I study the impact of past climates and environmental changes on tree species' distributions in Europe and in the Mediterranean areas over a period of time which encompasses the last glacial period and the Holocene.
I use fossil records to identify areas where tree species have persisted during the last glacial period, how they have adapted their range over the Holocene, how fast they could migrate and where they might persist over the next century under the ongoing global climate warming.
UPCOMING EVENTS
MY LATEST RESEARCH
Early Holocene greening of the Sahara requires Mediterranean winter rainfall
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The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in Early to Middle Holoceneclimate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa.
The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.
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Cheddadi, R., Carré, M., Nourelbait, M., François, L., Rhoujjati, A., Manay, R., Ochoa, D., Schefuß, E. 2021. Early Holocene Greening of the Sahara requires Mediterranean winter rainfall. Proceedings of the National Academy of Sciences, 118 (23), DOI: https://doi.org/10.1073/pnas.2024898118
Figure: Three simulation scenarios of net primary productivity in North Africa 9000 years ago using the CARAIB vegetation model: (A) simulation using the climatology of a climate model (HADCM3); (B) testing the hypothesis of an extension of the monsoon to North Africa with an additional 300 mm of precipitation in summer only; and (C) an increase in precipitation similar to (B) but with a different seasonal distribution: in winter north of 24°N, in summer south of 18°N, and gradually between 18° and 24°N with occurrence of precipitation in both summer and winter.
Priority conservation areas for Cedrus atlantica
in the Atlas Mountains, Morocco
Assessing biodiversity loss and species extinction is necessary to warn societyand raise awareness of the impacts of ongoing climate change. Prioritizing protected areas is the pragmatic and applicable management measure under the pressure of ongoing climate change and limited resources to conserve species at risk of extinction. We developed a novel conservation index (CI) to prioritize areas and populations of an endangered mountain tree species tha need protection in the face of ongoing climate change, as conservation of all populations may not be realistic. This CI integrates (1) mountain topography to identify potential refugial areas with suitable microclimates, (2) genetic diversity to assess the adaptive capacity of local populations, and (3) hypothetical climate change in the species' range. We applied this CI to Atlas cedar, an endemic and threatened species whose populations are scattered throughout the Moroccan mountains. This index provided a scale for 33 populations studied and suggests that genetically diverse populations located in rugged areas where future local climate may overlap with their current climatic niche should receive a higher conservation priority. This index may also be applicable to other mountain species with scattered populations and is likely to be more accurate if more precise climate data are used at the microrefugia scale.
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Cheddadi, R., Taberlet, P., Boyer, F., Coissac, E., Rhoujjati, A., Urbach, D., Remy, C., Khater, C., El Antry, S., Aoujdad, J., Carré, M., Ficetola, G.F. 2022. Priority conservation areas for Cedrus atlantica in the Atlas Mountains, Morocco. Conservation Science and Practice. E12680, 1-15. https://doi.org/10.1111/csp2.12680
Figure: Conservation index map showing recommended priorities for Atlas cedar populations from low (green) to high (red) priority