Pollen dating paints picture of our past

Another LifeThe feathery heads of flowering grasses quite change the look of the unmown bits of countryside, dusting over the fresh, spring greens of verges and meadows with the nondescript beiges of July.

The weight of pollen on the anthers of cocksfoot looks enough to bow down this sturdiest of grasses, but a breeze of a mere 10 kilometres per hour is enough to shake out its mass of male gametes and waft them off to find a female flower.

Grass pollen is what afflicts nearly all Irish victims of hayfever, but a glad immunity lets me off their misery. For me, "pollen count" has quite a different significance - one that takes me travelling to the past.

When, walking beside the lake, I picture the heathery birch forest that grew there in post-glacial times, the warm forest of pine and bracken that followed it, the saltmarsh that formed when swings in climate brought the ocean flooding in, there are facts to frame the pictures in my mind. The clearances and fires of Neolithic farmers, the "weeds" that took root among their crops - all this is on record in cores of peat extracted from the nearby bog.

It is 70 years since a Danish professor, Knud Jessen, famously brought his bog-drill to Ireland to demonstrate the scientific potential of ancient pollen grains trapped deep in the peat. Fresh out of college, the great naturalist Frank Mitchell became his courier and collaborator. Since then, pollen studies - now called palynology - have explored Ireland's landscape history from the late Ice Age right up to Tudor times.

An Ireland-wide database of all this work, now under assembly with EU funding by Trinity botanists Rob Marchant and Fraser Mitchell, has so far found some 360 sites scattered across the island.

Pollen grains are microscopic in size, but their outer cases are tough. Kept wet and protected from oxygen in peat or lake sediments, their species can still be identified after thousands of years. The cases vary in size and shape and have distinctive pores and patterns: under the microscope, they can look beautiful, like an ancient vase.

Identifying the grains, and interpreting their proportions at successive levels, has been refined over the decades. Pollen of different species differs in abundance and ability to travel in the wind.

Hazel once spread its golden catkins over the whole island, and its pollen from that period is 17 times more abundant than all other tree species put together. Scots pine pollen can blow in clouds for 1000 kilometres or more. And while the early cores gave the sequence of vegetation changes, caused by climate shifts, new plant arrivals, farming clearances, and so on, they were lacking in chronology.

This had to wait for radiocarbon dating. When it was developed at Queen's University, Belfast in the 1960s, in what became the Palaeoecology Centre, Alan Smith was able to show in Co Derry that a Neolithic clearance in the woods for short-lived fields could "reverberate" in the pollen record for about 350 years. Michael O'Connell in Connemara and Pete Coxon in Kerry found the same thing.

Radiocarbon dating has steadily improved, needing less and less material and producing more exact results. But new chronologies offered by tree-ring dating and studies of volcanic ash (also pioneered in Queen's) have dated pollen layers with even greater precision.

Ireland's bogs hold layers of tephra - ash blasted into the air in volcanic eruptions and finally falling to earth in glassy shards as fine as dust. Jonathan Pilcher and Valerie Hall have found tephra layers to match the eruptions of specific Icelandic volcanoes, recorded historically in 1104, 1362 and 1510. These mark time so sharply within the core of pollens that its patterns can be set against historical events - to see, for example, what happened to Irish agriculture after Black Death arrived in 1347.

Some results have shown people twisting the truth in contemporary documents. Tephra-dating helped Valerie Hall to revise the whole picture of Ulster's supposed woodland wealth and its rapid destruction 400 years ago. Sampling named sites in three counties, she found no pollen evidence, either for the highly-praised oakwoods or their felling.

On the bare and windswept Aran Islands, it is present-day impressions that are confounded. Michael O'Connell and Karen Molloy, of NUI Galway's palaeo-environmental research unit, have worked on a long core they extracted from the bed of a deep lake on Inishere.

Its 11,000-year pollen story has shown an island once covered with trees - oak, pine, elm, hazel, alder, birch and willow; then, later, the yew that became so prominent in the secondary woods of the west.

Some 70 years ago, botanist Robert Lloyd Praeger foresaw that pollen studies would bring "a flood of light" to exploration of the Irish landscape. In its computerised archive of ancient biodiversity, Trinity's pollen database will give this a new reality.