Profiles of water content and pore size in Sphagnum and peat, and their relation to peat bog ecology

Abstract

The bog mosses, Sphagnum, form a significant part of the total mass of plants in the world. Their rate of growth depends to a considerable extent on the supply of water to them, and different species occupy characteristic habitats which differ in their ability to supply water. We used the profiles of water content in almost undisturbed cores of two species to infer the size and distribution of spaces around the plants in an attempt to account for the observation that S. papillosum is usually found not far above the water table, while S. capilliforlium is usually found on hummocks well above the water table. Profiles of water content were recorded non-destructively from 30 cm diameter cores of Sphagnum and underlying peat, with use of the absorbance of the soft gamma radiation of $^{241}$Am. The distribution of water-fillable spaces of different size was inferred from profiles with the water table at different distances, to a maximum of 150 cm, below the surface. The larger spaces, which are the main path of water transport, are outside the plant cell walls: between leaves and between pendent branches and stems. The mean radius of such spaces around the hummock species S. capillifolium is smaller than that around S. papillosum. For a given depth of water table the water content of the apical tuft of branches, where growth occurs, is greater in the hummock species than it is in the lawn species. Of ecological importance is that, for a given water content in the apex, the water table is at a greater depth below the hummock species than it is below the lawn species. As the water table rises and falls, so the water content of both species shows hysteresis as large as the difference between them. The ecological significance of this and the need for measurements while water is flowing are discussed.