Drought events influence nutrient canopy exchanges and green leaf partitioning during senescence in a deciduous forest

The increase in the frequency and intensity of drought events expected in the coming decades in Western Europe may disturb forest biogeochemical cycles and create nutrient deficiencies in trees. One possible origin of nutrient deficiency is the disturbance of the partitioning of the green leaf pool during the leaf senescence period between resorption, foliar leaching and senesced leaves. However, the effects of drought events on this partitioning and the consequences for the maintenance of tree nutrition are poorly documented. An experiment in a beech forest in Meuse (France) was conducted to assess the effect of drought events on nutrient canopy exchanges and on the partitioning of the green leaf pool during the leaf senescence period. The aim was to identify potential nutritional consequences of droughts for trees. Monitoring nutrient dynamics, including resorption, chemistry of green and senesced leaves, foliar absorption and leaching in mature beech stands from 2012 to 2019 allowed us to compare the nutrient exchanges for three nondry and three dry years (i.e., with an intense drought event during the growing season). During dry years, we observed a decrease by almost a third of the potassium (K) partitioning to resorption (i.e. resorption efficiency), thus reducing the K reserve in trees for the next growing season. This result suggests that with the increased drought frequency and intensity expected for the coming decades, there will be a risk of potassium deficiency in trees, as already observed in a rainfall exclusion experiment on the same study site. Reduced foliar leaching and higher parititioning to the senesced leaves for K and phosphorus (P) were also observed. In addition, a slight increase in nitrogen (N) resorption efficiency occurred during dry years which is more likely to improve tree nutrition. The calcium (Ca) negative resorption decreased, with no apparent consequence in our study site. Our results show that nutrient exchanges in the canopy and the partitioning of the green leaf pool can be modified by drought events, and may have consequences on tree nutrition.