During the Carboniferous Period some 345-280m years ago, the continent of Pangaea began to drift northwards from its southern hemisphere origins, and also began to pivot 30 degrees to the west. Giant insects developed, amphibians evolved further and reptiles became more land-based. Plants were reproducing in an alternating manner of asexual and sexual methods across different generations (spores and seed respectively). Equisetum plants (includes Horsetails) became huge, forming Calamites in damp, swampy areas, and forming Cordaites (that had pollen sacs and ovules at branch tips - later forming the first conifers, such as ginkgos, during the Permian Period 280-225m years ago) in drier areas. Clubmosses grew to 30-45m in height. All of this development was fueled by the equatorial climate induced by the drift northwards of Pangaea, particularly at the northern-most end of the super-continent.

Coal measures were thus formed very readily from such large plants in what now constitutes Northern Europe (given this segment of Pangaea was first to travel over the equator), once the sea began to engulf the swamps of massive clubmosses, semi-composting them and later compacting them down with silt and clay to form lignite, and eventually forming coal under continued compression events. It took 20m of rotted 'forest' biomass to produce a 1m-thick coal measure, so given many European coal measures are hundreds of metres thick, the length of time required to create such coal measures would have been hugely significant. This also adds significance to our eagerness to burn such stored coal, releasing carbon that has been locked away for hundreds of millions of years in mere decades.

The southern area of Pangaea (Africa, Australia) contained smaller plants and thus smaller coal measures, only beginning to lay down larger coal measures (that never amounted to the extent of the earlier coal measures, as the steamy swamps that produced the huge clubmosses and subsequent coal measures no longer existed due to climate change) much later once the southern segment of Pangaea did reach the equator (during the Cretaceous and Tertiary Periods) when plants had evolved to develop roots and reproduce sexually via seed. Evolutionary-speaking, the development of reproduction via seed was critical to the survival of plants long-term, as germination could be delayed until conditions were desirable.

Building on the above a little, though nonetheless as a tangent, Europe has so few tree species as tree populations could not retreat southwards as the ice sheets encroached into their territory, given the east-west running mountain ranges that are the Alps and Pyrenees, and the area that is now the Mediterranean Sea. Because of these blockading landscapes, tree diversity is rather low within Europe, when compared to the Americas and Asia.

The UK's plant diversity is even more impoverished as when the ice sheets last retreated 12,000 years ago, the subsequent rising of sea levels bridged the gap to mainland Europe. Thus, only species that had colonised during the 6,000 years after the ice caps began to retreat are found today - any others that may have potentially once again reached these shores (I suspect sweet chestnut, plane, holm oak, etc, though definitely Norway maple and larch) were barred from doing given the mass of water in the way.

Source: Davis, M. (2015) A Dendrologist's Handbook. UK: The Dendrologist.