Seven millennia of Cedrus atlantica forest dynamics in the Western Rif Mountains (Morocco)

Abstract

Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carrière) is an endemic and relict conifer species from northwestern Africa, relatively drought-tolerant but also highly sensitive to recurrent summer heat stress. Cedar forests have undergone a dramatic range contraction in recent decades. The development of effective conservation strategies requires long-term perspectives to understand how forests have responded to past disturbances. We present a multi-proxy, high-resolution analysis of a 122 cm-deep fossil record (Merj Lkhil; LKH) located at 1213 m a.s.l. in Jbel Bou Hachem (Moroccan Rif), providing insights into the fragmentation of cedar stands. Cedrus likely formed extensive lowland populations during the final stages of the Late Glacial and began migrating upslope during the Greenlandian. It reached its maximum extent in the Rif around 7000 cal yr BP. Thereafter, increasing aridity, enhanced seasonality, and growing anthropogenic pressure triggered its long-term decline. This trajectory involved a vertical reorganization of montane ecosystems, with Cedrus progressively retreating within mid- and low-elevation forests, while deciduous oaks maintained a long-term co-dominance and Q. ilex L. gradually expanded, especially at lower elevations. Today, Cedrus is confined to isolated high-elevation stands in Jbel Bou Hachem. These relic populations should be prioritized for conservation under ongoing climate and land-use change.