PARENT SESSION
Contributed Oral Session 57: Polar and Alpine Ecology: Carbon and Nutrient Dynamics
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Below-ground responses to in situ CO₂ enrichment at the Swiss alpine treeline.

Handa, I. Tanya^*,1, Hagedorn, Frank², Hättenschwiler, Stephan^{1, 3}, ¹ University of Basel, Basel, Switzerland² Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland³ Center of Functional Ecology and Evolution (CEFE-CNRS), Montpellier, France

ABSTRACT- Plants are frequently observed to increase carbon allocation to below-ground sinks and to particularly 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. In a four-year free air CO₂ enrichment (FACE) experiment, we assessed below-ground responses to elevated CO₂ in a late successional treeline ecosystem in the Swiss Central Alps (2180 m asl) dominated by ericaceous dwarf shrubs (Vaccinium myrtillus, Vaccinium uliginosum, Empetrum hermaphroditum). Measurements included quantification of fine root growth using ingrowth root cores and parallel standing crop harvests, decomposition of roots using litter bags, and measurements of water extractable soil organic carbon and soil respiration. 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. However, elevated CO₂ resulted in 20% more water extractable soil organic carbon and marginally higher CO₂ concentrations in the soil air. The use of the stable isotope ¹³C permitted us to trace the new carbon entering the system through our CO₂ enrichment treatment. After four years, results indicated that only ca. 30% of the new carbon was incorporated into new roots indicating a rather slow root turnover. The relative contribution of new carbon to the dissolved organic carbon pool was ca. 10% and to the soil CO₂ efflux via root and microbial respiration was ca. 40%. Our data show that fine root growth may be much less stimulated by elevated CO₂ in late successional systems than in ecosystems with a rapidly expanding plant community biomass. The higher water extractable organic carbon measured at elevated CO₂ could have important implications for soil functioning by changing substrate availability for microorganisms or the potential for carbon loss from the system by leaching.

Key words: elevated CO₂, root growth, treeline, carbon cycling