Final Conclusions on Glacial Refugia

The post will attempt to give a well rounded review of the debate over cryptic glacial refugia in the Northern Hemisphere.

The consensus is that glacial refugia existed in the mountains of the mediterranean peninsulas in Iberia, Italy and the Balkans. This is where temperate tree species are thought to have persisted during full glaciations. However, there is another theory that cryptic glacial refugia existed in parts of northern Europe. For example the tree pollen records in northern Europe in the early postglacial period suggest huge migrations rates of trees of up to 1500m per year if they colonized from southern refugia. The theory of cryptic refugia involves some tree populations being able to survive in favourable microclimates during glaciations where the regional climate would not normally be suitable (Stewart & Lister 2001). How the population manage to survive are not fully understood, and many scientists (Tzedakis et al. 2003) put the presence of the early post glacial pollen to isolated long distance dispersal events.

Examples of cryptic populations can be seen today in certain areas in the current climate. In the central Sahara a total of 223 individuals of the conifer, Cupressus dupreziana, managed to persists in ravines, where rainfall averages at 30 mm per annum (subject to large variability) (Abdoun & Beddiaf 2002). This is similar in the way in which certain temperate tree species may have been able to survive in northern Europe during glaciations.

Techniques used to identify refugia are: pollen analysis, macrofossils and molecular techniques. Pollen analysis can be used to show the presence of species in an area. For species with low production and that are non wind pollinated, presence of the species can be concluded with confidence, however with wind pollinated species the pollen could have been produced outside the regional area. The records are quite vast and are an important tool in assessing species presence.

Macrofossils are the most precise way to identify species. Macrofossils have been recovered from paleolithic sites in northern Spain, dated between 17,000 and 15,000 years BP revealing a diverse assemblage of temperate and mediterranean species. This suggests a glacial refugium was present. Although this suggests the prescence of tree species, it does not make clear the type of vegetation (e.g. forest, woodland, steppe, steppe-tundra). Willis et al. (2000) studied macroscopic charcoal particles in glacial sediments in Hungry. They concluded that there were ‘oases’ in central Europe where temperate flora and fauna could survive the glacial conditions.

Molecular techniques involve the phylogeographic techniques discussed in a previous post. They involve studying, amongst others, species diversity as identifier of refugia. See ‘Glacial refugia - are they being too cryptic’ for a refresher on phylogeographic techniques. An example of how it is used is the prescence of Scot’s pine, Pinus sylvestris, in Scotland. High nuclear genetic diversity in scottish populations of P. sylvestris suggest a cryptic glacial refugium (Sinclair et al. 1999) during the last glaciation.

The evidence suggests that plant species persisted during glaciations in cryptic glacial refugia. While some academics still believe post glacial recolonization took place by long distance dispersal from southern refugia, I believe the cyptic glacial refugia hypothesis to be a better fit. 

The hypothesis manages to explain the huge migration rates had been inferred from colonization from southern refugia, which were highly unrealistic. This breakthrough has impact on future modelling of vegetation change due to climate change. After understanding how glacial refugia exists and the debates over post glacial colonization, the blog will move on to looking at the future and how vegetation modelling will be important for assessing climate impacts on species migration.