Leaf traits of Central-European beech (Fagus sylvatica) and oaks (Quercus petraea/robur): Effects of severe drought and long-term dynamics

In 2018—2020, Central-European forests suffered from extremely hot and dry summers. We used data from long-term forest monitoring of six stands of European beech (Fagus sylvatica) and two stands of oak (Quercus petraea/Q. robur) growing on different geological substrates in the climatically uniform state of Saarland (south-western Germany) for analyzing leaf traits of the period 2004—2021. We aimed at detecting overall effects of the drought on foliar morphology, nutrient concentrations, and injury, and long-term alterations in these traits. Across sites, drought resulted in a decrease in leaf size and specific leaf area (SLA) and increased fruiting in the beech and a decrease in the foliar nitrogen (N) concentrations in both tree genera. During drought, foliar calcium and manganese concentrations were lower and potassium (K) concentrations higher across the beech stands, whereas in the oak stands, drought led to a reduction in the foliar phosphorus (P) and magnesium (Mg) concentrations. High rates of anthropogenic N deposition during recent decades have resulted in high foliar N concentrations and low to deficient concentrations of P and, in the beech, of Mg. However, a significant (negative) long-term trend in leaf traits across the study sites was only found for the K concentration and necroses of the beech leaves. Foliar N correlated positively with SLA in the beech and with leaf size in the oak but was not related to herbivory. Chlorosis was the only leaf trait that, in the beech, correlated (negatively) with the climatic water balance. We conclude that even severe drought during three consecutive years does not seem to critically affect the nutrient supply to the two most important deciduous forest tree genera of Central Europe. In the beech, a decrease in leaf size and SLA might be used as an early indication of severe drought stress effects in regular monitoring programs.