Discussion on the palaeobotanical data
Several recent studies in North America (e.g. Jackson & Booth, 2002; Parshall, 2002; Lyford et al., 2003) show that landscape structure and climate variability play important roles in governing the pattern and pace of natural invasions, with cases of extremely rapid population expansions and cases of populations persisting for hundreds to thousands of years without expanding. These heterogeneous colonization patterns also characterize the postglacial expansion of European beech. In many European regions, pollen values below 2% occur throughout long sections of the postglacial profiles before actual expansion of beech takes place, suggesting that small populations of beech were unable to increase their role in forest communities for thousands of years. The macrofossil evidence strongly supports this conclusion. In several cases, wood of beech has been found at sites where its pollen representation was negligible (Fig. 4). The case of the Iberian Peninsula is particularly striking: fossil wood of beech is found in the Pyrenees, in the Basque territory and on the Cantabrian coast dating from the last glacial maximum, the late glacial period, and the first half of the postglacial period, at a time when the pollen record is lower than 2% and discontinuous. Although beech expanded in the region only after 4 kyr 14C-BP, it appears to have been present in a number of refugia in northern Spain since at least the full glacial period.
The analysis of the regional patterns of beech spread indicates that the expansion of this species was not simultaneous, suggesting that local/regional factors controlled this process. For example, the data from Bohemia, Moravia and Slovakia show differences in beech expansion of up to several thousands of years for populations a few tens of kilometres apart (Rybničková & Rybniček, 1996; Svobodováet al., 2001, 2002; Rybníček & Rybničková, 2002).
Similarly, in the Balkans, where no doubt exists as to the survival of beech during the last glacial period, the pollen data indicate a very late expansion of beech: in the north-western Pirin Mountains, low but appreciable frequencies of Fagus are recorded in the early Holocene (Tonkov, 2003a), at a time when other sites in the Pirin Mountains show only rare occurrences (Tonkov et al., 2002; Stefanova & Ammann, 2003). Beech expansion out of these small refugia is observed only after 4 kyr 14C-BP, and in the nearby Osogovo-Belasitsa mountain range only after 2.5 kyr 14C-BP (Tonkov, 2003a,b). Similar patterns are found at the northern fringes of the range of this species, where human activity also played an important role (Aaby, 1986; Latałowa, 1992; Björkman, 1996; Latałowa et al., 2004). In areas where low frequencies of beech pollen are recorded already at 5.5–5 kyr 14C-BP, the expansion in forest communities took place not earlier than c. 3–2.8 kyr 14C-BP (Latałowa, 1992; Jahns, 2000). According to Björkman (1996) beech still expands in southern Sweden, while in Poland, at its north-eastern range, it still moves east (Matuszkiewicz, 2002; Latałowa et al., 2004).
These observations show that Fagus did not always start to expand from the refuge areas at the beginning of the postglacial period: in many cases, its population increase has been either very slow or very late. As a result, the different refuge areas have contributed to different extents to the colonization of Europe. The populations expanding early and quickly, such as those from Slovenia, played a much more important role than the populations spreading late and slowly, for example those from Greece.
Such delayed population growth considerably complicates the identification of beech refuge areas in Europe. Small refugia might have remained completely undetected, especially if they have been overrun by the migration of stronger populations. The case of the Vosges illustrates this problem well: a single 8 kyr 14C-BP-old piece of wood suggests the existence of a refuge for beech (Fig. 4; 8.9–7.9 kyr cal-BP). The pollen records, indicating a local presence for beech 2000 years later, point instead to a migration pattern from the Swiss mountains into the Vosges.
Hence, beech appears to have survived in many regions of Europe during the last glacial period. The data collected in recent years confirm that the Italian and Balkan Peninsulas were refuge areas, as suggested by previous studies (Huntley & Birks, 1983; Huntley, 1988; Lang, 1992, 1994; Pott, 1997, 2000; Brewer, 2002). However, neither the Italian nor the Balkan Peninsulas were the source areas for the colonization of Europe by beech. In fact the Italian population migrating from the south reached the northern Apennines rather late (c. 5 kyr 14C-BP; 5.7 kyr cal-BP), when beech was already present in most of central Europe. The Balkan populations underwent a real expansion even later, c. 4 kyr 14C-BP (4.5 kyr cal-BP), and were distinctly separated from those in the Carpathians. The new evidence from the Iberian Peninsula demonstrates that there were refuge areas for beech, which expanded late in the Holocene and did not migrate northward. Contrary to what was generally believed, the palaeobotanical data show that none of the three southern peninsulas (Iberian, Italian and Balkan) was a source area for beech in central and northern Europe.
The macrofossil and pollen records from Slovenia and Istria indicate that beech survived there during the last glacial period. The diffusion of beech in the eastern Alps and in western Hungary already at the beginning of the postglacial period probably suggests that it survived in a rather large territory, from where it could spread rapidly into central Europe.
Pollen data also suggest the hypothesis of south Bohemia and south Moravia as refuge areas. These regions, separated from Slovenia by Austria and the Danube valley, were colonized by forest trees very early in the postglacial period. The modern vegetation of the Šumava Mountains and of the Bohemian Forest, which includes both relic boreal populations and species that presumably emigrated from the Alps, supports the hypothesis that these areas were the starting points for the vegetation development of central Europe during the postglacial period (Svobodováet al., 2001, 2002).
Similarly, the data from the Rhône valley in France indicate that beech probably survived the last glacial period in various localities in the region. In the Alpine region, not far from the site of Les Echets, where Fagus was present in the last interglacial period, the 2% threshold was reached between 7.5 and 7 kyr 14C-BP (8.4–7.9 kyr cal-BP).
In the Apuseni Mountains, beech is found before 6 kyr 14C-BP (6.8 kyr cal-BP), at a time when Fagus is not yet recorded anywhere in the Carpathians. However, even if the Apuseni were a refuge area, it was not an important source area for the colonization of the Carpathians, which were colonized instead from central Europe.
According to the palaeobotanical data, Slovenia, the eastern Alps, the French Alps and possibly south Moravia should be considered as the main source areas from which Fagus colonized central and northern Europe. All these regions are located at a latitude (between 45° N and 49° N) much higher than expected from previous palaeobotanical studies. This casts new light on the recent debate on the existence of northern refugia of temperate trees in Europe (Willis et al., 2000; Carcaillet & Vernet, 2001; Stewart & Lister, 2001; Tzedakis et al., 2002, 2003; Stewart, 2003; Willis & van Andel, 2004).
A comparison between the distribution of beech during the last interglacial period (the Eemian, 127–112 kyr BP) and that during the last glacial period (70–15 kyr BP) highlights unexpected concordances: in both cases, the extension of beech was very reduced and central-southern Italy, south-eastern France, south Moravia and Slovenia were survival areas. The abundance of beech during the forest phases following the Eemian, both in the Balkan and in the Iberian Peninsulas, suggests that Fagus may also have been present in these regions during the Eemian at a level undetectable by pollen (Fig. 3). As beech had only a very moderate spread in Europe during the Eemian, we can assume that the Eemian populations largely reflect the distribution of refuge areas for beech during the previous glacial period, corresponding to MIS 6 (approx. 170–130 kyr in its most severe phase). If so, a number of beech populations should have persisted throughout at least two glacial–interglacial cycles in central-southern Italy, south-eastern France, southern Moravia, and possibly the Balkan and Iberian Peninsulas. The residual populations of the last pleniglacial period might therefore show genetic differentiation resulting from hundreds of thousands of years of survival in isolation.
The fossil data indicate that beech expanded along particular geographical courses. In the first half of the postglacial period, beech mainly spread along the hilly and mountainous regions of central-southern Europe and it was only after 4 kyr 14C-BP (4.5 kyr cal-BP) that it started to colonize the lowlands of central-northern Europe. This behaviour indicates that the mountain chains were not geographical barriers for beech; on the contrary, in many cases they facilitated its survival and diffusion (cf. the Pyrenees, Cantabrian Mountains, Apennines, French Massif Central, Bohemian Mountains, Dinarian Range, Pirin Mountains and Rhodopes). Beech did not cross the highest chains (e.g. the Alps) but expanded along the slopes. In contrast, F. sylvatica appears to have been limited by large plains with continental climate and by important river valleys, for example the Hungarian plain, the Po valley and the lower Danube valley. This suggests that the common opinion that mountain ranges are important barriers causing the isolation of genetic lineages (Taberlet et al., 1998) should be revised, at least for beech.
The fossil data represented on the maps in Fig. 4 clearly show that the postglacial spread of Fagus was irreversible, or at least did not include important retreats, so that the modern distribution of European beech roughly corresponds to its maximum extension. In other words, the modern populations are likely to be the direct descendants of the populations that first settled in that area.