Five successive years of rainfall exclusion induce nutritional stress in a mature beech stand

Abstract

Faced with the intense and repeated drought episodes expected in the coming decades, the challenge is to adapt forest management to limit the risks of forest deterioration, especially for forest ecosystems most sensitive to drought such as those developed on shallow soil in mesic conditions. In addition, there is a lack of knowledge on nutrient cycles and notably on nutrient deficiencies induced by drought.The objective of this study is to observe the consequences of recurrent water shortages in a mature European beech forest on shallow limestone soil which is highly represented in the region. The response of beech trees to five consecutive years of spring and summer rainfall exclusion (0 mm of precipitation for three and half months per year) was quantified by measuring changes in stem growth and crown condition. Nutrient contents in green leaves, senescent leaves, holorganic horizons, fine roots and soil were also measured.After five years of spring and summer drought we observed a decrease in growth of 55% relative to control and a sharp deterioration in the crown condition with 67% leaf loss relative to local reference trees. Moreover, in 2021, two years after the end of the experiment, 33% of the trees in the rainfall exclusion experiment were found dead, while no tree mortality was observed in the control. Among the nutrients, potassium was the most affected, with a significant decrease in all compartments studied, and K deficiency was found in green leaves.Five consecutive years of water shortage on calcareous soil with limited extractable water and K reserves induced double water and nutrient stress, which could contribute to the vulnerability of trees to drought and the risk of mortality. To limit the impact of future drought events on this sensitive ecosystem, forest managers could consider K fertilization and/or rational and K-efficient management.