Sphagnum Macrostructure as an Indicator of Decay and Compaction in Peat Cores from an Ombrotrophic South Swedish Peat-Bog

(1) Decay and compaction can be determined from a quantified analysis of macrostructure of Sphagnum peat in short cores from ombrotrophic peat-bogs. (2) Because stems of Sphagnum fuscum were intact to a depth of 30 cm, compaction of peat in this region can be determined from the increase in the cumulative length of S. fuscum stems per unit peat volume with depth. (3) A comparison of measured bulk density for each depth in two S. fuscum hummock cores and that predicted from compaction data for the corresponding depth showed that decay in the hummock profile took place in three phases: an initial slow decay in the top few cm, a relatively rapid decay from about 5-15 cm, and a final slow decay from about 15 to 30 cm. (4) In hummock profiles the average length of stem differed among species in the same sample and decreased in the order S. fuscum > S. rubellum > S. balticum. This order indicates that stems of S. fuscum were the most decay resistant, stems of S. rubellum intermediate, and stems of S. balticum least resistant. The average stem length decreased significantly with depth only in S. balticum. The cumulative stem lengths per unit peat volume for these species showed the same order of decay resistance. (5) At all depths in the hollow profiles, average stem length of the hollow-growing Sphagnum species (S. cuspidatum, S. tenellum and S. balticum) was about half that of S. fuscum, the dominant hummock species, at corresponding depths in hummock profiles. Average stem length decreased very rapidly from the surface to about 20 cm. Furthermore, nearly all plants in the hollow peat were completely fragmented into stems and leaves at a depth of 7-10 cm, whereas most plants of hummock species, most notably S. fuscum, were completely intact to at least 25 cm. This may indicate that decay takes place more rapidly in hollows than in hummocks. (6) Preservation of Sphagnum macrostructure in hummocks differed greatly from that in hollows but changed in the same way with respect to the water table. Plants in both hummock and hollow profiles disintegrated rapidly and completely within the zone of water-table fluctuation. The macrostructural collapse suddenly lowers the hydraulic conductivity of the peat and thus initiates the onset of anoxia in the catotelm.