Growth and carbon relations of mature Picea abies trees under 5 years of free-air CO₂ enrichment

Are mature forests carbon limited? To explore this question, we exposed ca. 110-year-old, 40-m tall Picea abies trees to a 550-ppm CO₂ concentration in a mixed lowland forest in NW Switzerland. The site receives substantial soluble nitrogen (N) via atmospheric deposition, and thus, trees are unlikely N-limited. We used a construction crane to operate the free-air CO₂ release system and for canopy access. Here, we summarize the major results for growth and carbon (C) fluxes. Tissue ¹³C signals confirmed the effectiveness of the CO₂ enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pre-treatment signals. Over the five experimental years, needles retained their photosynthetic capacity and absorbed up to 37% more CO₂ under elevated (E) compared to ambient (A) conditions. However, we did not detect an effect on stem radial growth, branch apical growth and needle litter production. Neither stem nor soil CO₂ efflux was stimulated under elevated CO₂. The rate at which fine roots filled soil ingrowth cores did not significantly differ between A- and E-trees. Since trees showed no stomatal responses to elevated CO₂, sap flow remained unresponsive, both in the long run as well as during short-term CO₂ on–off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E-trees suggesting a CO₂-stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation (‘priming’). Synthesis. The lack of a single enhanced C sink to match the increased C uptake meant a missing C sink. Increased C transport to below-ground sinks was indicated by C transfer to ectomycorrhiza and on to neighbouring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO₂ concentrations and further atmospheric CO₂ enrichment will have at most subtle effects on growth, despite enhanced N availability.