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Response to Comment on "Buffered Tree Population Changes in a Quaternary Refugium: Evolutionary Implications"
Stewart (1) reiterates recently
published views on the possible existence of "cryptic" refugia in
northern Europe(2) and questions the role of southern
European refugiaas centers for speciation. In view of the implications
of theseissues for conservation strategies, critical evaluation of thegenetic and paleobotanical evidence is particularly important.
Establishing the northernmost extent of glacial refugia in Europe
is not straightforward, because a species' distributionpattern is
likely to comprise larger southern populations andprogressively
smaller, northern peripheral ones (3).Moreover, postglacial
expansion may include advances and extirpationsdue to climatic
oscillations, and thus northern remnants of anearlier advance may form
the "secondary" refugia for a subsequentwave of expansion
(4). Separating the effect of theseevents and
resolving the location of the northern edge duringglacial maxima
requires spatially detailed genetic and fossilevidence, which are
still largely unavailable. It is also importantto consider the
individual histories of species: Making inferencesabout refugia of
temperate trees based on evidence from freshwaterfish, land snails,
sedges, or rockferns may be of limited value.
Most paleoecologists accept that small populations of conifers and
birch survived north of the Alps during the Last GlacialMaximum (LGM).
Stewart (1) challenges the dominant paradigmby invoking the
occurrence of refugia of temperate trees withseriously disjunct
distributions north of the Iberian, Italian,and Balkan peninsulas. The
impetus for this claim originates fromthe existence of fossils of
temperate mammals--which today areassociated with deciduous
woodlands--in glacial deposits in northernEurope. We question the
strength of this association because thesemammals are found in a
variety of habitats including open, montane,and coniferous
environments (5, 6). In addition,no clear genetic
evidence exists for disjunct refugia far northof the main refugial
range of temperate trees in Europe. Finally,the paleobotanical
evidence cited by Stewart (2) doesnot stand up to close
scrutiny. The occurrences of temperate treesin (2)
were either not LGM in age (19 to 23 ka), or weredated indirectly and their chronological framework is unclear.
We suggest that the existence of northern refugia can be tested
using continuous pollen records spanning the last glacialnorth of the
Alps. If small populations of temperate deciduoustrees survived in
these regions, they should have been able toexpand during the 2000- to
3000-year interstadial phases thatpunctuated the period between 45 and
11.5 ka. No such increaseis evident from French pollen records, which
only show expansionof pine and birch and some spruce and larch
(7, 8).Rather, the available evidence indicates that
postglacial colonizationby temperate trees did not originate north of
about 45°N and thattheir glacial survival and migration histories
may have differedfrom the examples cited in (1). Where that
leaves temperatemammals with northern European glacial records is
unclear, butbehavioral accommodation and facultative adaptation
(9)may be easier to invoke than trying to force the
paleobotanicalrecord to fit the mammalian story.
The second issue raised in (1) is that southern European
refugia of temperate trees could not be both sources fornorthward
postglacial colonization and crucibles of speciation,because they
could not remain sufficiently isolated for geneticdivergence to
accumulate. We suggest that the two roles are notincompatible. The
history of a refugium can be viewed as the continuouspresence of tree
populations with intermittent pulses of outwardrange expansion.
Encounters with other expanding genomes fromdifferent geographical
areas would take place farther afield andlead to the formation of
hybrid zones (10). The keysto long-term isolation are that
(i) a part of the refugial populationremains behind providing
continuity; and (ii) during intervalsof climatic deterioration, the
colonizing genomes do not returnto their point of origin, but rather
degrade in situ (11).Whether the refugial populations we
identified in the Pindus Mountains(12) contributed to the
recolonization of Greece butwere blocked in the north by a leading
edge expansion of Balkanpopulations (10), or also
contributed to northward expansionbeyond Greece, is irrelevant to the
issue of long-term isolation.In either case, there was no reverse
movement during the populationcontraction phase at the onset of a
stadial or a glacial period,thus preserving the distinctive character
of the tree populationsthat persisted in refugia. Any genetic
differentiation that mayhave accumulated in northern European
temperate tree populationsduring interglacials was ultimately lost in
the ensuing glacialphase (11).
P. C. Tzedakis
School of Geography University of Leeds Leeds, LS2 9JT, UK E-mail:
P.Tzedakis{at}geog.leeds.ac.uk I. T. Lawson
Department of Geography and
Environment University of Aberdeen Aberdeen, AB24 3UF, UK M. R. Frogley
Centre for Environmental
Research School of Chemistry, Physics and Environmental Science University of Sussex Falmer, Brighton, BN1 9QJ, UK G. M. Hewitt
School of Biological
Sciences University of East Anglia Norwich, NR4 7TJ, UK R. C. Preece
Godwin Institute for Quaternary
Research Department of Zoology University of Cambridge Cambridge, CB2 3EJ, UK
A. M. Lister, in Past and Future Rapid
Environmental Changes: The Spatial and Evolutionary Responses of
Terrestrial Biota, B. Huntley et al., Eds. (Springer,
Berlin, 1997), pp. 287-296. [CrossRef] [Medline]
