The Return of the Forest: how trees reconquered the European continent after the last Ice Age

People who are interested in the ecology of European forests know that the current climatic gradient has a big influence in shaping the distribution and diversity of tree species in the continent. Shrubs and evergreen broadleaves in the southern Mediterranean areas, broadleaf and mixed forests in temperate regions, conifers in north and in the Alps. However, this is just a snapshot. Forests in Europe have varied greatly over the last million years due to the alternating cycle of glacial and interglacial period. Research on this topic is carried out to better understand how trees spread back after the Ice Age.

In this post I would like to discuss a topic that have always fascinated me and it is related to the past. Between 25 000 and 18 000 years ago, large part of Europe was not more than a layer of ice. The entire Scandinavia, half of Britain, Switzerland, northern Italy and Eastern Europe were covered by blankets of ice. In Central Europe you could have perhaps been able to walk without crampons, but you would have rarely encounter a tree, as the landscape looked like current south-west Greenland. These zones stretching from Northern Germany to Northern Spain were called ‘mammoth steppe’, an ecosystem that, although treeless, supported the presence of a large fauna (not for nothing called Megafauna). The question is how did European forest tree survived these cold and long-lasting conditions? Where did they survive? And how did they climb back north?

Vegetation of Europe today and during the last glacial maximum. Modified from: https://goo.gl/oZpAXW

To understand and reconstruct the history of forests over the past millennia, scientists use different techniques based on fossil records. The most widely used for vegetation reconstruction is certainly the pollen analysis. This technique was invented in 1916 by Lennard von Post, a Swedish geologist who had the idea to study the pollen that has been preserved in lakes and bogs and to create stratigraphical diagrams showing how the percentage of different pollen types vary with depth, and thus with time. When many sequences are available, it is then possible to create the so called ‘isopollen’ maps, which show us the pollen percentage of a certain species, which can be then related to the presence/absence of the species in the territory.

Isopollen maps of Quercus (oak) pollen percentages across Europe for 12 000, 10 000, 6 000 and 2 000 years before present (BP). Figure modified from Birks and Tinner (2016) .

READ MORE: Aliens are Coming! Threatening invasion or new opportunities from introduced tree species in Europe?

The Return of the Forest

The debate on ‘where’ temperate trees have survived during the latest glacial maximal (circa 21 000 years ago) is still open. The classic theory is that most species were restricted to refugia in the Balkans, Iberian and Italian peninsula. This is called the ‘southern refugia hypothesis’. However, in recent years several authors suggested that trees, particularly of species with a boreal distribution such as birch, spruce and pine, were distributed much more broadly in the southern part of Central Europe (the ‘northern refugia hypothesis’). This might have helped to speed up the process of recolonization of the northern part of the continent. An interesting modelling study published few years ago in Journal of Ecology indicates that there might have been suitable conditions of northern refugia for several species.

Representation of the distribution of major vegetation types and forest trees along a hypothetical transect from the Mediterranean to the Arctic Ocean. Panel (a) represent the interglacial period (i.e., present time), panel (b) the Last Glacial Maximum (LGM) according to the southern refugia hyphothesis and panel (c) the LGM according to the northern refugia hypothesis. Source: Birks and Willis (2008)

About 16 000 years ago, climate started to warm up. This can be observed also in pollen data, which suggest a quick response of the vegetation to the climatic changes. Trees started to spread from their Ice Age refugia (whether southern or northern ones). And pollen data suggest that they all did it at their own pace. For example, deciduous oaks seem to have reached southern Norway about 7000 years ago but beech took it slower, as it colonized Britain only about 3000 years ago. These differences may be also due to the different methods of seed dispersal. For example, species with wind-dispersed seeds such birch and Scots pine might have spread faster than others. But the recolonization might also have been favored by animals and humans (yes, Homo sapiens was already around at that time!) who may have carried acorns and fruits further north.

Finally, about 5 000 years ago, European trees reached their widest extension towards north, and the alpine tree lines started to look like the one we see today. Some species, however, continued their slow spread , like spruce into Scandinavia and beech and maple in Britain (which might have got some help to cross the English Channel!). The rest is modern history: human started farming, clearing and managing forests, shaping the European landscape as we see now. An interesting book talk about these aspects in great detail.

READ MORE: Ecosystem services, mountain forests and climate change

 

Mediterranean refugia: the case of Silver fir

I have already written about the ecological and economic importance of silver fir in Europe (see my previous post about silver fir and climate change). Although this species is in the list of species that might have been present in northern refugia, there is a general agreement that silver fir survived the last glacial maximum mostly in the Apennines (Italian peninsula) and in the southern Balkans. Until now it was a mystery exactly where the species found the ideal conditions to survive.

A recent genetic study published in Journal of Biogeography examined more than thousands trees from different locations in the Apennines showing evidences of the presence of three refugia in these area. The study demonstrated that trees from these populations, who are at the southern edge of the distribution of the species (see figure below), have similar gene pools than populations in the northern Apennines and in the Alps and seem to have differentiated before the last glacial maximum.

The sampled populations and the distribution of silver fir (green). Source: Piotti et al (2017)

These rear edge populations are currently quite isolated from the larger alpine and Central European populations and there are high evidences that are the remains of the last glacial refugia. Until now, the locations of these refugia were only hypothesized. In addition, the researchers found genetic similarities with Eastern European populations, suggesting a trans-Adriatic connection between silver fir populations of southern Italy and the Balkans.

Among the genetic characteristics of these populations, there are variants associated with a high adaptability, a feature that might be important in the context of adaptation to the future climate change.  This highlight the importance to preserve these tree populations for climate change adaptation (e.g., assisted population migration). And it also confirms the famous quote from Confucius Study the Past if you would define the future.

Sources used for the post:


Main photo: The Aletsch Glacier in the Swiss Alps. Author of the photo: Marco Mina

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