Where Did the Forests Go During the Ice Age? Somewhere Warmer, and Then Back
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Species survived the cold periods in scattered pockets where conditions remained tolerable, and the modern distribution of European wildlife is largely a record of where those pockets were.
What the pockets were
During the coldest phases of the ice ages, ice sheets covered northern Europe and much of the ground beyond them was open tundra or cold steppe where forest could not grow. Species needing woodland did not adapt in place and did not die out entirely, since they persisted in areas where conditions remained suitable, chiefly the southern peninsulas of Iberia, Italy and the Balkans, where mountains and coasts created sheltered pockets. Those areas were not uniformly warm either, and the surviving populations were small, scattered and isolated from each other for thousands of years at a time.
What the isolation did
Long separation in small populations left permanent marks:
- •Populations in different areas diverged genetically
- •Small numbers lost genetic variation through chance alone
- •Some diverged far enough to become separate species
- •Each area accumulated its own distinctive variants
- •Those areas still hold far more variation than the north
- •Species found nowhere else survive in several of them
How the recolonisation is traced
When the ice retreated, populations spread north again, and genetic evidence reconstructs the routes in detail. A population expanding rapidly into empty ground carries only the variants held by the individuals at its leading edge, so variation is progressively lost with distance from the starting point, which produces a clear gradient. Comparing that gradient across many species reveals which southern area supplied which part of northern Europe, and the answers differ by species, with some routes blocked by the Alps and the Pyrenees for one animal and not another. Where two expanding populations met, they formed suture zones, narrow bands where distinct genetic types meet and interbreed, and several such zones run across central Europe for many species at once.
The argument about northern pockets
The tidy picture of southern peninsulas supplying everything has been challenged, and the dispute is worth knowing about. Genetic patterns in several cold-tolerant species, including some trees, small mammals and insects, are difficult to explain by expansion from the far south alone, and suggest populations survived much further north in sheltered spots within the tundra, in river valleys, on south-facing slopes and near the coast. Fossil pollen and beetle remains from such places support that for some species. The current view is that both happened, with warmth-demanding species confined to the south and hardier ones persisting in scattered northern pockets, which makes the recolonisation pattern species-specific.
Why it matters now
The concept has become central to conservation planning as climate changes again. Southern peninsulas hold disproportionate genetic diversity, which makes them priorities for protection even where populations are not currently threatened, since the variation there cannot be recovered elsewhere. The historical pattern also shows that species can move substantial distances given time and connected habitat, and that the current landscape is far more fragmented than the one they crossed before. And warming rather than cooling means the pockets that matter now are cool ones, at altitude and on north-facing slopes, which are the places species are already retreating into and which have no further ground above them.
The takeaway
Woodland species survived the cold phases in scattered southern pockets, chiefly in Iberia, Italy and the Balkans, isolated for thousands of years, which is why those areas still hold far more genetic variation than the north. Rapid expansion afterwards lost variation with distance, producing gradients that reveal which pocket supplied which region. The pockets that matter under warming are cool ones, with nothing above them.