Jacques-Louis de Beaulieu

Identification of refugia and post-glacial colonisation routes of European white oaks based on chloroplast DNA and fossil pollen evidence

Abstract The geographic distribution throughout Europe of each of 32 chloroplast DNA variants belonging to eight white oak species sampled from 2613 populations is presented. Clear-cut geographic patterns were revealed by the survey. These distributions, together with the available palynological information, were used to infer colonisation routes out of the glacial period refugia. In western Europe in particular, movements out of the Iberian and the Italian Peninsulas can be clearly identified. Separate refugia are also present in eastern Balkans, whereas further west in this peninsula similarities with Italy were evident. Movements resulting in the exchange of haplotypes between refugia both during the present interglacial and probably also during earlier glacial cycles were therefore inferred. The consequences of these past exchanges is that phylogenetically divergent haplotypes have sometimes followed very similar colonisation routes, limiting somewhat the phylogeographic structure. Cases of geographic disjunction in the present-day distribution of haplotypes are also apparent and could have been induced by the existence of rapid climatic changes at the end of the glacial period (specifically the Younger Dryas cold period), which resulted in range restriction following an early warm period during which oak first expanded from its primary refugia. This cold phase was followed by a new period of expansion at the outset of the Holocene, involving in some cases ‘secondary’ refugia. It is expected that these short climate oscillations would have led to a partial reshuffling of haplotype distribution. Early association between haplotypes and oak species are also suggested by the data, although extensive introgression among species has ultimately largely blurred the pattern. This implies that colonisation routes may have been initially constrained by the ecological characteristics of the species hosting each chloroplast variant. We suggest for instance that two oak species distributed in the north of the Iberian Peninsula ( Quercus petraea and Q. pubescens ) are recent post-glacial immigrants there. When considered together, conclusions on the location of glacial period refugia and the colonisation routes derived from molecular information and fossil pollen data appear to be both largely compatible and complementary.

Holocene seasonality changes in the central Mediterranean region reconstructed from the pollen sequences of Lake Accesa (Italy) and Tenaghi Philippon (Greece)

This study presents pollen-based climate reconstructions of Holocene temperature and precipitation seasonality for two high-resolution pollen sequences from the central (Lake Accesa, central Italy) and eastern Mediterranean (Tenaghi Philippon, Greece) regions. The quantitative climate reconstruction uses multiple methods to provide an improved assessment of the uncertainties involved in palaeoclimate reconstructions. The multimethod approach comprises Partial Least Squares regression, Weighted Average Partial Least Squares regression, the Modern Analogues Technique, and the Non-Metric-Multidimensional Scaling/Generalized Additive Model method. We find two distinct climate intervals during the Holocene. The first is a moist period from 9500 to 7800 cal. BP characterised by wet winters and dry summers, resulting in a strongly seasonal hydrological contrast (stronger than today) that is interrupted by a short-lived event around 8200 cal. BP. This event is characterised by wet winters and summers at Accesa whereas at Tenaghi Philippon the signal is stronger, reversing the established seasonal pattern, with dry winters and wet summers. The second interval represents a later aridification phase, with a reduced seasonal contrast and lower overall precipitation, lasting from 7800 to 5000 cal. BP. Present-day Mediterranean conditions were established between 2500 and 2000 cal. BP. Many studies show the Holocene to have a complex pattern of climatic change across the Mediterranean regions. Our results confirm the traditional understanding of an evolution from wetter (early Holocene) to drier climatic conditions (late Holocene), but highlight the role of changing seasonality during this time. Our data yield new insights into the aspect of seasonality changes, and explain the apparent discrepancies between the previously available climate information based on pollen, lake-levels and isotopes by invoking changes in precipitation seasonality.

An attempt at correlation between the Velay pollen sequence and the Middle Pleistocene stratigraphy from central Europe

Abstract Long continuous lacustrine sequences constitute on the continent a precious tool for coupling the long term continental environmental changes with deep sea and ice core records, using the ‘count from the top’ method. Moreover, they can contribute to long distance correlation on the continent itself and thus help to classify discontinuous sedimentary records. Palynostratigraphical correlation is proposed here between the Velay long sequence and the late Middle Pleistocene series from Central Europe with special attention to temperate episodes. The similarities between the Praclaux and the Holsteinian Interglacial suggest that they are both contemporaneous with marine isotopic stage (MIS) 11, even if absolute dates are still divergent. The evidence of two major warm periods with an interglacial type of vegetation between the Holsteinian and the Eemian is confirmed by the Velay record.

High-resolution record of climate stability in France during the last interglacial period

The last interglacial period (127–110 kyr ago) has been considered to be an analogue to the present interglacial period, the Holocene, which may help us to understand present climate evolution. But whereas Holocene climate has been essentially stable in Europe, variability in climate during the last interglacial period has remained unresolved, because climate reconstructions from ice cores, continental records and marine sediment cores give conflicting results for this period. Here we present a high-resolution multi-proxy lacustrine record of climate change during the last interglacial period, based on oxygen isotopes in diatom silica, diatom assemblages and pollen–climate transfer functions from the Ribains maar in France. Contrary to a previous study, our data do not show a cold event interrupting the warm interglacial climate. Instead, we find an early temperature maximum with a transition to a colder climate about halfway through the sequence. The end of the interglacial period is clearly marked by an abrupt change in all proxy records. Our study confirms that in southwestern Europe the last interglacial period was a time of climatic stability and is therefore still likely to represent a useful analogue for the present climate.

European vegetation during Marine Oxygen Isotope Stage-3

European vegetation during representative “warm” and “cold” intervals of stage-3 was inferred from pollen analytical data. The inferred vegetation differs in character and spatial pattern from that of both fully glacial and fully interglacial conditions and exhibits contrasts between warm and cold intervals, consistent with other evidence for stage-3 palaeoenvironmental fluctuations. European vegetation thus appears to have been an integral component of millennial environmental fluctuations during stage-3; vegetation responded to this scale of environmental change and through feedback mechanisms may have had effects upon the environment. The pollen-inferred vegetation was compared with vegetation simulated using the BIOME 3.5 vegetation model for climatic conditions simulated using a regional climate model (RegCM2) nested within a coupled global climate and vegetation model (GENESIS-BIOME). Despite some discrepancies in detail, both approaches capture the principal features of the present vegetation of Europe. The simulated vegetation for stage-3 differs markedly from that inferred from pollen analytical data, implying substantial discrepancy between the simulated climate and that actually prevailing. Sensitivity analyses indicate that the simulated climate is too warm and probably has too short a winter season. These discrepancies may reflect incorrect specification of sea surface temperature or sea-ice conditions and may be exacerbated by vegetation–climate feedback in the coupled global model.

Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka

We present a high-resolution and independently dated multiproxy lake sediment record from the paleolake at Les Echets in southeastern France that displays synchronous changes in independent limnic and terrestrial ecosystem proxies, in concert with millennial-scale climate oscillations during the last glacial period. Distinct lake-level fluctuations, low lake organic productivity, and open, treeless vegetation indicate cold and dry conditions in response to Heinrich events. Alternating phases of higher and low lake organic productivity, stratified surface waters and long-lasting lake ice cover, decreased or increased catchment erosion, and tree-dominated or herb-dominated vegetation resemble Dansgaard-Oeschger interstadialstadial variability. Transitions between different ecological states occurred in as little as 40–230 yr and seem to have been controlled by the position of the Polar Front. Ecosystem response after 30 ka suggests that local climate conditions became more important. Our results demonstrate that all parts of the terrestrial system responded to the abrupt and dramatic climatic changes associated with Dansgaard-Oeschger and Heinrich events, and that regional factors modulated ecosystem response.

Vegetation dynamics in north-western Mediterranean regions: Instability of the Mediterranean bioclimate

Abstract Pollen and plant macrofossil records from a selection of sites in Spain, France and Italy are used to explore the origin and the development of the Mediterranean vegetation. The role of evergreen and summergreen taxa is discussed. The expansion of sclerophyllous trees during the Pleistocene interglacials is well correlated with the orbital forcing. During the Holocene, the large number of data illustrates latitudinal and longitudinal differences in vegetation dynamics. Multidisciplinary studies identify short terms climatic fluctuations. The debate is still open between those who attribute an increase of sclerophyllous forests during the late Holocene to a trend toward aridity and those who consider this dynamic as human-induced.

The transition from temperate phases to stadials in the long upper Pleistocene sequence from Les Echets (France)

Abstract From two sections of the long sequence of Les Echets, corresponding to the end of the Eemian and to the end of St-Germain II interstadial respectively, detailed pollen diagrams have been established which provide information about the climatic conditions prevailing during the transitions from very temperate periods to the following cold stadials.

Indian summer monsoon variations could have affected the early Holocene woodland expansion in the Near East

Postglacial expansion of deciduous oak woodlands of the Zagros—Anti-Taurus Mountains, a major biome of the Near East, was delayed until the middle Holocene at ~6300 cal. yr BP. The current hypotheses explain this delay as a consequence of a regional aridity during the early Holocene, slow migration rates of forest trees, and/or a long history of land use and agro-pastoralism in this region. In the present paper, support is given to a hypothesis that suggests different precipitation seasonalities during the early Holocene compared with the late Holocene. The oak species of the Zagros—Anti-Taurus Mts, particularly Quercus brantii Lindl., are strongly dependent on spring precipitation for regeneration and are sensitive to a long dry season. Detailed analysis of modern atmospheric circulation patterns in SW Asia during the late spring suggests that the Indian Summer Monsoon (ISM) intensification can modify the amount of late spring and/or early summer rainfall in western/northwestern Iran and eastern Anatolia, which could in turn have controlled the development of the Zagros—Anti-Taurus deciduous oak woodlands. During the early Holocene, the northwestward shift of the Inter-Tropical Convergence Zone (ITCZ) could have displaced the subtropical anticyclonic belt or associated high pressure ridges to the northwest. The latter could, in turn, have prevented the southeastward penetration of low pressure systems originating from the North Atlantic and Black Sea regions. Such atmospheric configuration could have reduced or eliminated the spring precipitation creating a typical Mediterranean continental climate characterized by winter-dominated precipitation. This scenario highlights the complexity of biome response to climate system interactions in transitional climatic and biogeographical regions.

Reconstruction and paleoclimatic interpretation of Holocene lake-level changes in Lac de Saint-Léger, Haute-Provence, southeast France

Abstract Lac de Saint-Leger is located in the transitional region between Central Europe and the Mediterranean. The reconstruction of the past lake-level changes was based on plant macrofossil and mollusc analyses of two stratigraphic transects correlated by pollen analysis. Past changes in lake level were quantitatively reconstructed from changes in the recorded level of overgrowth. In the beginning of the Holocene the lake level was high; however, a progressive lowering occurred from about 7500 B.P. Between about 4500 B.P. and 4000 B.P. there was a moderate rise in lake level. Subsequently the lowering was continued and culminated between about 3300 B.P. and 2500 B.P. From the reconstruction there then occurred a progressive rise in lake level until the present. The lake-level changes recorded in Lac de Saint-Leger are compared with a compilation of lake-level data from southern France. The major long-term changes can be supposed to be regionally significant and related to climatic changes. To obtain a basis for paleoclimatic interpretation of the changes in lake level, the reference pollen diagram from Lac de Saint-Leger was used for a quantitative reconstruction of paleoclimate. During the middle and later part of the Holocene there is a reasonable consistency between the recorded pattern of lake-level changes and the paleoclimatic reconstruction. During the earlier part of the Holocene there are some disagreements, which may be explained by the shortage in adequate modern pollen analogues. The paleoclimatic reconstruction indicates that the major changes in lake level were mostly related to changes in precipitation.