Drought, axe and goats. More variable and synchronized growth forecasts worsening dieback in Moroccan Atlas cedar forests

Understanding how climate warming and land-use changes determine the vulnerability of forests to drought is critical. However, we still lack: (i) robust quantifications of long-term growth changes during aridification processes, (ii) links between growth decline, changes in forest cover, stand structure and soil conditions, and (iii) forecasts of growth variability to projected climate warming. We investigated tree-ring records over the past 400-700 years, quantified changes in grazing area and forest cover during the 20th century, sampled current stand structure, and analyzed soil organic carbon δ¹³C and total nitrogen δ¹⁵N of Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carrière) Moroccan forests to characterize their dieback. Atlas cedar forests experienced massive dieback after the 1970s, particularly in the xeric High Atlas region. Forest cover increased in the less xeric regions (Middle Atlas and Rif) by almost 20%, while it decreased about 18% in the High Atlas, where soil δ¹³C and δ¹⁵N showed evidences of grazing. Growth declined and became more variable in response to recent droughts. The relative growth reduction (54%) was higher in the Middle Atlas than elsewhere (Rif, 32%; High Atlas, 36%). Growth synchrony between forests located within the Middle and High Atlas regions increased after the 1970s. Simulations based on a worst-case emission scenario and rapid warming forecast a stronger limitation of growth by low soil moisture in all regions, but particularly in the Middle Atlas and after the mid-21st century. Climate warming is expected to strengthen growth synchronization preceding dieback of conifer forests in xeric regions. The likelihood of similar dieback episodes is further exacerbated by historical degradation of these forests.