A PALEOECOLOGICAL PERSPECTIVE OF VEGETATION SUCCESSION ON RAISED BOG MICROFORMS

Abstract

Peat cores were taken from eight primary and one secondary raised bog sites throughout the United Kingdom in order to determine the pathways of succession between the microform (hummock, ridge, lawn, hollow, and pool) plant communities of the mid to late Holocene. All of these sites have undergone drainage and/or cutting in the recent past, at least. The plant communities were reconstructed by using plant macrofossil analysis, and they are defined primarily by the dominant species of Sphagnum. While plant macrofossils may be used to infer hydrological conditions, an alternative proxy, using testate amoebae, was utilized. This group of organisms was used to avoid circular arguments when relating the vegetation changes to hydrological events. Consequently, a conceptual model of late Holocene succession on British raised mires is constructed that elucidates succession between the microform plant communities according to changes in the water table depth, its stability, quality, and physical disturbance of the vegetation cover. The general sequence of ombrogenous communities was elucidated to be, with increasing height above the water tables, S. cuspidatum and S. auriculatum > Sphagnum sect. Cymbifolia taxa > Sphagnum sect. Acutifolia taxa > a return of Sphagnum sect. Cymbifolia taxa > non-Sphagnum mosses. A major pathway toward degradation of the mire is apparent in modern times, and recovery is typically via a phase of dominance by S. tenellum prior to the establishment of either S. magellanicum or S. papillosum. Although this disturbance is of a large magnitude, it was found that equable or more severe perturbations to the water table had occurred in the past, especially at coastal sites where marine regression led to the establishment of Sphagnum-poor communities dominated by Calluna and Eriophorum vaginatum. The long- term persistence of hummock–hollow vegetation communities is demonstrated, but the appearance of persistent, more homogeneous phases suggests that this surface topography is not always well developed.