Internal factors controlling peatland-lake ecosystem development
To examine nonclimatic factors controlling small—peatland development we expanded the classical model of peatland development by hypothesizing the existence of three horizontal zones. We then tested five predictions derived from the expanded model. Prediction 1: Peat Strata. There are two vertical strata of peat, an upper layer consisting of peat formed in the floating or grounded mat (mat peat) and a lower layer of peat formed in the floating mat, but dropped from the side or bottom of the mat (debris peat). Underlying these strata are lake sediments originating in the water column. Prediction 2: Strata Boundary. Chamaedaphne calyculata provides the framework for mat growth, and therefore stems and roots of this plant form the boundary between the two peat strata. The lower boundary of mat peat is continuous from the lake edge to the upland. Prediction 3: Strata Thickness. Thickness of mat peat increases as a function of the depth of the original basin and the length of time peat has been accumulating. Prediction 4: Peat Density. The density of peat increases with distance from the lake edge up to a zone in which peat has reached a maximum density and mean density is constant. Prediction 5: Vertical Accumulation Rate. The long—term rate of vertical accumulation (in grams per square metre per year) decreases with distance from the lake edge. To test these predictions we used data collected on peat stratigraphy, lake—edge vegetation, and peat age and density in Fallison Bog, a 5.5—ha peatland—lake system in northern Wisconsin, and to a lesser extent from 13 other peatlands in the region. Each prediction was confirmed, except that thickness of mat peat appears to be independent of peat accumulation time. We estimated the vertical growth rate of the mat by radiocarbon dating of C. calyculata stems. By dating twigs at different locations in the mat we estimated that the rate of vertical peat accumulation ranges from 34—75 g°m—2°yr—1, depending on location within the peatland. The relationships among peat densities, vertical accumulation rates, and distance from the lake edge suggest that three horizontal zones occur in Fallison Bog: (1) a zone of thickening near the lake edge where vertical accumulation of organic matter thickness the floating mat, (2) a zone of compaction farther from the lake edge where vertical accumulation compacts underlying peat, and (3) a zone of equilibrium farthest from the lake edge where peat has reached a maximum density (100—110 kg/m3 in Fallison Bog) and, in the absence of a perched water table, no peat accumulates. Our results underscore the importance of spatial dynamics in peatland development.