Biodiversity in extreme environments at “Critical Risk” of extinction: the ancient cypress trees of the Sahara (Cupressus dupreziana A. Camus)

Although the Sahara is the largest desert on Earth, it harbours a unique relict flora of exceptional biogeographical value. Among its most remarkable spe- cies is Cupressus dupreziana, the Saharan desert cypress or Tarout, endemic to the Tassili n’Ajjer Mountains of southern Algeria and currently regarded as one of the most endangered tree species worldwide. Although long known to Tuareg populations, the species was not scientifically described until 1926, having initially been confused with other conifers. Subsequent studies have unequivocally demonstrated its distinctiveness from Cupressus sempervirens and C. atlantica based on genetic divergence, reproductive biology—most notably paternal apomixis—and morphological characteristics. The species persists under extremely harsh conditions, occupying hyper-arid highlands shaped by ancient oueds (ancient dry riverbeds) and gorges that function as microrefugia. This region preserves remnants of a formerly humid Holocene flora, including the desert olive (Olea europaea subsp. laperrinei) and the Sa- haran myrtle (Myrtus nivellei). Many individuals are estimated to be between 600 and 2,500 years old; however, natural regeneration is almost entirely absent due to extreme aridity and very low seed viability. The “Tarout Project”, involving researchers from Italy, Algeria, Spain, and Switzerland, has documented a recent and rapid decline of the species. The most recent complete inventory (1998–2001) recorded 233 living individuals of C. dupreziana, forming a single, highly fragmented population distributed over approximately 120 km², with at least 12 confirmed mortalities. The main drivers of this decline include prolonged drought (2019–2023), human disturbance, livestock grazing, physiological stress, pathogen activity, and biomechanical instability. Current efforts focus on population reassess- ment, health diagnostics, genetic and phytopathological analyses, germplasm conservation, and the implementation of urgent measures aimed at restoring both individuals and their habitats. This study provides the first detailed documentation of a recent mortality episode affecting the species and explores its likely causes.