Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project

Bader Martin K.-F. 1,2, Leuzinger Sebastian 1,3, Keel Sonja G. 1,4, Siegwolf Rolf T.W. 5, Hagedorn Frank 6, Schleppi Patrick 6, Körner Christian 1

1 Institute of Botany, University of Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland
2 New Zealand Forest Research Institute (SCION), Te Papa Tipu Innovation Park, 49 Sala Street, Rotorua 3046, New Zealand
3 Earth and Oceanic Sciences Research Institute, Auckland University of Technology, 31-33 Symonds Street, Auckland, New Zealand
4 Physics Institute and Oeschger Centre for Climate Change Research, Climate and Environmental Physics, University of Bern, Sidlerstrasse 5, Bern CH-3012, Switzerland
5 Laboratory for Atmospheric Chemistry, Paul Scherrer Institute, CH-5323 Villigen PSI, Switzerland
6 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstr. 111, CH-8903 Birmensdorf, Switzerland

J. Ecol. 101 (2013): 2509-2519

DOI: 10.1111/1365-2745.12149


Rapidly increasing atmospheric CO2 is not only changing the climate system but may also affect the biosphere directly through stimulation of plant growth and ecosystem carbon and nutrient cycling. Although forest ecosystems play a critical role in the global carbon cycle, experimental information on forest responses to rising CO2 is scarce, due to the sheer size of trees.
Here, we present a synthesis of the only study world-wide where a diverse set of mature broadleaved trees growing in a natural forest has been exposed to future atmospheric CO2 levels (c. 550 ppm) by free air CO2 enrichment (FACE). We show that litter production, leaf traits and radial growth across five hardwood species remained unaffected by elevated CO2 over 8 years.
CO2 enrichment reduced tree water consumption resulting in detectable soil moisture savings. Soil air CO2 and dissolved inorganic carbon both increased suggesting enhanced below-ground activity. Carbon release to the rhizosphere and/or higher soil moisture primed nitrification and nitrate leaching under elevated CO2; however, the export of dissolved organic carbon remained unaltered.
Synthesis. Our findings provide no evidence for carbon-limitation in five central European hardwood trees at current ambient CO2 concentrations. The results of this long-term study challenge the idea of a universal CO2 fertilization effect on forests, as commonly assumed in climate–carbon cycle models.

Keywords: CO2 fertilization, coupled climate–carbon cycle model, ecosystem carbon cycling, elevated CO2, free air CO2 enrichment (FACE), global carbon cycle