Thursday, 13 November 2014

How did oaks repopulate Europe?

In my post about oaks I talked about the oak tree and how useful this tree was and still is to people. In this post I would like to explain why I believe that people were as useful to the oak trees as the oak trees were useful to people. I believe that the influence that people had on the distribution of oaks in Europe could have been far greater then it is currently accepted. I think that the northward spreading of oaks from their glacial refugiums after the last ice age was actually the result of the northward spreading of humans from the same refugiums. I believe that it was humans who brought the oaks to the north of Europe. Let me explain why I believe that that was the case.

As I said in my last post, fossil pollen evidence in Europe indicates that during the last Glacial Maximum (20 000 years ago), oak species were confined to three main Pleistocene refugiums in southern Europe: Iberia, Italy and the Balkans. Then around 17,000 BC, the global temperature started to increase which resulted in melting of the great Northern Hemisphere ice sheets. This melt down lasted until about 4,000 BC, and the result was a global warming and the increase in sea levels. But this increase in temperature, warming and sea level increase was not uniform. If we look at the global sea levels, we see that during the period of the big melt down, 17,000 BC to about 4,000 BC, they rose by a total of more than 120 meters. This means that if the temperature increase was uniform, the average rate of sea-level rise would have been roughly 1 meter per century. However, when we look at the actual sea level rise figures, we see that the gradual sea level rise at an average rate of 1 meter per century was interrupted by two periods with rates of rise up to 2.5 meters per century, during period 13 000 - 11 000 BC, and period 11 000 -  9 000 BC. The first of these jumps in the amount of ice-sheet melt water entering the world ocean coincides with the beginning of a period of global climate warming called the Bølling-Allerød period. The Bølling-Allerød period was a warm and moist period that occurred during the final stages of the last glacial period. This warm period began around 12,700 BC and ended abruptly around to 10,700 BC. This is the beginning of the cold period called Younger Dryas, also known as "the big freeze". This cold period is thought to have been caused by the collapse of the North American ice sheets. The temperatures in the North Hemisphere were reduced back to near-glacial levels within a decade. Younger Dryas lasted between 10,800 and 9500 BC, when we see the second big jump in temperature and resulting sea level rise rate. 

During Younger Dryas period, most of Eurasia and North America had been covered by tundra with swaths of taiga.The tundra biome is the coldest of all the biomes. Tundra comes from the Finnish word "tunturi", meaning treeless plain. It is noted for its frost-molded landscapes, extremely low temperatures, little precipitation, poor nutrients, and short growing seasons. 

Taiga is a high latitude northern hemisphere biome consisting mostly of conifer (fir) and birch forest. Temperature regimes within some of the taiga areas are among the lowest on Earth, since there is a continental aspect to the interior portions of the taiga, making it colder than many locations in the polar desert to the north. Extreme minimums in the taiga are typically lower than those of the tundra. 

So this is what most of the north of Europe was like during the Younger Drias.

Younger Drias was succedded by the Preboreal stage of the Holocene epoch. Preboreal stage lasted from 8,300 to 7,000 BC. It is the first stage of the Holocene epoch. The Preboreal started with an abrubt climate change which resulted in rapid rise of the global temperature and rapid rise of sea waters.

During the Preboreal period, large quantities of tree pollen began to replace the pollen of open-land species, as the most mobile and flexible arboreal (tree) species colonised their way northward, replacing the ice-age tundra plants. Foremost among them were the birches, accompanied by rowan also known as mountain-ash (known as jasen in Serbian) and aspen (known as jasika in Serbian).

This is a rowan tree:

This is an aspen tree:

Especially sensitive to temperature changes and moving northward almost immediately were the dwarf and shrub juniper (known as kleka in Serbian) respectively, which reached a maximum density in the Preboreal, before their niches were shaded out. This is a juniper bush:

Pine soon followed, for which reason the resulting open woodland is often called a birch or a pine-birch forest:

In the yet warmer early Boreal period (7000 - 5500 BC) hazel and pine expanded into the birch woodlands to such a degree that palynologists refer to the resulting ecology as the hazel-pine forest. I am not sure exactly what these forests used to look like because they don't exist any more, but maybe they looked something like this. This is old hazel forest from Britain:

In the late Boreal it was supplanted by the spread of a deciduous forest called the mixed-oak forest. Pine, birch and hazel were reduced in favour of oaks, elm, lime (linden) and alder. The former tundra was now closed by a canopy of dense forest.

Those interested in climatic and vegetative conditions in Europe over the last 150,000 years will find this web page quite interesting.

It is currently accepted that oak started to emerge from refugiums in southern Europe as the ice caps began to retreat at the end of the Younger Drias around 9500 BC. Oaks arrived in North Central Europe by the 8500 BC and to Eastern Britain by 7500 BC. 

But how did the oak forests expand from the southern Europe into Central Europe and Northern Europe? 

How do forests expand? A tree grows out of a seed. This seed falls from the tree branch on the ground and if it falls onto a fertile soil it sprouts into another tree. If the seed falls straight down from the branch the next tree generation will sprout at the distance of the furthest branch reach from the tree trunk, which is maximum 36 meters.

If the seed is light and can be carried by wind, and is falling from a big height during a strong wind, it can be carried away maybe up to the maximum tree height distance from the trunk, which could be up to a 120 meters and maybe even more.

If the tree grows near water and produces fruit or seed which can float on water then the seed can end up anywhere down stream from the tree. If the tree produces fruit which has tiny indigestible seeds, and if the fruit is eaten whole by birds or animals or humans, then the seed can be spread far and wide and deposited during defecation. 

Knowing all this we can understand why biologists believe that forests expand in general at a pace between 250-500 meters per year.

But I believe that the above estimate does not apply to oaks. Oak seed is acorn, and acorns are among some of the heaviest tree seeds in northern hemisphere. So oak seeds can't be carried away very far by the wind so the maximum acorn fall distance from the tree trunk is not more than few meters from the edge of the tree crown. This means that the next generation of oaks sprouting from the fallen acorns will sprout at maximum 40 meters from the parent tree. Oak produces edible seeds, acorns, which are encased in a protective hard shell.

Animals which eat acorns actually eat the seed. To get to the seed they have to break the protective shell and then they chew and eat the seed itself. Once the unprotected chewed seed eds up in a digestive system, it has not chance of sprouting again. There are three creatures that do collect whole acorns, carry then away from the tree and then bury them. These creatures are squirrels, jay birds and people.

Jays are strikingly coloured members of the crow family. 

Like the other members of the crow family, they are shown to be extremely intelligent, and in fact to be as good at problem solving as a seven year old human at a problem solving task. It can be found over a vast region from Western Europe and north-west Africa to the Indian Subcontinent and further to the eastern seaboard of Asia and down into south-east Asia.

Its diet includes a wide range of invertebrates including many pest insects, beech mast and other seeds, fruits such as blackberries and rowan berries, young birds and eggs, bats, and small rodents. Its favourite food is acorns, and jey birds are often found near oaks. Jays collect and bury acorn in large cashes throughout autumn which they then use as food sources during the winter. A single bird can bury up to a several thousand acorns each year, playing a crucial role in the spread of oak woodlands.

Even though they are the birds of the forest edge, they are unlikely to venture into the open as they are very poor fliers and can be caught easily by any raptor bird. So jay birds probably contributed to the spread of oaks through the original hazel-pine forests but they were hardly responsible for spreading oaks into the open grasslands. Also how far would a jay bird carry the acorns from the oak tree before burying them? Jay birds are extremely territorial and would defend their territory from all intruders. This type of behaviour means that jays are unlikely to spread the acorns too far from the oak under which they found the acorns. I could not find any precise data about the territory radius of jay birds, but I would say that it is definitely not bigger than one kilometer. So the furthest jay birds would spread the acorns from the oak tree would be up to few kilometers and only through the already existing forest.

Squirrels are agile tree dwelling members of the rodent family.

They are found in many regions of the world, including Europe, Asia and the Americas. There are many subspecies of which the most wide spread is the red squirrel or Eurasian red squirrel which is common throughout Eurasia.

Squirrels primarily eat nuts and seeds, but will also eat berries, fungus and insects. Squirrels horde food in small amounts in several locations when it's abundant. Although the red squirrel remembers where it created caches at a better-than-chance level, its spatial memory is not very accurate and durable. It therefore will often have to search for its food caches when it needs them, and many are never found again. These forgotten caches of seeds, if they were buried in the ground, instead become seedlings.

Arboreal predators which pray on squirrels include small mammals such as the pine marten, wild cats, and the stoat, which preys on nestlings as well as foxes which will pray on grown animals in the open as well as birds, including owls and raptors such as the goshawk and buzzards. Squirrels are extremely vulnerable on the ground and in the grassland which is why they never venture too far from trees.

The squirrel foraging home range consist of several acres that overlap the home ranges of other squirrels. Squirrels do not defend these territories. However, there is a dominance hierarchy with others of the same species. In general, older males are dominant over females and younger squirrels. Although they seldom stray farther than few hundred meters from their nest in any one season, they are known to travel up to a few kilometres to get to a good nut tree.So the furthest squirrels would be able to spread the acorns from the oak tree would be a few kilometres and only through the already existing forest.

So neither squirrels nor jay birds were very likely to take acorns out of the forest into the open grassland and bury them there. But people are actually likely to do exactly that as human settlements were usually out in the open near the forest edge.

People would collect the acorns in the forest and then bring them to their settlement and store them in pits dug in the ground. Any acorn dropped anywhere along the way could sprout into a new oak tree. What is more, people could have carried acorns with them during their seasonal hunting or fishing migrations up north in the spring and down south in the autumn or in and out of the steppe while chasing large game. Those acorns could have been brought as the source of starch food, as acorns remain in good condition for a long time and are easy to transport. In my post about acorns in archaeology i presented enough evidence that acorns were a staple starch food of most European Mesolithic and Neolithic communities. So Mesolithic people traveling across Europe with bags of acorns is not a possibility any more but a certainty. Any acorns lost or thrown away would have sprouted along the migration or hunting routes and then spread from there radially.

Oaks are said to have spread originally along the river valleys and then from the river valleys they spread into the surrounding hills. So it is presumed that oaks spread along these river valleys through normal forest creep on the same side of the river and through acorns falling into the water and then being carried away by the current down stream and diagonally across to the other side of the river. In theory this sounds plausible. In practise this is impossible to happen as acorns don't float. Actually the only acorns that do float are fungus infested, worm ridden dead acorns. The way to separate good acorns from bad ones is to dunk them into water and throw away any acorn that floats. So acorns which fall from the oak tree into the water sink straight down to the bottom and would never sprout.

Even if by any chance any of the good acorns did end up floating, they would have been carried down stream, southward not northward, as all rivers flowing just above the glacial refugiums flow southwards. But oaks did spread northwards along the big rivers. They managed to cross from the Balkans into the Panonian basin across giant rivers like Danube and Sava which completely cut the Balkans off from the rest of Europe.

The only place where Balkans is connected to the rest of Europe by land is in Slovenia where we find the Alps. The maximum elevation on which oaks are found is about 1500 meters above the sea level so oaks would have had a great problem finding their way from the Balkans into the Central Europe through the Alps by themselves even with the help of the squirrels and jay birds.

But acorns could have easily been carried up and across the big rivers in dugout canoes by Mesolithic people who lived along the European coast and major rivers and who used rivers as waterways along which they traveled into the center of the European continent. These Mesolithic fishermen could have carried acorns up rivers as the source of starch food. And these acorns could have been dropped by accident or discarded anywhere along the river and on either sides of the river, where they would have sprouted into young new oaks. This would have allowed oaks to quickly spread from the Balkans refugium into the Central Europe and to be already growing in the Main river valley between 8500 and 8000 BC.

This is the Hohenheim chronology of the Oak finds in Germany. You can see that we find oaks in Rhine and Main valleys even before 8000 BC:

If the Main valley oaks came from the Balkan peninsula then they had to cross about 1500 km from the north Balkans to Main valley. During that trip they had to cross high mountains (Alps) and big rivers (Danube, Sava).  If the Main valley oaks came from the Iberian peninsula then they also had to cross about 1500 km from the north Iberia to Main valley. During that trip they had to cross high mountains (Pyrenees‎) and big rivers (Garonne, Rhone, Loire). If the oaks started their expansion from the Balkans or the Iberia immediately after the end of the Younger Dryas, around 9500 BC, that would mean that they had to move at the speed of over 1 km a year to get to the Main river valley on time. These must have been some very speedy, all swimming, all mountain climbing, all singing, all dancing oaks....Not like the oaks we find today...

We have the same problem if we look at the chronology of the British oaks. The first oaks were found to have arrived in Britain around 7500 BC. From there it took oaks 2000 years to reach Scotland. Based on genetic evidence, most British oaks came from the Iberian refugium.

The direct shortest distance between north Spain and southern Britain is about 1500 km. Oaks are supposed to have started spreading at some stage after 9500 BC and have arrived to Britain 2000 years later. If we accept what biologists are saying, that oak forests expand in general at maximum pace of 500 meters per year, it would take oaks 3000 years to get to Britain which means that they would not have made it in time to be found in Britain at 7500 BC. 

But the path that oaks had to travel from Iberia to Britain was not straight and there were many high mountains and rivers which oaks needed to cross on their way up north.

This would all extend the distance the oaks needed to cross on their march to Britain to about 2500 km. This means that oaks would need at least 5000 years to expand from Iberia to Britain by expanding at the rate of 500 meters a year.

To make things worse according to latest data the Rhine, combined with the Schelde and Meuse, flowed to the south into the English Channel.

The above 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. This would extend the distance that oaks had to cover between the north Iberia and south of Britain even more to maybe 3000 km. This means that oaks would need at least 6000 years to expand from Iberia to Britain by expanding at the rate of 500 meters a year.

But this is all based on a completely wrong premise that oak forest edge can move outwards at the rate between 250 and 500 meters a year, which is actually impossible. Oak forest edge can't move at the rate of even a 250 meter per year. The reason for that is that new oak tree will not produce any acorns until the tree is at least 25 years old. If we take this into account we get the real natural oak forest expansion speed.

If the acorn just falls down and sprouts it can't fall more than the maximum height or diameter of the tree which is not more than 120 meters. At best, if we take into account a possibility of an adventurous jay bird or a squirrel, which would take an acorn as far away as possible from the oak tree to bury it, that would still mean that the acorn would end up no more than few kilometres from the tree trunk. If the acorn then managed to sprout into a new oak, the expansion of the edge of the oak forest would then stop for next 25 years until the new oaks growing at the new edge of the oak forest start producing their own acorns. This means that the oak forests edge would not move 500 meters a year but at most 5000 meters (5 km) every 25 years. This would mean that even if we accept that the distance that oaks needed to cover to get from the north Iberia to the southern England or from the Balkans to the Main valley was only 1500 km, it would have taken oaks 12 500 years to cover that distance by "natural" means.

But if people brought oaks with them, travelling in their dugout canoes along the Atlantic coast, then oaks could have arrived to Britain from their Iberian refugium within a few years. Equally if people brought oaks with them, travelling in their dugout canoes up the river Danube and then down the river Rhine, then oaks could have arrived to Britain from their Balkan refugium within a few years. 

This means that there is a strong possibility that oaks did not spread naturally back into northern Europe from the Mediterranean refugiums . There is a strong possibility that oaks were imported into Britain and Ireland, and into the rest of northern Europe by the migrating hunters-gatherers and fishermen. For these Mesolithic boat people rivers were not barriers.  They actually used rivers as their main migration pathways. This is why we find that the oak expansion pattern is along river valleys and then radially into the hills. 

There is actually a direct proof that acorns were were spread into northern Europe from their Mediterranean refugiums by people. And it can be found in Walsh Marches. Based on genetic evidence, most British oaks came from the Iberian refugium. But not all. The oak population from the border between England and Wales (the Welsh Marches) possessed a haplotype which is common in the Balkan area. This means that it is possible that Britain had been recolonized by oaks from more than one refugium. How did these Balkan oaks arrive to England and Wales? The distance between Balkans and Wales is at least 2500 km. Just imagine how long it would have taken Balkan oaks to arrive to Wales if they were to be spread only by acorn drop, or by squirrels and jay birds...And by the way the closest oaks possessing the same genetic type as Welsh oaks, are scattered individuals along the north coast of France and in a clustered population near Rennes, 300 and 450 km distance from the Welsh Marches. This completely removes the possibility that the Welsh Marches oaks arrived by means of forest creep. They had to be brought to Wales from the Balkans by people.  The only question is when.

It's even not unthinkable that the Mesolithic people has deliberately sown acorns. It's one of the easiest things to do. You stick the acorn into the ground and forget about it. The young trees do not require attention and can take care of themselves. It takes 25 years until new oaks started to produce acorns. But once they do start to produce acorns, they would continue producing them for hundreds of years and would give one of the biggest yields per year of any edible plant. I believe that this must have been known to Mesolithic people and there is more and more evidence that Mesolithic people deliberately planted and cultivated "wild" crops millenniums before "agriculture" was invented. 

I will talk about cultivation versus agriculture in one of my next posts. Until then have fun. 


  1. Really great explanation. One thought to me is Jays (or perhaps squirreks or others) actually stealibg cached acorns and therefore moving and increasing the natural distance travelled by acorns. Although i also think as you say, humans did move acorns 000' s of km' is really the way. Ted Green talks of 'Viking Bubble Wrap'...that is when Vikings rauded they would carry off their ill gotten gains. Sometimes, perhaps putting them into urns, pottery jars etc and using acorna as a shock absorber to protect the delicate goods.

  2. I would not rule jays.

    "The maximum elevation on which oaks are found is about 1500 meters above the sea level"

    From Wiki:
    "A typical forest bird park. Frequently occurs in deciduous and mixed with a rich structure (especially oaks), but also in small forests between fields and meadows, thickets, clearings, plantings midfield, quite often in parks, orchards and gardens, semi-open areas with groups of trees, from the the upper limit of lowland forest in the mountains."

    "Even though they are the birds of the forest edge, they are unlikely to venture into the open as they are very poor fliers and can be caught easily by any raptor bird. So jay birds probably contributed to the spread of oaks through the original hazel-pine forests but they were hardly responsible for spreading oaks into the open grasslands. Also how far would a jay bird carry the acorns from the oak tree before burying them? Jay birds are extremely territorial and would defend their territory from all intruders. This type of behaviour means that jays are unlikely to spread the acorns too far from the oak under which they found the acorns. I could not find any precise data about the territory radius of jay birds, but I would say that it is definitely not bigger than one kilometer. So the furthest jay birds would spread the acorns from the oak tree would be up to few kilometers and only through the already existing forest. "

    From Wiki
    "Autumn is very active, then collects food supplies for the winter, especially acorns. Jays can then be combined into a family or larger flock, consisting mainly of young birds, and move to smaller or larger distances (average approx. 600 km). In a beautiful, sunny autumn days, you can sometimes see individuals northern trek south. Then the sky a flying jay see one after another, which, though flying alone remain in sight. It happens that saw local individuals stop in a place to rest and to satisfy hunger in the forests of beech and oak (at this time the fruits of these trees are the main food). Most birds, however, leads strictly sedentary. This autumn activity, but not leading to the departure, was the source of the creation of saying "choose how Jay overseas" (some only migrate to southern Europe, but did not fly away)."

    These birds are much more efficient in planting oak trees than people.I live in the city over 2000 meters in a straight line from the forest. Jay I see around the house like they hide acorns in the garden and pots on the balcony:-). If I had not plucked these "seedlings", I would have oak wood rather than the garden :-) .They are very smart and hard working, and what is important , the river crossing is not a problem for them.

    pozdrawiam jaropl

    1. Hi Jaropl. Thanks for your comment. I didn't discount Jays. I think they are very important secondary dispersers of acorns together with squirrels, and this is why I talk about them in my article. But they could not reach north of Europe fast enough for oaks to appear in fossil records when they do.... I actually give the full maths in the article...Only people could have done it.

  3. This comment has been removed by the author.

  4. Thanks!
    Are you writting about Querqus robur?
    Some Q. robur produce acorns when they are five-ten years old.
    Congratulations, it's a good article.

  5. Hazelnuts have a spotty distribution in central Scandinavia, and are often found in stone age sites, and also in piles when harvesting peat. I there is a strong likelihood humans actively managed and spread hazelnuts as well!

    1. I couldn't agree more. I have been meaning to write an article about the first plant species which spread into Europe after the last Ice age. Most of them are edible and very very useful. And can't be spread by wind and birds... Strange don't you think?

  6. Very interesting article! Here in North-Easternn North America the White-tailed deer (Odocoileus virginianus) feeds on oak acorns and spread them. I believe that the european roe deer also feeds on oak acorns so perhaps you should add this mammal to the list.

    1. Thank you very much Roger. This is very interesting. Has it been confirmed that the deer actually spread the acorns or just eat them? Can acorn survive in the digestive tract of the deer? Is it still able to sprout when it comes out the other end. I presume they do chew the acorn.