It's not just animals that hide from the cold...

The last two pieces of research that I summarised present evidence for animals in refugia, but it’s not just animals that are suspected to have survived in refugial areas. It was once thought that trees only survived glacial periods in scattered Mediterranean refugia (Southern refugial hypothesis), but over the last few years there has been evidence to suggest that tree species may have been more widely distributed in Northern Europe (Northern refugial hypothesis) – I think I’m beginning to see a pattern developing here!

I have found a nice piece of work by Svenning et al (2008) in which they investigate the Northern refugial and Southern refugial hypothesis by estimating the possible distributions of a number of tree species in the Last Glacial Maximum using species distribution modelling. They decided to focus on species central to the refugial debate so included 7 boreal and 15 nemoral (forest species) widespread European tree species in their investigation. The species distribution modelling was based on data concerning the current distribution of the chosen tree species and climate which they then projected onto two Last Glacial Maximum climate simulations for Europe – if they can work out the relationship between present day species distribution and climate, then they can infer past distributions of the same species from climate simulations of the past.

What were their findings?

 

Their model results showed that suitable conditions existed for the boreal tree species across Central and Eastern Europe (see Fig. 1 for an example of 2 of these species’ distributions).

Source: Svenning et al (2008)

However, they found that suitable climatic conditions for the nemoral tree species were generally restricted to the Mediterranean and Black Sea regions during the Last Glacial Maximum (see Fig. 2 for an example of 2 of these species’ distributions).

Source: Svenning et al (2008)

Their research findings suggest that the response of nemoral tree species to climatic changes around the Last Glacial Maximum can be best explained using the Southern refugial hypothesis. The boreal tree species, however, seem to have responded in a way better explained by the Northern refugial hypothesis. The authors, therefore, push for a reassessment of the view of Last Glacial Maximum vegetation in Europe as largely treeless.

Significance of these results…

Well, firstly, it adds to the growing amount of evidence supporting the existence of Northern refugia. They also help explain the occurrence of mammal fossils with forest, tundra and steppe species in Central and Eastern Europe during the Last Glacial Maximum. Finally, they help explain the phylogeographic evidence for the persistence of many boreal species at latitudes north of the Mediterranean. Of course, when looking at this study you have to think about the limitations of the models used.

I think this example is particularly interesting as it investigates both the Southern and Northern refugial hypothesis, rather than just presenting evidence to prove just one or the other. I also think it’s interesting to see how such different research methods – from mitochondrial DNA data used in the previous examples, to species distribution modelling used in this study – can all be used to produce evidence that supports the existence of Northern refugia.