Effects of Elevated CO₂ and Nitrogen Deposition on Natural Regeneration Processes of Cut-Over Ombrotrophic Peat Bogs in the Swiss Jura Mountains

Abstract

In the Swiss Jura mountains most of the remaining ombrotrophic peat bogs have been exploited to some extent for peat. In these sites, natural regeneration processes are taking place. The dominant process is paludification, where a cut over drained surface is colonised by key species, usually either Polytrichum strictum or Eriophortum vaginatum. These early colonisers of bare peat surfaces create microclimatic conditions that enable the re-colonisation of Sphagnum mosses, usually S.fallax. In later stages of the succession S. fallax grows to form a continuous carpet and the key species gradually suffer from competition for light availability. We studied the effect of elevated CO₂ (560 ppm) and nitrogen deposition (30 kg ha ^-1 year ^-1) on the competition between Sphagnum fallax and Polytrichum strictum year in a three years field experiment (EU project BERI — Bog Ecosystem Research Initiative) using miniFACE systems (small size Free Air Carbon dioxide Enrichment). The cover and growth in length of the two species was monitored. The height difference between the emerging Polytrichum and the top of the Sphagnum mosses was also recorded at regular intervals. Effect Of CO₂: Sphagnum cover increased in the first year but this trend was not confirmed subsequently, whereas Polytrichum cover was not affected by elevated CO₂ Both Sphagnum and Polytrichum had a reduced growth in length under elevated CO₂. However, the growth of Sphagnum was less reduced than that of Polytrichum and therefore the height difference between Sphagnum and Polytrichum decreased. Effect of N: Sphagnum cover declined and Polytrichum cover doubled over the three years period in the high N plots. Sphagnum growth in length was not significant affected by N, but Polytrichum grew more in the high N plots. As a results the height difference between Sphagnum and Polytrichum increased. These results suggest that elevated CO₂ and nitrogen deposition may have contrasting effects on bog regeneration. The positive effect of elevated CO₂ on Sphagnum mosses may be counterbalanced by higher N deposition levels.