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To understand the effects of drought events on the forest biogeochemical cycles and tree nutrition, highly instrumented experiments have been set up in a mature beech forest of North-Eastern France. The experiments allowed to calculate the nutrients fluxes and stocks within various compartments of the ecosystems (i.e., perennial biomass, green leaves, litterfall, soil, fine roots, atmospheric deposition, throughfall, stemflow and soil solution) and to consider three degrees of droughts (Low – L, Medium – M and High – H). The L degree was assessed through the observation of natural droughts while the M and H degrees were assessed thanks to rainfall exlusion experiments. This work showed that the disruptions of biogeochemical cycles by droughts are mainly indirectly induced by the evolution of the biomass production. Moreover the drought impacts were often accentuated by the drought degree. Thus, the M degree appeared to slightly reduce total biomass production, due to important decrease of aboveground productions that were partly compensated by an increased fine root production, also observed in the H degree. In addition, the calculated nutrient flux and stocks in the M degree suggested no drought effect on nutrients needs for biomass production while a decrease in leaf biomass production induced lower resorption fluxes. Potassium seemed to be the most drought sensitive nutrient with an important decrease of its resorption efficiency with the drought degree and a decrease of its availability in the first soil layers measured for the H degree. A K deficiency in leaves for the H degree was also measured. This K deficiency may be the direct consequence of maintaining the K need for biomass production associated with insufficient resorption efficiency and low-K availability in the soil. A nutrient stress, coupled with water stress, in a context of intense and repeated drought events may accentuate forest dieback in the coming decades.
