1 Swiss Federal Research Institute WSL, Zürcherstr. 111, CH-8903 Birmensdorf, Switzerlad
2 Institute of Terrestrial Ecosystems (ITES), ETH Zürich, Universitätstrasse 16, CH-8092 Zürich, Switzerland
3 Institute of Forest Botany and Tree Physiology, Chair of Tree Physiology, University of Freiburg, Georges-Köhler-Allee 53/54, DE-79110 Freiburg, Germany
Being tolerant to heat and drought, oaks are promising candidates for future forestry in view of climate change in Central Europe. Air warming is expected to increase and drought decrease soil N availability and thus N supply to the trees. Here, we conducted a model ecosystem experiment, in which mixed stands of young oaks (Quercus robur, Q. petraea and Q. pubescens) were grown on two different soils and subjected to four climate treatments during three growing seasons: air warming by 1 to 2 °C, drought periods (average reduction of precipitation by 43% to 60%), a combination of these two treatments, and a control. In contrast to our hypotheses, neither air warming nor drought significantly affected N availability, whereas total amounts, vertical distribution and availability of soil N showed substantial differences between the two soils. While air warming had no effect on tree growth and N accumulation, the drought treatment reduced tree growth and increased or tended to increase N accumulation in the reduced biomass, indicating that growth was not limited by N. Furthermore, 15N-labelling revealed that this accumulation was associated with an increased uptake of nitrate. On the basis of our results, climate change effects on N dynamics are expected to be less important in oak stands than reduced soil water availability.
Keywords: Quercus petraea, Quercus pubescens, Quercus robur, 15N tracer, ammonium, nitrate, recovery rate, climate change