1 Institute of Botany, University of Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland
2 Institute of Soil, Water and Environmental Sciences, ARO Volcani Center, Beit Dagan 50250, Israel
3 New Zealand Forest Research Institute (SCION), Te Papa Tipu Innovation Park, 8 Sala Street, 3046 Rotorua, New Zealand
4 Institute for Applied Ecology New Zealand, School of Applied Sciences, Auckland University of Technology, 31-33 Symonds Street, Auckland 1142, New Zealand
5 Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmennsdorf, Switzerland
6 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, PSI, CH-5323 Villigen, Switzerland
Are mature forests carbon limited? To explore this question, we exposed ca. 110 year-old, 40 m tall Picea abies trees to a 550 ppm CO2 concentration in a mixed lowland forest in NW Switzerland. The site receives substantial soluble nitrogen (N) via atmospheric deposition, and thus, trees are unlikely N-limited. We used a construction crane to operate the free air CO2 release system and for canopy access. Here we summarize the major results for growth and carbon (C) fluxes.
Tissue 13C signals confirmed the effectiveness of the CO2 enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pretreatment signals. Over the 5 experimental years, needles retained their photosynthetic capacity and absorbed up to 37% more CO2 under elevated (E) compared to ambient (A) conditions. However we did not detect an effect on stem radial growth, branch apical growth, and needle litter production.Neither stem nor soil CO2 efflux was stimulated under elevated CO2. The rate at which fine roots filled soil ingrowth cores did not significantly differ between A and E trees.
Since trees showed no stomatal responses to elevated CO2, sap flow remained unresponsive, both in the long run as well as during short-term CO2 on-off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E-trees suggesting a CO2-stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation (‘priming’).
Synthesis. The lack of a single enhanced C sink to match theincreased C uptake meant a missing C sink. Increased C transport to belowground sinks was indicated by C transfer to ectomycorrhiza and on to neighboring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO2 concentrations and further atmospheric CO2 enrichment will have at most subtle effects on growth, despite enhanced N availability.
Keywords: conifers, FACE, forest, elevated CO2, carbon isotopes, height profile, wood anatomy