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

Summary Rapidly increasing atmospheric CO 2 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 CO 2 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 CO 2 levels ( c . 550 ppm) by free‐air CO 2 enrichment (FACE). We show that litter production, leaf traits and radial growth across the studied hardwood species remained unaffected by elevated CO 2 over 8 years. CO 2 enrichment reduced tree water consumption resulting in detectable soil moisture savings. Soil air CO 2 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 CO 2 ; 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 CO 2 concentrations. The results of this long‐term study challenge the idea of a universal CO 2 fertilization effect on forests, as commonly assumed in climate–carbon cycle models.