MIGRATION RATES

At temperate latitudes tree species have adjusted their ranges substantially in response to past climate changes. Their maximum migration rates, based on fossil pollen data, were estimated higher than those simulated by theoretical models. This discrepancy is (partially) related to the location and number of areas (glacial refugia) from which species propagate.

In order to contribute to this debate I used both fossil pollen records and modern genetic data to estimate the migration rates of silver fir (figure). The fossil pollen records allow to locate the putative glacial refugia from which species may spread. The genetic data allow (1) the exclusion of those refugial areas from which the species did not spread or spread only locally during the Holocene and (2) the delineation of the areas over which the species spread from each glacial refugium.
This study shows that integrating fossil pollen records and DNA data may provide more accurate estimates of the species migration rates.

Migration rates (m/yr) of Abies alba from pollen data solely (a); from both pollen and genetic data for (b) the northern Apennine and (c) Balkan haplotypes within each time-slice.

GLACIAL REFUGIA & SPECIES PHYLOGEOGRAPHY

During the last glacial period the ranges of all temperate tree species were geographically limited due to severe climatic conditions. Species had to survive as isolated populations in glacial refugia from which they recolonized larger areas during the post-glacial period. The impact of species isolation in glacial refugia on their modern genetic diversity is clearly established. Reconstructing the migration pathways of different genotypes may help us for a better evaluation of the species response to future climate changes.

Within this research topic I aim at reconstructing the migration pathways of tree species and the migration timing since the last glacial period in Europe using both fossil records and genetic data. As an illustration of this research topic, I have shown that the long-term isolation of Pinus sylvestris (figure) in a few glacial refugia and its migrational process during the Holocene have played a major role in shaping its modern genetic diversity in Europe.

Migration timing (isolines) and routes (arrows) of Pinus sylvestris in Europe inferred from palaeobotanical and modern genetic data.The arrows and their colors correspond to the migration routes of different haplotypes of the species.

PAST CLIMATES IMPACTS ON SPECIES

Using statistical methods we can estimate different climate variables from the fossil pollen records. Climate time series can help us understanding how species have reacted to past climate changes and evaluating their potential of persisting in their modern range or not during the next century.

As an illustration of this research topic, we have quantified the mean January temperature and both winter and summer precipitation from three fossil pollen records collected in Lebanon. These three climate time series were interpreted in terms of drivers of persistence of tree species in the Eastern Mediterranean. In this case we observed that the low amplitude of temperature change between the last glacial period and the Holocene as well as the change in the seasonal distribution of moisture throughout the year instead of an occurrence mainly during the winter season seem to have helped tree species to persist in situ.

Reconstructed mean January temperature from the fossil pollen records of Al Jourd, Aammiq and Chamsine compared to the dO18 values from the NGRIP ice core with correlation coefficients (r2) between the reconstructed temperature of each record and NGRIP d180 values.

CONSERVATION EFFORTS FOR ATLAS CEDARS

Cedrus atlantica is an endangered species according to the IUCN Red List. Today we observe that this species has a highly fragmented range, its populations densities are decreasing and its lower treeline is shifting at higher elevations. Environmental studies in northern Africa show that past climate changes had a significant impact on the occurrences of Atlas cedars and on their overall genetic diversity.

This research topic involves palaeoecology, genetics and vegetation modeling. As a palaeoecologist I am investigating the past environmental and climatic changes that have impacted the dynamics of Atlas cedar and I expect to identify the drivers that are causing the current regression of the species range. Geneticists are carrying a DNA survey on the modern populations for mapping the overall genetic diversity. Vegetation modelers are performing experiments to test the causes of the species' range shifts through time. My goal is that this multidisciplinary approach will help a better preservation of the Atlas cedars in Morocco.

Populations of Morrocan Atlas cedar sampled (map) for a genetic study and a phylogenetic tree showing genetic distances between populations.