Aridification increases growth resistance of Atlas cedar forests in NW Algeria

A warmer climate will increase aridity and threaten forest persistence in xeric areas. This is the case of some Atlas cedar (Cedrus atlantica) forests showing recent growth decline, dieback and high mortality rates in North Africa. A lower resistance to drought, manifested as stronger growth loss, could increase the drought-related mortality risk in these forests as has been found for gymnosperms worldwide. It could be also expected that changes in dryness are linked to large-scale atmospheric circulation patterns such as the North Atlantic Oscillation (NAO). We tested these hypotheses by analyzing growth resilience indices (resilience, recovery, resistance) derived from tree-ring data. We sampled nine Atlas cedar plots located in north-western Algeria, where climate shifted towards drier conditions in the 1980s. In these forests, drier winters, associated to positive NAO phases, constrained tree growth and resistance, two outputs of the drought impact. During dry years, the resilience index decreased as elevation increased. The association between cedar growth resistance and drought severity is strengthening: the drier the climate conditions, the lower the growth resistance. Resistance also showed a significant higher temporal variability in 8 out of 9 plots as drought intensified. These findings have allowed identifying the increase in the temporal variability of growth resistance as a new early-warning signal of drought stress.