1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
2 Forest Ecology, Swiss Federal Institute of Technology (ETH), CH-8092 Zurich, Switzerland
Human activities have drastically increased nitrogen (N) inputs to natural and near-natural terrestrial ecosystems such that critical loads are now exceeded in many regions of the world. This implies that these ecosystems are shifting from natural N limitation to eutrophication or even N saturation. This process is expected to modify the growth of forests and thus, along with management, to affect their carbon (C) sequestration. Yet, knowledge of the physiological mechanisms underlying tree response to N inputs, especially in the long term, is still lacking. In this study, we used tree-ring patterns and a dual stable isotope approach (δ13C and δ18O) to investigate tree growth responses and the underlying physiological reactions in a long-term, low-dose N addition experiment (+23 kg N ha-1 a-1). This experiment is conducted since 14 years in a mountain Picea abies forest in Alptal, Switzerland, using a paired-catchment design. Tree stem C sequestration increased by about 22%, with an N use efficiency of ca. 8 kg additional C in tree stems per kg N added. Neither earlywood nor latewood δ13C values changed significantly compared to the control, indicating that the intrinsic water use efficiency (A/gs) did not change due to N addition. Further, the isotopic signal of δ18O in early- and latewood showed no significant response to the treatment, indicating that neither stomatal conductance nor leaf-level photosynthesis changed significantly. Foliar analyses showed that needle N concentration significantly increased in the fourth to seventh treatment years, accompanied by increased dry mass and area per needle, and by increased tree height growth. Later, N concentration and height growth returned close to background values, while dry mass and area per needle remained high. Our results support the hypothesis that enhanced stem growth caused by N addition is mainly due to an increased leaf area index (LAI). Higher LAI implies that more photosynthetically active radiation is absorbed and therefore canopy-level photosynthesis is increased. We conclude that models assuming that N deposition increases tree growth through higher leaf-level photosynthesis may be mechanistically inaccurate, at least in forest canopies that are not (yet) completely closed.
Keywords: nitrogen deposition, carbon seqestration, carbon isotopes, oxygen sotopes, leaf area dynamics, basal area increment