Europe: Trees or No Trees? A New Emerging Synthesis

The study of biodiversity during the Pleistocene offers many opportunities to assess the impacts of climate change.  

GOAT WILLOW (Salix Caprea) now believed to
have had wide boreal distribution during the LGM

Scientists are fascinated by distributions of European trees in the coldest stages of the last full glacial, especially at isolated locations where some species survived to influence their long-term ancestry and current distributions. For fauna and flora species, the location of their European refugium during the coldest glacial maximum remains a teasing puzzle.  

Emerging evidence from fossil proxies, paleoclimatic modelling and genetic research suggests the traditional paradigm of trees being restricted to S. Europe, especially the 3 southern peninsulas (Iberian, Balkan and Italian) is questionable.  

Evidence includes 151 carbon-14 dated and identified pieces of macrofossil charcoal wood from 40 sites in central and eastern Europe that suggest that during the last full glacial, there were populations of conifers with some deciduous trees growing much further north and east than previously believed.  

Plotted against a new high-resolution millennial time-scale for the interval ∼32–∼16 ka BP in Greenland our evidence shows that coniferous as well as some broadleaf trees were continuously present throughout those interstadial/stadial cycles for which there are adequate data.Willis (2004)

Southern vs. northern refugia hypotheses were investigated by estimating the potential LGM distributions of 7 boreal and 15 nemoral widespread European tree species using species distribution modelling. The models were calibrated using data for modern species distributions and climate and projected onto two LGM climate simulations for Europe. Five modelling variants were implemented.

Broadly consistent results were obtained irrespective of the climate simulation and modelling variant used. Results indicated that LGM climatic conditions suitable for boreal species existed across Central and Eastern Europe and into the Russian Plain. 

In contrast, suitable climatic conditions for nemoral tree species were largely restricted to the Mediterranean and Black Sea regions. Large proportions of these northern and southern regions would have been suitable for a number of boreal or boreal plus nemoral tree species, respectively.

Synthesis:  It is clear that the view of the LGM landscape in Europe as largely treeless, especially north of the Alps, needs to be revised. Trees were probably much more widespread during the LGM than hitherto thought, although patchily distributed at low densities due to low atmospheric CO₂ concentrations and high wind-speeds. 

The findings presented here help explain the occurrence of mammal assemblages with mixtures of forest, tundra and steppe species at many localities in southern Central and Eastern Europe during the LGM, as well as the phylogeographic evidence for the extra-Mediterranean persistence of many boreal species.
Svenning (2008)

Trembling Aspen - Populus Tremuloides propagates by root sprouts, to form enormous clone coloni of one single organism  - Utah's Pando Populus colony has the distinction of being both the oldest (80,000 years old) and heaviest living organism at 6 million kg. 

The northern refugia hypothesis is that trees were distributed much more widely in Europe during the LGM.  According to this hypothesis, trees occurred not only across Southern Europe, but also in the southern parts of Central Europe, a proposal is mainly based on evidence from pollen and plant macrofossil evidence but is also receiving support from phylogeographic studies of certain species (e.g. Silver Burch (Betula pendula and Betula pubescens), Trembling Aspen (Populus tremuloides, and willow (Salix caprea).  These species all show wide boreal pleistocene distributions.  

Betula Pendula -Tiver, Russia

The phylogeographic evidence for nemoral (woodland) tree species appears to be more consistent with the southern refugia hypothesis.

Survival for the non-boreal beech tree Fagus sylvatica  as far north as the Carpathians has been suggested. Late-glacial and post-glacial increases in the number of pollen sites where Fagus was locally present suggests that the area occupied by beech populations expanded exponentially from the glacial refugia for a duration of over 10,000 years, until about 3500 years before present.  Magri (2008)

Glacial refuge areas should be expected to harbor a large fraction of the intraspecific biodiversity of the temperate biota. Chloroplast DNA variation was studied in 22 widespread European trees and shrubs sampled in the same forests. Most species had genetically divergent populations in Mediterranean regions, especially those with low seed dispersal abilities. Petit (2003)

Glacial Refugia were hot spots but not melting pots of genetic diversity .... genetically most diverse populations were not located in the south but at intermediate latitudes, a likely consequence of the admixture of divergent lineages colonizing the continent from separate refugium.

Multispecies genetic divergence of each of the 25 European forests studied. Higher than average values in black circles, lower than average, white circles.  For all forests, divergence levels represented by connecting lines, with continous black lines indicating comparatively high divergence, dotted lines, intermediate divergence.  Altitude is indicated by grey shading (lightest gray 250-500 m with gray intensifying as altitude increases from 500-1000 and over 1000 m).  Past sea levels are indicated by black dotted lines Petit (2003)