news and update ISSN 1948‐6596 commentary Tree refugia and slow forest development in response to post‐ LGM warming in North‐Eastern European Russia Though the Postglacial period counts its thousands of years, it was not indefinitely long, and few plants that merely scatter their seed could advance more than a yard in a year […]. The oak, to gain its present most northerly position in North Britain after being driven out by the cold, probably had to travel fully six hundred miles, and this without external aid would take some‐ thing like a million years. (Clement Reid 1899, p. 25) Driven by an urge to understand how the compo‐ sition of the British flora arose, Clement Reid was puzzled by the discrepancy between the apparent low dispersal capacity of big seeded trees and the high postglacial migration rates deduced from fos‐ sil evidence. Possible explanations resolving Reid’s paradox, as it was later termed, were fast migra‐ tion by long‐distance dispersal events or glacial survival of small scattered populations at shel‐ tered sites further north than evidenced by the fossil record (Reid 1899, Skellam 1951). More than 100 years have passed since the early writings of Reid, but two questions leading from his thoughts are still debated: How far north did tree species survive the Last Glacial Maximum (LGM, ca. 21 kya)? How fast did tree species spread and thus current forest structure develop during post‐LGM warming? A new paleoecological study by Valiranta et al. (2011, Journal of Biogeography) provides, in addition to several other recent studies (e.g., Bin‐ ney et al. 2009, Heikkila et al. 2009), insight into these unresolved questions. Valiranta et al. (2011) summarized macrofossil and stomata records from five sites at the late‐glacial–early Holocene transition at the European Russian arctic tree line; an area east of the margin of the Scandinavian ice sheet and west of the Ural Mountains (Fig. 1). Their data evidenced the presence of Norway spruce (Picea abies) and tree birch (Betula pen‐ dula/pubescens) during the early Holocene (11,500–10,000 cal. yr BP) at all five sites. At one site, two radiocarbon dates even suggested the presence of these taxa up to 1000 yrs prior to the beginning of the Holocene. Historical biogeogra‐ phy, however, depends on reliable chronologies of fossil records. Ideally, each site should have a well established chronology based on an adequate number of radiocarbon dated fragile and precisely identified terrestrial plant remains that show no signs of redeposition. In this respect, the chro‐ nologies of Valiranta et al. (2011) were not opti‐ mal and the late‐glacial presence of P. abies and tree Betula can be questioned, but their early Holocene records were well documented. Hence, the smoking gun, i.e., direct palaeoecological proof of LGM‐populations, might be evasive, but scattered glacial and widespread early Holocene fossil records indeed reflect consistent patterns of early expansions in North‐Eastern European Rus‐ sia. Furthermore, independent climate reconstruc‐ tions are crucial when evaluating the possible LGM survival of boreal tree populations so far north. Glacier‐based climate models do indeed indicate relatively modest LGM temperature de‐ pressions in the arctic Ural Mountains (Allen et al. 2008). This might have facilitated survival of trees. Besides, the ability to produce krummholz forms (Oberg & Kullman 2011) or other strategies for non‐reproductive persistence through millennia (Bhagwat & Willis 2008) may have been decisive. Nevertheless, as concluded by Valiranta et al. (2011), it certainly seems likely that postglacial development of high‐latitude forests occurred through expansion of local tree populations rather than through re‐colonisation from distant south‐ ern refugia. Using molecular markers, recent phy‐ logeographic studies for both P. abies and Betula support glacial survival in north‐eastern Europe (Maliouchenko et al. 2007, Tollefsrud et al. 2008). For example, P. abies shows distinct gene pools of similar within‐population genetic diversity for northern and central‐southern Europe (Tollefsrud et al. 2008): a pattern that supports west expan‐ sion from such north‐eastern refugia and not re‐ colonisation of northern Europe from the south. For decades, pollen data have suggested early frontiers of biogeography 2.4, 2011 — © 2011 the authors; journal compilation © 2011 The International Biogeography Society
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Jan 2, 2017
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