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The mystery of the British oak

oak
A magnificent British oak, with his brother behind.

We know that the European oak (Quercus Robur and Quercus Petraea - typical British oaks) survived the ice age in the south of France and the north of Spain (DNA revealed that 98% came from that region). Just after the Younger Dryas period, around 8000 BC, climatic conditions in the north of Europe became suitable for oaks to grow there.

Most scientists suppose that the oak spread itself naturally to the north. But there are concerns about the arrival of the oak in Britain and Ireland. We know that Ireland was cut off from the British mainland by the rising sea almost immediately after the first stage of warming up of the climate, around 12000 BC. There is no way acorns could have travelled over the Irish Sea by themselves. Even in that case, they would have landed on a salty beach which would have prevented any successful germination. Therefore, we can safely accept that big trees with rather heavy seeds such as oak and beech were imported into Ireland by humans. But did the oak reach Britain by itself or was the tree imported?

7200BC-North-Sea
source: Karl-Ernst Behre, "Eine neue Meeresspiegelkurve für die südlich Nordsee", 2003

Path of the oak

According to the map above, the Rhine, combined with the Schelde and Meuse, flowed to the south into the English Channel. This map shows the latest insights into how the North Sea was formed. Previously, scientists thought that the Rhine-Meuse-Schelde flowed to the north eventually joining the Ouse. But that idea proved to be wrong. One of the reasons for the southerly course of the Rhine was the forebulge that existed in the north of Holland. The weight of the ice cap upon Scandinavia pushed the earth's crust down and the crust bulged up to the south of it. Once the ice had gone, this bulge gradually flattened. Since 18000 BC, the north of Holland has sunk some 30m. The south of Britain also used to be higher than today.

As the water rose quickly we can safely assume that the Channel extended fast to the north, rendering the distance from the south of France longer by each passing day. We previously supposed that the oak had some 1500 years to reach Britain via the land-bridge Calais-Dover. But this latest information tells us that the land-bridge at Dover was already gone way before the real end of the Ice Age. We must now suppose that Britain remained reachable by land for some 800 years, not much more, and that the detour to reach it became longer every day.

So the oak had barely some 800 years to spread itself from the south of France to Britain, if the North Sea did not fill itself too quickly. As this distance can be calculated as some 1000 km in a straight line, around 8000 BC and at least an estimated 400 km more around 7200 BC. 800 years for 1400 km means that the oak spread at an average pace of at least 1750 metres per year (a bit more than a mile), also in a theoretical straight line, which of course was not the case.

Biologists accept the fact that forests expand in general at a pace between 250-500 metres per year. Note that this is an average and valid for birch and other light-seeded trees. Acorns however, are among the heaviest tree seeds in Europe and on top of that, oaks need suitable soil to grow. An oak will not grow well everywhere. The soil must have been, so to say, prepared by other trees and then oaks 'take over'. Oaks are known to be climax vegetation. We know that jays eat acorns and that they spread them. But no serious study has been done about how far and in which direction the jays disperse acorns. Does the territory of the jays comprise parts of the forest without oaks? Or do they limit their territory to those parts where oaks do grow? Were they responsible for the fast spreading of oaks? Is a spreading with a pace of 2 km average per year credible?

We know that native Americans in the Californian region did not practice agriculture before the white man came, but also that they 'cultivated' oaks, for instance by clearing with controlled fire the shrubs, the undergrowth in the oak forests. Acorns are edible, but it is necessary to wash out the excess tannin with (a lot of) water. It is well possible that agriculture in western Europe was not entirely new to the indigenous population when the real farmers moved in. The native Europeans could have practised similar habits. They could have carried acorns with them during the Younger Dryas seasonal migrations (and shortly after) to the north. Those acorns must have been considered to be reserve food, in case the hunt was not successful. Acorns remain in good condition for a long time. At the end of the season, they threw away most of the redundant acorns, and then travelled south, for a new acorn harvest awaited them. At the beginning of the Holocene, the climate warmed up considerably and so the wasted acorns had their chance to grow on the spot.

There is a strong possibility that this is how oaks were imported into Britain and Ireland. Not the farmers imported the oak, the migrating hunters-gatherers did. It's even not unthinkable that they deliberately sow acorns. It's after all one of the easiest things to do and the young trees do not require attention. After a few years local acorns could then be harvested. They also must have been very aware of the excellent quality of the oak wood. Acorns from mature trees give one of the highest food yields per acre of any “crop”. Planting acorns was probably an excellent exercise with maximum yield for minimum effort.

There is more: a small proportion (2%) of the British oaks originated from the Balkan region, the ice-age refuge of Ivan. The authors of the DNA study propose that the oaks with 'Balkan-DNA' were imported by humans. Those oaks, I propose, were imported by the first farmers. They colonized the south of France (and imported Occitan) and some moved on to the British isles, with 'their' oaks.


Reference:
Distribution of chloroplast DNA variation in British oaks - the influence of post glacial colonisation and human management.
J. E. Cottrell, et al.