Temporal scale and the accumulation of peat in a Sphagnum bog

We examined short-term (decadal) and long-term (millenial) processes of peat accumulation, and the links between them, in a Sphagnum bog in continental Canada. A previously published model of bog growth was fitted to age profiles of the oxic acrotelm (surface, <60 cm thick) and the underlying, anoxic catotelm (210 cm thick). Approximately 5300 years of accumulation were represented in a radiocarbon-dated core that extended to the base of the deepest part of the peat deposit. The model estimated that the overall rate at which material entered long-term storage in the catotelm was 60-66 g . m(-2). a(-1). Although the decay rate coefficient was near zero, the bog stopped accumulating peat within the past 1500 years, resulting from either a decrease in the rate of transfer of material from the acrotelm or an increase in the rate of decay of material at the top of the catotelm. The model of bog growth estimated recent inputs to the acrotelm (90-930 g . m(-2). a(-1)) that were twofold higher than published field measurements of aboveground productivity, and decay rate coefficients (0.005-0.040 a(-1)) that were 10-fold lower than published litter-bag measurements of mass loss. The pattern of mass loss over time, approximated from nitrogen concentration data, deviated from the pattern predicted by exponential models of decay. Calculations of the balance between additions to and losses from the acrotelm suggest that the amount of material transferred to the underlying catotelm differs among microhabitats. Such spatial variability in short-term processes is incompatible with long-term processes determining the position of the acrotelm-catotelm boundary. We discuss the applicability of the model to continental peatlands and suggest ways to improve modelling of short-term autogenic processes.