After The Ice Age
For the past million years, the British climate has been mainly cold with shorter warmer intervals each lasting some tens of thousands of years. During the warm spells between glaciations or ice ages, plants and animals have migrated in and out of the British Isles as sea levels have risen and fallen to cut off or link the British mainland and Ireland with Continental Europe.
After the last Ice Age, plants began to naturally re-colonise Britain as the ice sheets melted to leave a cold and barren landscape. Around 10,000 years ago, the climate began to warm and trees began to return. Natural spread through seeding continued in a south to north wave until about 6,000 years ago when melt water filled the English Channel and cut off these islands. About 33 species of medium and large trees made it across by then and form our native tree species nowadays.
More: Pollen analysis provides a fascinating insight into the ebb and flow of tree species here.
“Trees & Woodlands in the British Landscape”. (1993) D. Rackham. J.M. Dent & Sons.