The decline of Algerian Cedrus atlantica forests is driven by a climate shift towards drier conditions

Some north-African Atlas cedar (Cedrus atlantica) forests are in decline, following decades of anthropogenic pressure and repeated drought events. We investigated if the recent decline episodes of these forests are linked to precipitation and temperature shifts, leading to a reduction in tree radial growth and climate-growth uncouplings. Tree-ring width chronologies of Atlas cedar in north-western Algeria allow the identification of climate and growth shifts in these vulnerable Mediterranean forests. Such chronologies, built for six sites, showed common patterns of year-to-year variability during the period 1910-2006. The growth at north-facing sites declined from the 1980s until 2006, whereas the growth at mid-elevation sites declined from the early 20th century until the 1940s, remained stable until the 1980s and then declined until 2006. Cool and wet spring conditions enhanced cedar growth. Sites with fast-growing trees, where growth was strongly reduced by dry summer conditions, showed the most-pronounced negative trends. However, a clear climate-growth uncoupling was observed after the 1970s, when the climate rapidly warmed. We also detected a negative growth shift in the 1980s, when mortality increased. This coincided with changes in early-warning signals of the growth series, such as an increase in the first-order autocorrelation of tree-ring width. All these lines of evidence indicate that the 1980s climate shift towards warmer and drier conditions triggered a shift in cedar growth. The use of radial-growth series as early-warning signals should be further investigated in this species and in other drought-sensitive conifers, given the aridification trends expected for the Mediterranean Basin.