Abstract: The Atlas cedar forests (Cedrus atlantica (Endl.) Manetti ex Carrière) are among the most emblematic and threatened 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.
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 1617 m a.s.l. in the Middle Atlas region of Morocco. Utilizing this new fossil record alongside other records from the Middle Atlas, we traced the range dynamics 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 increased and its seasonal distribution changed. Throughout the last glacial period, the Atlas cedar likely persisted at lower elevations or as isolated populations near our study area. It was not until the Northgrippian, when conditions became warmer and wetter, that the cedar was able to migrate upslope and stablish locally.
More favorable 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 range shift 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 range dynamics 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 prioritize habitat protection, minimise human pressure, and sustain ecosystem resilience.
