6] foundations of modern distributions 173 



Postglacial^ Changes 



Climatic change has affected plant distribution right up to the 

 present time, and indeed such changes and their effects on plant 

 life are still going on and presumably will always continue. The 

 postglacial sequence of changes is best known and perhaps most 

 marked in temperate Europe, where the sequence has been briefly 

 as follows : 



(i) the earliest deposits following the last glacial recession show 

 evidence of an arctic-subarctic-arctic sequence of vegetation-types 

 characterized by Dwarf Birch [Betula nana agg.), shrubby Willows 

 {Salix spp.), and Mountain Avens {Dryas octopetala s.l.),- developed 

 under probably rather dry as well as cold conditions except around 

 the middle of the period, which persisted for example in the British 

 Isles until about ii,ooo years ago. This ' Subarctic' period was 

 followed about a millennium later by 



(2) a ' Pre-Boreal ' period of variable but milder climate, char- 

 acterized by Scots Pine [Pinus sy/vestris), Birch [Betula piibescens s.l.), 

 and Elm (Ulmus), with Spruce (Picea) dominant in some eastern 

 regions. This was in turn followed by 



(3) a ' Boreal ' period of relatively warm and dry ' continental ' 

 climate which towards its end supported mixed hardwood forest — - 

 particularly of Oak {Ouerciis) with abundant associated Hazel 

 {Corylus) in the temperate belt, and persisting there until probably 

 seven or eight thousand years ago. Thereafter followed 



(4) a still warmer but wet ' Atlantic ' period characterized by 

 mixed Oak and Lime (Tilia) forest, constituting the so-called 

 ' climatic optimum ' {i.e. for northwestern Europe) that lasted until 

 5,000 or fewer years ago. This was in turn followed by 



(5) the more continental, drier ' Sub-Boreal ', which lasted until 

 about 2,500 years ago and in which there occurred a reduction of 

 bog growth but an increase of Conifers and the entry of Beech 



^ It is said that we are still not out of the Pleistocene epoch and that, con- 

 sequently, we should not speak of the Present or Recent or post-Pleistocene ; 

 from our point of view, and although because of variability in different places it 

 cannot be satisfactorily defined, the time since the last great glacial recession is 

 all postglacial, and seems best so termed (informally, with a small ' p '). While 

 suggesting this course, Professor R. F. Flint confirms (voce) that the terms Recent 

 and Holocene, which are often used for postglacial time, have also not been 

 properly defined. 



" Also, Professor Gunnar Erdtman informs me (voce), by such ' pioneer ' plants 

 as species of Artemisia, Heliauthetmim, members of the Chenopodiaceae, and 

 even Ephedra. 

 G 



