Ecologists See The Primeval Wood For The Trees

The paper by Dr Fraser Mitchell of Trinity College Dublin provides important new evidence about the nature of ancient woodlands in temperate Europe, which has been the source of much controversy among forest ecologists. In 2000, the Dutch ecologist Frans Vera challenged the prevailing ecological view of ancient woodland as closed canopy forest by arguing that ancient woodlands would have resembled modern parkland because of the action of large grazing animals such as aurochs (primitive cattle), tarpan (wild horses), deer and wild boar.

There are no stands of primeval virgin woodland in Europe today, so ecologists analyse tree pollen, several thousands of years old, preserved in lakes and peat bogs to reconstruct primeval forest. But the analysis is open to interpretation, as Dr Mitchell explains: "As every hay fever sufferer knows, pollen blows about in the wind and this mixing of pollen from different sources places some imprecision on our reconstructions. This has fuelled debate among European ecologists as to how pollen data should be interpreted in relation to Vera's hypothesis. The crux of the debate is: did the forest control the grazers or did the grazers control the forest? The traditional view implies that forest structure dictated the carrying capacity of grazing animals whereas Vera's hypothesis dictates that the density of grazing animals controlled forest structure."

Modern ecologists can easily assess the impact of grazing on forests by using fences to exclude animals from areas under study. Dr Mitchell mimicked this experiment in primeval Ireland: "I could adopt this approach with primeval forests because all large grazing animals, with the exception of wild boar, were absent from Ireland in primeval times. The island of Ireland was like a huge grazing exclosure which can be compared to the rest of Europe where large grazing animals roamed. I could find no significant difference in the relevant primeval pollen data from Ireland compared to the rest of Europe. In other words, the presence or absence of large grazing animals had no measurable impact. My paper concludes that grazing animals did not exert a significant impact on primeval forest structure," he explains.

Understanding what primeval forests were like is important because woodland conservation policy in Europe seeks to recreate such habitats. "Current European forest conservation policy promotes closed canopy deciduous forest, but this policy would be misguided if the primeval forest was indeed more open, especially where the conservation aim is to maintain the forest close to its natural condition prior to human impact," Dr Mitchell says.