Studying spring's surges

Like brush-strokes in a watercolour, light-filled shades of green wash over the acre as trees break into leaf. How dark and tired are the spires of spruce against the luminous, lettucegreen mist of hawthorn, larch and birch!

If I am specially tuned to this, it is because I have been attending at the very birth of green - of half-a-dozen, totally different greens. For several springs now, we have joined in a Budburst Survey run by a UCD research botanist, Thomas Cummins, and this year it has been my turn, straight after breakfast each morning, to measure the rain for the Met Office and check the buds for Tom.

Cummins' survey has run since 1994 and may well, over time, show a response to global warming or even some regional change in environment: it's a long-term project, open to any surprise. This sort of study is called phenology, but its purpose, for the present score or so of participants, is almost incidental: it's simply a great pleasure to watch and record the darling, arboreal surge of spring.

We visit the same trees and branches each morning, from March to mid-June and mark a card with a number for the stage of their progress, from the very first swelling of tight-clenched winter buds to the full four leaves out and a confetti of bud scales on the ground. Such intense acquaintance gives a quite new experience of trees - the astonishing bud-burst of the sycamore, for example, flourishing fat catkins in rosy bracts, exotic as orchids; or the sudden, overnight leaves of lime, poised on their ruby twigs like freshly-emerged green butterflies.

I had never actually seen, until a week ago, that oaks, too, can have little catkins, spilling out ahead of the leaves. These flowers, I hope, will make our first acorns, on a tree that we planted itself as a sprouted acorn, all of 17 years ago. Nor had I registered that our young oaks burst their buds at such different points in spring. Earliest of all was the "John Healy Memorial Oak", grown from an acorn that memorable Mayoman put in his pocket in Strasbourg.

This summer, I mean to sort out once and for all which sort of oak is which among those we're growing and make a secret mark that I can glance at when I'm asked. According to the books there are plenty of differences between the sessile oak, Quercus petraea, and the pedunculate oak, Q. robur. The first is Ireland's national tree, the hardy, wiry oak of rocky, acid mountainsides; the other the "noble" oak of fertile, limy lowlands and big house demesnes. Outside of such giveaway settings, the differences lie in such things as waviness of leafshape, length of stalks on leaves and acorns, hairiness of undersides. But while they are certainly two species, every degree of intermediate hybrid exists so that, in some localities, even botanists can be foxed.

But where did Ireland's oaks come from? And does it matter that most of those being planted in Ireland, even in "conservation" schemes, have been grown from French and Dutch acorns?

The idea that oaks, like so many other species, arrived in Ireland from Britain is probably still firmly rooted, so to speak, in the public mind. The late Frank Mitchell may have helped this along in the first edition of his classic Reading the Irish Landscape, when he pictured "organized oakwoods advancing across a land-bridge". But he noted, too, that oaks were growing in the south of Ireland 9000 years ago, and that fact has since been fitted into a rather different picture.

At the maximum spread of the Ice Age glaciers, about 20,000 years ago, oak was pushed south to Spain, Italy and the Balkans. Genetic research by Trinity's Department of Botany, working with Teagasc in Kinsealy, now suggests that our Irish oaks came from Iberia and that the Italian and Balkans races failed to reach Britain or Ireland.

Indeed, Trinity's Dr Fraser Mitchell believes that most plants migrating into Ireland probably avoided the Irish Sea and bypassed Britain altogether. They could, he thinks, have migrated straight from the Atlantic coast of France to Ireland's south coast by way of the "forebulge" landbridge that rolled northwards, over thousands of years, as the earth's crust rebounded from the retreating weight of ice.

Radiocarbon dating of fossil pollens trapped in peat and lake muds make it possible to map the migration of trees over time. For example, it took the oaks five centuries to get from Cork to Lough Neagh, competing with elms all the way through the midlands and slowing right down as they neared the cold northern coast.

These "isochrone" maps raise other questions about the way trees spread. Both in Ireland and Europe they migrated at a faster rate than would match the first fall of seed at the advancing fringe, or dispersal of seeds on the wind. All the species were spreading at more than 3.5 km per generation, which certainly seems too rapid a progress for trees producing heavy hazel-nuts, acorns and beech-mast. In America, the extinct passenger pigeon has been credited with helping oaks to cross Lake Michigan, and Fraser Mitchell wants more research into "assisted migration" which may, after all, have helped some trees to jump the Irish Sea.

Fossil oak pollens don't easily sort into species, so the relative histories of sessile and pedunculate oaks in Ireland are somewhat obscure, especially since both species (and notably Q. robur) have been so widely planted. But we do have remnants of ancient woods in which old stands of each occur, and these are now a source for rearing native nursery stock for the big Millennium planting schemes.

At Charleville Castle in Offaly, home of the venerable pedunculate Charleville Oak, voluntary collections organised by Crann last October bagged more than half a million acorns. But they can't be kept for more than 12 months and good "mast" years tend to be scarce in Ireland's uncertain climate. Heavy rain during flowering in April and May can wash the pollen to the ground.

I look out to our first catkins, dancing and dripping in another icy shower, and try not to picture the raindrops turning gold.

Thomas Cummins, who runs the Budburst Survey, can be contacted at Contact by E-mail: thomas.cummins@ucd.ie.