Increased growth variability and lower post-drought recovery characterize Atlas cedar decline

Tree growth decline and forest die-off are widely reported in climate-warming hotspots such as the Mediterranean Basin. There, several studies have been conducted in European conifer forests, but research on widespread Atlas cedar ( Cedrus atlantica ) die-off in xeric mountain sites of the Maghreb is still limited. African Atlas cedar die-off sites could provide valuable diagnostic signals by analyzing the radial growth series of declining trees currently showing high crown defoliation. We investigated whether declining and healthy (not defoliated) Atlas cedar trees differed in their growth rates, growth variability, post-drought recovery and responsiveness to climate, in the “Theniet El Had” National Park, northwestern Algeria. We also searched for shifts in drought severity and growth to explain the recent die-off. The study area showed a rapid aridification due to rising temperatures after the 1980s, when a shift towards warmer conditions started as the North Atlantic Oscillation (NAO) showed persistent positive phases. Such a climate shift led to another growth shift in the 1980s when growth rates abruptly dropped. Currently declining trees presented lower growth rates and temporal growth stability (higher growth variability) than healthy trees. Superposed epoch analyses showed that declining trees presented a significant negative growth departure during dry years. Declining trees showed a higher resistance but lower recovery and relative resilience than healthy trees. Dry and warm conditions in the previous autumn and during the current growing season constrained growth. In declining trees, growth was more limited by May water deficit than in healthy trees. Healthy trees showed shifts in the drought-growth correlations in the 1970s and 1980s. Trees currently classified as declining were characterized by lower growth stability during the climate shift associated with the studied die-off.