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

1. 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. 2. 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 the studied hardwood species remained unaffected by elevated CO₂ over 8 years. 3. 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. 4. 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.