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Pollen History found in Bogs

Each year pollen produced by the vegetation growing on the bog surface and the vegetation of the surrounding landscape is trapped in the upper peat layer of the bog. In fact there is so much pollen trapped in the peat of a bog that the poet Seamus Heaney's description of them as "pollen bins" is very appropriate.

These huge quantities of pollen grains can only be seen with the help of a microscope. Counting the different types of pollen grains in a sample of peat gives an idea of the vegetation present in the landscape of Ireland at a given time in the past. So by looking at the pollen content of successive samples down through the peat profile, it is just like turning the pages of a history book. You can record and see the changes in the vegetation history of an area. In this way the history of the vegetation of Ireland has been reconstructed since the last Ice Age.

A generalised pollen diagram for the last 10,000 years. Only six species curves are presented. These are Birch, Pine, Hazel, Elm, Grass and Plantain. The pollen diagram is divided into three zones. During the first zone from 10,000 to 7,000 years ago trees such as Birch, Pine and Hazel invaded the country. The second zone from 7,000 to 5,000, is the woodland phase marked by large amounts of Elm pollen. The final phase is from 5,000 to the present day beginning with the "Elm decline" and characterised by increases in pollen of grass and Plantain, indicative of farming activities. This zone ends with the reintroduction of species like Pine 300 years ago.

About 10,000 years ago Ireland was coming out of the grips of an ice age. The types of pollen found in the lake muds beneath the peat mass shows that the country was covered with tundra vegetation, composed of low growing cushion plants and hardy shrubs, similar to that found in northern Canada and Iceland today.

As the climate warmed up the ice melted. This allowed temperate, relatively frost sensitive trees to return to Ireland from the continent, by migrating over the land connections that existed between Ireland, Britain and Europe. Birch was the first tree to colonise the barren expanse of Ireland, followed by Pine and Hazel. By 7,000 years ago the woodlands of Ireland had become established, with Elm, Oak, Alder and Hazel growing on the fertile soils and Pine and Birch on poorer soils.

A peat monolith being collected from a peat bank in blanket bog in county Mayo. Once the monolith is freed from the peat bank it is wrapped in plastic to maintain the moisture content and returned to the laboratory for analysis where it is stored in a refrigerator or freezer.

At this time it is possible to imagine most of the country covered by woodland, interrupted by shallow lakes, which were becoming infilled with Reed and fen communities. By 7,000 years ago this infilling process of the lakes in the Midlands, caused by the accumulation of dead plant remains was complete and the Bog Moss (Sphagnum) became abundant on the surface of the bog. The arrival of this moss marks the beginning of acid peat formation and the start of raised bog development. In western Ireland, blanket peat development had also started by this time in low lying depressions in the landscape.

Pollen grains as seen on a "pollen slide" following extraction of the grains from a peat sample using chemical methods. 1: Hazel pollen; 2: Pine pollen; 3: grass pollen grain.

From 7,000 to 5,000 years ago "silence" reigned in the woodlands until the slash and crack of the first farmers, the Neolithic or Stone Age farmers was heard. These people cleared Elm and Hazel from the good soils to make room for their animals and to grow crops. This clearance of woodland may be associated with the sudden decline in the pollen of Elm, known in scientific circles as the "Elm decline" and the appearance of the pollen of weeds such as Plantain, Dandelion and Dock. Similarly these farmers also cleared the Pine woods on the poorer soils in the west of Ireland using stone axes to fell the trees which were then burned along with the underbrush to create agricultural land.

The clearance of Irish woodlands for agriculture initiated by Stone Age farmers continued for a further 5,000 years and resulted in the relatively treeless landscape familiar to us today. In the pollen records, this gradual reduction in trees is associated with the continuous presence of grass, cereal and weed pollen such as Plantain and Dandelion. In peat samples close to the surface of the bog it is possible to detect the pollen of exotic species including conifers, Beech and Lime, trees introduced to Ireland from abroad during the Tudor plantations of the late 16th century.

	The unearthing of the mysterious history preserved in our peatlands, tell us of a past unrecorded in history books. If bogs are allowed to continue their growth, the future impacts of man on the environment will also be preserved in the living peaty pages o this unique history book. Catherine O'Connell
Ling heather pollen grains.