1. Plants are frequently observed to increase carbon allocation to below-ground sinks and particularly, to accelerate fine root turnover in response to rising atmospheric CO₂ concentration. While these strong below-ground responses have predominantly been observed in rapidly expanding systems, late successional plant communities have rarely been studied.
2. In an ongoing free air CO₂ enrichment (FACE) experiment, we assessed below-ground responses to elevated CO₂ after 4 years, in a treeline ecosystem in the Swiss Central Alps (2180 m a.s.l.) dominated by a late successional ericaceous dwarf shrub community (Vaccinium myrtillus, V. uliginosum, Empetrum hermaphroditum), and a sparse overstorey of 30-year-old Larix decidua and Pinus uncinata trees. Measurements included quantification of fine root growth using ingrowth root cores and parallel standing crop harvests and decomposition of roots using litter bags.
3. Elevated CO₂ did not stimulate root growth of the treated vegetation (although some significant above-ground growth responses were observed), nor did altered root decomposition occur. Root quality measurements indicated that elevated CO₂ resulted in significantly higher starch concentrations, but no change in N concentration, or root dehydrogenase activity.
4. The use of the stable isotope δ¹³C permitted us to trace the new carbon entering the system through our CO₂ enrichment treatment. We observed that only c. 30% of new root biomass (< 2 mm) was formed by new carbon indicating a rather slow root turnover in this system.
5. Our data show that fine root growth may be much less stimulated by elevated CO₂ in systems with late successional elements than has been reported in ecosystems with a rapidly expanding plant community biomass.