The last Ice Age in Ireland ended about 10,000 years ago and, as the ice retreated, the island began to be colonised with plant and animal species again. Ireland came to be dominated by woodland about 7000 to 8000 years before the present (BP), when the climate was at its optimum for tree growth. Traces of these forests can still be found as the bog oak which is common in the peat deposits of the island. It was from the uplands that the trees first disappeared. There is some dispute as to the causes of this. It is possible that changes in climate leading to increased wetness favoured the formation of the blanket bogs that still cover much of the hills and mountains of Ireland. Some commentators blame the loss of trees and the growth of blanket bog on the actions of people. People had come to Ireland about 8000 BP and there is evidence in the pollen found in peat deposits that the vegetation started to change. About 5500 BP Neolithic farmers started to clear the thinner forests on the hillsides. These forests were dominated by elm (Ulmus species). Bronze age peoples cleared further areas and this is thought to be the reason for, in about 4000 BP, the growth of areas of blanket bog.
Despite their extensive deforestation, much of the native forest, particularly in the lowlands, remained intact until the Plantation period of the 17th century when large numbers of English and Scottish planters cleared much of the surviving woodlands. Again the drive was to provide timber for the new housing, fuel for the population and to clear land for crops and for grazing. The timber was also used in industries for making barrels and for making into charcoal for iron production. By the 18th Century timber had to be imported into Ireland.
Ireland now is estimated to have 4% forest cover, and much of this is recently planted rows of foreign trees such as Sitka spruce, of limited value for native wildlife. A few patches of woodland remain which may be remnants of the native forests, although some may be the result of planting and management over many years, by estate owners for example. These patches of woodland display enormous diversity. As Tomlinson says
" Rock, soil and slope changes, among other environmental variables, have led to the creation of distinct types of woodland - for example, ashwoods in some of the limestone areas, native oakwoods on basalt, granite and other rock types - while many areas of mixed woodland are also found." (Tomlinson, R., Vegetation, in Cruickshank, J.G. and Wilcock, D.N.(1982) Northern Ireland Environment and Natural Resources, QUB and NUU)
In Northern Ireland the fragments of the great native forests include Banagher Glen Nature Reserve in County Derry, which is mostly sessile oakwood (Quercus petraea). At Glenshesk, in the Glens of Antrim, are oak woods with an understorey of hazel (Corylus avellana). In County Down, the Rostrevor Nature Reserve has 79% mature oaks with a scattering of ash (Fraxinus excelsior), cherry (Prunus avium), rowan (Sorbus aucuparia) and sycamore (Acer pseudoplatanus). The last is not a native plant, originating in southern Europe. The Marble Arch Nature Reserve, in County Fermanagh has woodland dominated by ash.
Breen Wood Nature Reserve (see photographs and QTVR movie) in County Antrim is another example of an oak woodland which may be native. Although alder and birch can be found close to the stream which runs though the wood and in damp hollows, the wood is dominated by sessile oak (Q. petraea), with some pendunculate oak (Q. robur). Rowan (Sorbus aucuparia), hawthorn (Crataegus monogyna), holly (Ilex aquifolium) and hazel (Corylus avellana) form the understorey. The ground layer is composed of many varieties of ferns, grasses and other flowering plants. Most of the native forests were lost because of the pressures of agriculture and industry. The reason that Breen Wood survived is not known. However local people point to the name of the wood. It means "Fairy Palace" in Gaelic and may have been applied to a earth fort or rath which was once located there. Since it was very unlucky to interfere with land associated with the fairies, this may have been why Breen Wood was spared, while the woodland around it was destroyed.