Wind-controlled linear patterning and cyclic succession in Tasmanian Sphagnum mires

Abstract

1. We investigated the characteristics and causes of striped patterning in minerotrophic Sphagnum cristatum mires on the western Central Plateau in Tasmania by surveying the mire vegetation, surface and subsurface topography, analysing their topographic exposure to the prevailing strong winds relative to non-striped mires, stratigraphic analysis, measurements of wind speed and direction across and along ridges and swales in typical synoptic conditions, and measurement of growth rates and asymmetry of shrubs. 2. The stripes were orientated south-west–north-east, independent of the aspect of the general slope. The shrub Richea scoparia was dominant on the north-west-facing slopes of the low ridges that formed the stripes. Sphagnum cristatum was dominant on the south-east-facing ridge slopes and the restiad, Baloskion australe was dominant in the swales. The stripes were independent of the topography of the underlying block stream. Striped mires were more exposed to both north-westerly and south-westerly winds than non-striped mires. 3. Strong north-westerly winds formed rotors behind each ridge, with wind speed being highest on the north-west-facing ridge slope and lowest on the south-east-facing ridge slope. The annual growth of R. scoparia inversely reflected this variation in wind speed. South-westerly winds were stronger in the swales than on the ridges and stronger on the south-west extremity of the ridges than further to the north-east. Asymmetry in R. scoparia on the ridges reflected this variation in wind speed. 4. We infer that the ridges slowly migrate in a cyclic successional pattern as the result of differential growth rates of R. scoparia in response to variation in the speed of drying north-westerly winds. The stripes appear to result from training of shrubs by strong, cold south-westerly winds, which also inhibit the growth of R. scoparia in the swales. 5. Synthesis. There are no other documented cases of wind-controlled patterning and cyclic succession in mires, and no case of exposure to two distinct wind directions being responsible for patterning in any vegetation type. The distinctiveness of the process and the form merit recognition of these striped mires as outstanding in their universal value under the world heritage area criteria