Résumé
The Atlas cedar forests (Cedrus atlantica (Endl.) Manetti ex Carrière) are among the most emblematic ecosystems of the Mediterranean region. Although this conifer species is relatively resistant to drought, it is still quite vulnerable to long-lasting and frequent droughts and prolonged heat stress due to its limited ability to withstand high temperatures. Today, it is found in fragmented areas of the North African mountains, at elevations ranging from 1,200 to 2,500 meters above sea level (m a.s.l.) where it thrives in increasingly dry conditions and experiences significant seasonal hydroclimatic stress.To understand the historical relationship between Atlas cedar forests and climate change, we performed an in-depth multiproxy paleoecological study on a newly obtained sediment core from the N’Harcha mire, located at 1,617 m a.s.l. in the Middle Atlas region of Morocco. Utilizing this new fossil records alongside other records from the Middle Atlas, we traced the expansion patterns and biogeographic processes that have influenced the distribution of Atlas cedar since the last postglacial period.Our study shows that steppe landscapes were predominant during the cold and arid Younger Dryas period (ca. 12,000 years ago). This was followed by a gradual transition to evergreen sclerophyllous forests as temperatures rose and both the annual precipitation and its seasonal distribution increased. Throughout the last glacial period, the Atlas cedar likely persisted in small populations at lower elevations or as isolated populations near our study area. It wasn't until the Northgrippian, when conditions became warmer and wetter, that the cedar was able to migrate upslope and expand locally.More favourable climatic conditions facilitated the establishment of a cedar-dominated forests during the early Meghalayan. In more recent millennia, the conifer belt has gradually contracted, while the prevalence of evergreen Quercus has increased, likely due to growing aridity and intensified human activity. This trend has ultimately led to a decline in forest cover and a resurgence of steppe elements, despite the notable structural resilience of cedar populations to both climatic and anthropogenic disturbances. This resilience, however, operates over long timescales and does not always allow for the recovery of populations once they have collapsed locally.The comparison of N’Harcha with other records from the Middle Atlas suggests that the postglacial expansion of Cedrus atlantica was neither uniform nor synchronous. It likely followed a mosaic-like pattern, with marked spatial and temporal variability. These findings underscore the need to interpret the Holocene expansion of Cedrus not as a single regional process, but rather as a complex, stepwise forest reorganization across Morocco’s mountain ranges.In the context of ongoing climate change, the shift from Cedrus forests toward open, stress-tolerant assemblages—through processes of matorralization and steppization—raises urgent conservation concerns. Mountain refugia may represent the last strongholds for this heat-sensitive conifer, highlighting the need to prioritise habitat protection, minimise human pressure, and sustain ecosystem resilience.