leaf like fronds were described separate to its woody trunk (called Callixylon) (4). This Callixylon could grow to impressive sizes, with fossils one meter in diameter and ten meters in length often being unearthed. Yet it was incorrectly identified as a Late Devonian conifer in when described in 1911. It took nearly 50 years for the link between Callixylon and Archaeopteris to be made. A chance discovery of a section of Archeaopteris with parts of the woody stem attached by Charles Beck allowed both Archaeopteris to be given the title of tree and the establishment of the extinct group of plants the progymnosperms (4). He was able to section an immaculately preserved sample and show that the attached wood was indeed Callixylon.

Archaeopteris hibernica fossil

Archaeopteris plants were non-seed bearing, producing spores much like modern day ferns on fronds that were arranged on branches. These extruded horizontally and in a helical pattern from a single trunk (5). Branching patterns were quite complex and longer living in comparison to other plants at the time allowed Archaeopteris to occupy more space, more efficiently. Indeed, it shares many of these features phylogenetically with modern seed plants (5). Its success was such that it was the pre-eminent vegetation of forests in the Late Devonian period though to the Mississippian.

Recreation of Archaeopteris hibernica

In old Devonian Ireland, A. hibernica was dominant. Wonderfully preserved examples of the fronds have been recovered from the Devonian-Carboniferous Kiltorcan Formation in Co. Kilkenny. Study of this area reveals a swamp like environment, with meandering streams feeding into pools surrounded by dense growth of A. hibernica (6).

References:

1. Teagasc.,2014. Irish Forests -  Annual Statistics. http://www.teagasc.ie/forestry/docs/stats/Ireland%20Forests%20-%20Annual%20Statistics%20Report%20-%20issue%202014.pdf
2. Pilcher and Hall, 2004. Flora Hibernica. Collins Press, Cork.
3. Clayton et al. 1979. Journal of Earth Sciences. Vol. 2, pp. 161-183.
4. Bora, 2010. Principles of Paleobotany. Mittal Publications.
5. Meyer-Berthaud et al. 1999. Nature 398 pp. 700-701.
6. Jarvis, 2000. Geological Society, London, Special Publications 180 pp. 333-341.