Fine root responses of mature deciduous forest trees to free air carbon dioxide enrichment (FACE)

1. Elevated atmospheric carbon dioxide (CO2) concentrations have often been reported to increase carbon allocation below-ground, particularly to fine root production. However, for trees these responses have primarily been studied in young expanding systems while the evidence for late successional systems that have reached steady state above- and below-ground is very limited. 2. At the Swiss Canopy Crane (SCC) experimental site, we assessed whether elevated CO2 affects fine root biomass, fine root expansion and fine root C and N concentration under mature deciduous trees (c. 100 years) exposed to 7 years of free air CO2 enrichment (FACE) in a typical near-natural central European forest. 3. After 5 and 6 years of CO2 enrichment, both, the soil core and ingrowth core method yielded similar reductions in biomass of c. –30% under elevated CO2 for live fine roots > 1 mm diameter. In year 7 of the experiment, when fine root biomass was re-assessed at peak season, there was no significant CO2-effect detectable. C and N concentrations in newly produced fine roots remained unaffected by elevated CO2. Soil moisture under CO2 exposed trees was significantly increased during rainless periods. 4. The isotopic label introduced into the system by canopy enrichment with 13C-depleted CO2 allowed us to trace the newly assimilated carbon. After 6 years of growth at 550 ppm CO2, recent fine roots (> 1 mm, ingrowth cores) of CO2-enriched trees consisted of 51% new carbon, suggesting a rather slow root turnover and/or slow mixing of old and new carbon in these trees. 5. Reduced tree water consumption under elevated CO2 and resultant soil water savings might cause these trees to reduce their fine root investments in a future CO2-enriched atmosphere. 6. Our findings and those from other multi-year experiments indicate that fine root mass in late successional systems may also be unaffected or even suppressed instead of being stimulated by elevated CO2.