Rachid Cheddadi

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.

Reconstructing biomes from palaeoecological data: a general method and its application to European pollen data at 0 and 6 ka

Biome models allow the results of experiments with atmospheric general circulation models to be translated into global maps of potential natural vegetation. The use of biome models as a diagnostic tool for palaeoclimate simulations can yield maps that are directly comparable with palaeoecological (pollen and plant macrofossil) records provided these records are “biomized”, i.e. assigned to biomes in a consistent way. This article describes a method for the objective biomization of pollen samples based on fuzzy logic. Pollen types (taxa) are assigned to one or more plant functional types (PFTs), then affinity scores are calculated for each biome in turn based on its list of characteristic PFTs. The pollen sample is assigned to the biome to which it has the highest affinity, subject to a tie-breaking rule. Modern pollen data from surface samples, reflecting present vegetation across Europe, are used to validate the method. Pollen data from dated sediment cores are then used to reconstruct European vegetation patterns for 6 ka. The reconstruction shows systematic differences from present that are consistent with previous interpretations. The method has proved robust with respect to human impacts on vegetation, and provides a rational way to interpret combinations of pollen types that do not have present-day analogs. The method demands minimal prior information and is therefore equally suitable for use in other regions with richer floras, and/or lower densities of available modern and fossil pollen samples, than Europe.

Fifty thousand years of Arctic vegetation and megafaunal diet

Although it is generally agreed that the Arctic flora is among the youngest and least diverse on Earth, the processes that shaped it are poorly understood. Here we present 50 thousand years (kyr) of Arctic vegetation history, derived from the first large-scale ancient DNA metabarcoding study of circumpolar plant diversity. For this interval we also explore nematode diversity as a proxy for modelling vegetation cover and soil quality, and diets of herbivorous megafaunal mammals, many of which became extinct around 10 kyr bp (before present). For much of the period investigated, Arctic vegetation consisted of dry steppe-tundra dominated by forbs (non-graminoid herbaceous vascular plants). During the Last Glacial Maximum (25–15 kyr bp), diversity declined markedly, although forbs remained dominant. Much changed after 10 kyr bp, with the appearance of moist tundra dominated by woody plants and graminoids. Our analyses indicate that both graminoids and forbs would have featured in megafaunal diets. As such, our findings question the predominance of a Late Quaternary graminoid-dominated Arctic mammoth steppe.

Genetic consequences of glacial survival and postglacial colonization in Norway spruce: combined analysis of mitochondrial DNA and fossil pollen

Norway spruce (Picea abies [L.] Karst.) is a broadly distributed European conifer tree whose history has been intensively studied by means of fossil records to infer the location of full‐glacial refugia and the main routes of postglacial colonization. Here we use recently compiled fossil pollen data as a template to examine how past demographic events have influenced the species’ modern genetic diversity. Variation was assessed in the mitochondrial nad1 gene containing two minisatellite regions. Among the 369 populations (4876 trees) assayed, 28 mitochondrial variants were identified. The patterns of population subdivision superimposed on interpolated fossil pollen distributions indicate that survival in separate refugia and postglacial colonization has led to significant structuring of genetic variation in the southern range of the species. The populations in the northern range, on the other hand, showed a shallow genetic structure consistent with the fossil pollen data, suggesting that the vast northern range was colonized from a single refugium. Although the genetic diversity decreased away from the putative refugia, there were large differences between different colonization routes. In the Alps, the diversity decreased over short distances, probably as a result of population bottlenecks caused by the presence of competing tree species. In northern Europe, the diversity was maintained across large areas, corroborating fossil pollen data in suggesting that colonization took place at high population densities. The genetic diversity increased north of the Carpathians, probably as a result of admixture of expanding populations from two separate refugia.

The climate in Western Europe during the last Glacial/Interglacial cycle derived from pollen and insect remains

Using the pollen sequence of La Grande Pile XX (France), we review problems with the application of transfer functions in paleoclimatic reconstructions. One of them is to find modern analogues for the herbaceous vegetation of the cold periods. We propose a method to distinguish between steppes and tundra vegetations for which the moderns are only partial analogues of the glacial periods. Another method to solve these problems is based on constraining by insect remains. The two methods provide coherent reconstructions. The results are also compared with other paleodata. There is a good correlation with the six cold Heinrich events between 70 and 15 ka B.P. A cooling event during the Eemian period (marked by high percentages of Taxas) at about 125 ka B.P. needs still to be correlated with high resolution ocean and ice cores.

Was the climate of the Eemian stable? A quantitative climate reconstruction from seven European pollen records

The aim of the present study is to estimate the range of the climatic variability during the Eemian interglacial, which lasted about 10,000 years (marine isotopic stage 5e). The modem pollen analogue technique is applied to seven high resolution pollen records from France and poland to infer the annual precipitation and the mean temperature of the coldest month. The succession of pollen taxa and the reconstructed climate can be interpreted coherently. The warmest winter temperatures are centred in the first three millennia of the Eemian interglacial, during the mixed oak forest phase with Quercus and Corylus as dominant trees. A rapid shift to cooler winter temperatures of about 6 degrees to 10 degrees C occurred between 4000 and 5000 years after the beginning of the Eemian, related to the spread of the Carpinus forest. This shift is more obvious for the reconstructed temperatures than for precipitation and is unique and irreversible for the whole Eemian period. Following this climatic shift of the Eemian, variations of temperature and precipitation during the fast 5000 years were only slight with an amplitude of about 2 degrees to 4 degrees C and 200 to 400 mm/yr. The estimated temperature changes were certainly not as strong as those reconstructed for the stage 6/5e termination or the transition 5e/5d. This is consistent with the constantly high ratio of tree pollen throughout the Eemian, indicative of a succession of temperate forest types. This gradual transition between different forest landscapes can be related to intrinsic competition between the species rather than to a drastic climatic change

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.

Inverse vegetation modeling by Monte Carlo sampling to reconstruct palaeoclimates under changed precipitation seasonality and CO2 conditions: application to glacial climate in Mediterranean region

Abstract Atmospheric CO 2 concentration has greatly fluctuated during the Quaternary. These variations have influenced the vegetation changes. The assumption that the relationship vegetation–climate sensu stricto was constant through time should be reconsidered taking into account the impact of the atmospheric CO 2 content on the plants. Here we propose to use a process-based vegetation model (BIOME3) in an inverse mode to reconstruct from pollen data the most probable climate under precipitation seasonality change and under lowered CO 2 concentration in the biosphere. Appropriate tools to match the model outputs with the pollen data are developed to generate a probability distribution associated with the reconstruction (Monte Carlo sampling and neural network techniques). The method is validated with modern pollen samples from Greece and Italy: it proves to be able to reconstruct modern climate with a more or less large error bar from pollen data. The error bar depends in fact on the tolerance of the vegetation to the corresponding climatic variable. The application to six pollen assemblages from Greece and Italy, representing the last glacial maximum (LGM: 18 000 14 C-year B.P.), is done into three experiments: (1) modern CO 2 concentration; (2) LGM CO 2 concentration; (3) LGM CO 2 concentration and high winter precipitation. The latter experiment is motivated by evidence of high lake-levels in Greece during the LGM which has been attributed to winter rainfall. These experiments show that winter was ca. 15–20°C colder than the present, in agreement with previous climate reconstruction. The apparent discrepancy between the high lake-levels and the steppe vegetation during the LGM, can be explained by an increase of the winter precipitation (which leads to high lake level) while the summer season is mild and dry (which affects the vegetation). The summer temperature has three stable states (−16°C, −10°C, −2°C), but the warmest one is the most probable if we take into account the lowered CO 2 and the high lake-levels.

Climate in northern Eurasia 6000 years ago reconstructed from pollen data

Using a climatic calibration based on the scores of the plant functional types (PFTs) calculated for 1245 surface pollen spectra, the climate at 6 ka BP has been reconstructed for a set of 116 pollen spectra from the former Soviet Union and Mongolia. The results are presented as maps of climatic anomalies and maps of probability classes showing the significance of these differences from the modern climate. The reconstructed patterns are spatially coherent, but have confidence levels that vary from region to region, due to the often-large error ranges. At 6 ka, the winters were more than 2oC warmer than today north of 50oN, with a high significance east of the Urals. Summers were also more than 2oC warmer than today with a high level of confidence north of the Polar Circle and in central Mongolia. In the mid-latitudes of Siberia, in northern Kazakhstan and around the Black and the Caspian seas, 6 ka summers were significantly cooler than today. The reconstructed moisture availability (ratio of actual to equilibrium evapotranspiration) was more than 10% higher than today in the Ukraine, southern Russia and northern Mongolia, and more than 10% lower than today in central Mongolia. This pattern corresponds partly with that of the water budget (annual precipitation minus evaporation) reconstructed from lake level records.

A method to determine warm and cool steppe biomes from pollen data; application to the Mediterranean and Kazakhstan regions

An objective method for the assignment of pollen spectra to appropriate biomes has been published recently. The aim of this paper is to improve the distinction between warm and cool steppes, thus refining vegetation and climate reconstruction, particularly during the Last Glacial Maximum. A set of modern pollen spectra from the Mediterranean and Kazakhstan regions, dominated today by open vegetation types, has been analysed statistically in order to relate pollen taxa abundances to warm acid cool grass/shrub plant functional types (PFTs). A statistical test using modern pollen data shows that the method is able to distinguish between cool and warm steppe biomes with a high degree of confidence. The method has been applied to two fossil pollen records. The results of this exercise showed that cool steppe dominated in central Greece between 18 000 and 13 000 yr BP, while in western Iran the vegetation was at the boundary between cool and warm steppes. These vegetation types were replaced by warm mixed forest in Greece and warm steppe in Iran after that time span