Introduction

Peatlands are unbalanced ecosystems where plant production exceeds decomposition of organic material. As a result, considerable quantities of organic material, or peat, accumulate over long periods of time: millennia. This organic material is composed primarily of plant fragments remaining after partial decomposition of the plants that at one time lived on the surface of the peatland. Decomposition occurs through the action of micro-organisms that have the ability to utilize dead plant components as sources of carbon for respiration (Thormann & Bayley 1997) in both the upper, aerobic peat column (the acrotelm) and the lower, anaerobic peat (the catotelm) (Ingram 1978, Clymo 1984, Wieder et al. 1990, Kuhry & Vitt 1996). Labile cell contents, cellulose, and hemicellulose are more readily available sources of carbon than recalcitrant fractions that contain lignin-like compounds, with these latter compounds being concentrated in peat by decomposition (Williams et al. 1998, Turetsky et al. 2000). The vascular plant-dominated, tree, shrub, and herb layers produce less biomass (Campbell et al. 2000) and decompose more readily than the bryophyte-dominated ground layer (Moore 1989). Surfaces of northern peatlands are almost always completely covered by a continuous mat of moss (National Wetlands Working Group 1988, Vitt 1990), and the large amount of biomass contained in this layer is composed of cell wall material that decomposes slowly. This slow decomposition, coupled with water-saturated, anaerobic conditions in the peat, cool climate, and a cool moist growing season conducive to bryophyte growth, allows organic matter to accumulate over large areas.