This is National Tree Week. An initiative since 1985 of the Tree Council of Ireland, in association with Coillte, the special event to celebrate trees takes place March 4th-11th.
Trees are not a natural group; they are just extra tall species and examples of them occur in many plant families. At least five metres tall is usually taken as the qualifying height that a mature adult plant must attain before it can be regarded a tree.
It must also stand erect on a single trunk, so Ivy scrambling up a pole to a height equalling or more than 5m does not qualify for the title 'tree'.
It is believed that the last ice age probably wiped out most life forms on the island of Ireland due to snow, ice and long periods of intense cold. If that was the case, it follows that the plants and animals that we have today must have arrived via the great post-glacial colonisation.
Twenty-two species of tree that got here under their own steam when the climate improved are regarded as natives: Alder, Ash, Aspen, Bird Cherry, Blackthorn, Crab Apple, Downy Birch, Hawthorn, Hazel, Holly, Juniper, Pedunculate Oak, Rowan or Mountain Ash, Sessile Oak, Silver Birch, Scots Pine, Strawberry Tree or Arbutus, Whitebeam, Wild Cherry, Willow, Wych Elm and Yew.
The seeds of Hazel and the two oaks may have been brought here by birds. The seeds of other trees, such as Willow and the two birches, are so light that they were possibly blown here from Britain by easterly winds. How many of the other trees got here is a mystery.
Since so much water was tied up in ice and snow, it is known that sea level was much lower at the end of the last ice age than it is today. To account for all our creeping insects and slow-moving slugs and snails, it is believed that we must have been linked to Britain and mainland Europe by dry land bridges.
As the ice melted, the sea rose, the land bridges were flooded, and Ireland became an island. When people arrived, they brought other non-native trees such as beech, sycamore, horse chestnut, spruce, larch and fir.
For information about events that are taking place near you for National Tree Week and/or advice on how to pick and plant a tree, see the helpful webpages of the Tree Council of Ireland at https://treecouncil.ie/.