Sticking out the LGM

In my last post I discussed Sommer and Zachos (2009) paper where they pointed out that only the refugia that existed during the Last Glacial Maximum are important in terms of recolonisation and the origin of modern temperate species populations. I have, therefore, found a study by Kelly et al (2009) that focuses on the significance of a LGM refugial area in southwest England. The authors analysed a pollen record from Bodmin Moor in southwest England and achieved results consistent with the cryptic Northern refugia hypothesis, with small scattered tree populations surviving under locally favourable conditions in the LGM (which opposes the prevailing view of unglaciated regions in Europe being characterised by treeless, steppe-tundra communities). They found elevated levels of pollen from coniferous taxa (e.g. spruce trees) as well as lower but persistent levels of broad-leaf taxa (e.g. alder) pollen roughly at the time of the LGM, prompting the conclusion that the area acted as a cryptic Northern refugia for temperate tree species during the LGM. The pollen diagram, along with temperature estimates from oxygen isotope data, is presented in Figure 1.

Source: Kelly et al (2009)

Provan and Bennett (2008) suggest that other palaecological data and the results of climate modelling may be used to back up propositions such as Kelly et al (2009)’s. In terms of palaecological data, fossil evidence indicating refugia of temperate mammals in southwest England has been put forward (see Stewart and Lister 2001), as well as plant macrofossil evidence of refugia in Northern Europe. One thing, however, is that no plant macrofossil evidence has been found in this area of southwest England and, as you might be able to remember from last year’s Reconstructing Past Environments course, pollen data can be ambiguous without plant macrofossil data to back it up. The authors then draw on the climate modelling research of Svenning et al (2008) to show that the potential LGM ranges of the species proposed to have survived in refugia further support their findings.

Something that caught my attention when reading this article was that Kelly et al (2009) suggested that moisture availability may have been the reason for the persistence of conifer trees in these proposed refugial areas. They then go on to say that if this was the case then cryptic Northern refugia are likely to occur where precipitation or moisture levels are higher, for example in the sheltered valleys of Bodmin Moor. Temperature has been the limiting factor for quite a lot of the refugial areas discussed so far, so it’s interesting to see a specific example where moisture availability is reason for certain species existing in refugia. Perhaps this blog should have been called ‘hiding from the cold/dry/anything else unfavourable’ – doesn’t have the same ring to it though, does it?

In general, the findings of this study support the existence of cryptic Northern refugia at Bodmin Moor. There are, however, some uncertainties with the pollen data which I will discuss shortly. The LGM is thought to be the time when the ranges of temperate species were most contracted, so if a refugium existed for temperate species during the LGM (as this study shows) then there should be no reason why it should not act as a source of recolonisation when climate conditions became favourable again?