Significance: Explaining the greening of the Sahara during the Holocene has been a challenge for decades. A strengthening of the African monsoon caused by increased summer insolation is usually cited to explain why the Sahara was vegetated from 14,000 to 5,000 y ago. Here, we provide a unique climate record of quantified winter, spring, and summer precipitation in Morocco over the past 18,500 y, and numeric simulations, which show that moisture contributions from the Mediterranean Sea and the North Atlantic Ocean in winter, were as important as the expanded summer monsoon for the greening of the Sahara during the African humid period. The findings of this study will help to better understand and simulate climate variability over northern Africa.

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.

The threatened conifer Abies cilicica currently persists in Lebanon in geographically isolated forest patches. The impact of demographic and evolutionary processes on population genetic diversity and structure were assessed using 10 nuclear microsatellite loci. All remnant 15 local populations revealed a low genetic variation but a high recent effective population size. F_ST-based measures of population genetic differentiation revealed a low spatial genetic structure, but Bayesian analysis of population structure identified a significant Northeast-Southwest population structure. Populations showed significant but weak isolation-by-distance, indicating non-equilibrium conditions between dispersal and genetic drift. Bayesian assignment tests detected an asymmetric Northeast-Southwest migration involving some long-distance dispersal events. We suggest that the persistence and Northeast-Southwest geographic structure of Abies cilicica in Lebanon is the result of at least two demographic processes during its recent evolutionary history: (1) recent migration to currently marginal populations and (2) local persistence through altitudinal shifts along a mountainous topography. These results might help us better understand the mechanisms involved in the species response to expected climate change.

The aim of this study is to reconstruct the impact of past environmental changes in the Moroccan Rif mountainous ecosystems. We collected an 8.5 m sedimentary core in the western part of the Rif and analyzed its palynological content, micro-remains of fossil charcoal, particle size, and content of several chemical elements. The chronology of the record is based on ten ¹⁴C datings that indicate a continuous time span of 9,000 years. This multidisciplinary approach allowed us to reconstruct both the environmental evolution of the catchment area and the vegetation dynamics in the surrounding landscape. Besides the fossil record, we collected a series of surface samples along a transect running from Talassemtane National Park, where fir populations are currently protected, to the study site in order to determine the dispersal capacity of different plant taxa. Based on the modern observations, the fossil pollen data show that despite its closer range today to the study site than Cedrus atlantica (closest populations at ca. 20 km), Abies maroccana (populations at ca. 10 km) did not reach the studied site during the Holocene. At the same time, Cedrus atlantica was present and even dominated the landscape between 8,000 and 6,000 BP. After 6,000 BP, other species more tolerant to annual drought, such as evergreen oaks, expanded gradually. The early Holocene period was wet enough to promote a maximum expansion of cedar forest. The Atlas cedar disappears from the fossil record after 2,000 BP due to an increased human pressure. Particle size analysis and chemical elements are particularly relevant for reconstructing the landscape changes and for depicting the human impact. Their changes after 2,000 BP are well correlated with the forest cover decline.

We quantified the degree to which the relationship between the geographic distribution of three major European tree species, Abies alba, Fagus sylvatica and Picea abies and January temperature (Tjan) has remained stable over the past 10,000 years. We used an extended data-set of fossil pollen records over Europe to reconstruct spatial variation in Tjan values for each 1,000-year time slice between 10,000 and 3,000 years BP (before present). We evaluated the relationships between the occurrences of the three species at each time slice and the spatially interpolated Tjan values, and compared these to their modern temperature ranges. Our results reveal that Fagus sylvatica and Picea abies experienced Tjan ranges during the Holocene that differ from those of the present, while Abies alba occurred over a Tjan range that is comparable to its modern one. Our data suggest the need for re-evaluation of the assumption of stable climate tolerances at a scale of several thousand years. The temperature range instability in our observed data independently validates similar results based exclusively on modelled Holocene temperatures. Our study complements previous studies that used modelled data by identifying variation in frequencies of occurrence of populations within the limits of suitable climate. However, substantial changes that were observed in the realized thermal niches over the Holocene tend to suggest that predicting future species distributions should not solely be based on modern realized niches, and needs to account for the past variation in the climate variables that drive species ranges.