Alexandra-Jane Henrot

Growth of subtropical forests in Miocene Europe: The roles of carbon dioxide and Antarctic ice volume

The middle Miocene is a crucial period for the evolution of apes, and it corresponds to their appearance in Europe. The dispersion of apes was made possible by tectonic changes and the expansion of their habitat, (sub-) tropical forest, in Europe. The context in which the middle Miocene climatic optimum occurred still lacks constraints in terms of atmospheric p CO 2 and ice-sheet volume and extent. Using a coupled atmosphere-ocean general circulation model (GCM) and dynamic vegetation model, we investigated the sensitivity of Miocene climate and vegetation to p CO 2 levels and Antarctic ice-sheet configurations. Our results indicate that higher than present p CO 2 is necessary to simulate subtropical forest in Western and Central Europe during the middle Miocene, but that a threshold at high p CO 2 makes subtropical forest partly collapse. Moreover, removing ice over Antarctica modifies oceanic circulation and induces warmer and slightly wetter conditions in Europe, which are consistent with the expansion of subtropical forest. These results suggest that a small East Antarctic Ice Sheet (25% of present-day ice volume) together with higher than present p CO 2 values are in better agreement with available European middle Miocene data.

Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco

This study reconstructs and interprets the changing range of Atlas cedar in northern Morocco over the last 9,000 years. A synthesis of fossil pollen records indicated that Atlas cedars occupied a wider range at lower elevations during the mid-Holocene than today. The mid-Holocene geographical expansion reflected low winter temperatures and higher water availability over the whole range of the Rif Mountains relative to modern conditions. A trend of increasing aridity observed after 6000 years BP progressively reduced the range of Atlas cedar and prompted its migration towards elevations above 1400 masl. To assess the impact of climate change on cedar populations over the last decades, we performed a transient model simulation for the period between 1960 and 2010. Our simulation showed that the range of Atlas cedar decreased by about 75% over the last 50 years and that the eastern populations of the range in the Rif Mountains were even more threatened by the overall lack of water availability than the western ones. Today, Atlas cedar populations in the Rif Mountains are persisting in restricted and isolated areas (Jbel Kelti, Talassemtane, Jbel Tiziren, Oursane, Tidighine) that we consider to be modern microrefugia. Conservation of these isolated populations is essential for the future survival of the species, preserving polymorphisms and the potential for population recovery under different climatic conditions.

Combining multiple statistical methods to evaluate the performance of process-based vegetation models across three forest stands

Abstract Process-based vegetation models are crucial tools to better understand biosphere-atmosphere exchanges and ecophysiological responses to climate change. In this contribution the performance of two global dynamic vegetation models, i.e. CARAIB and ISBACC, and one stand-scale forest model, i.e. 4C, was compared to long-term observed net ecosystem carbon exchange (NEE) time series from eddy covariance monitoring stations at three old-grown European beech (Fagus sylvatica L.) forest stands. Residual analysis, wavelet analysis and singular spectrum analysis were used beside conventional scalar statistical measures to assess model performance with the aim of defining future targets for model improvement. We found that the most important errors for all three models occurred at the edges of the observed NEE distribution and the model errors were correlated with environmental variables on a daily scale. These observations point to possible projection issues under more extreme future climate conditions. Recurrent patterns in the residuals over the course of the year were linked to the approach to simulate phenology and physiological evolution during leaf development and senescence. Substantial model errors occurred on the multi-annual time scale, possibly caused by the lack of inclusion of management actions and disturbances. Other crucial processes defined were the forest structure and the vertical light partitioning through the canopy. Further, model errors were shown not to be transmitted from one time scale to another. We proved that models should be evaluated across multiple sites, preferably using multiple evaluation methods, to identify processes that request reconsideration.