Organic soils on Mt. Sprent, south west Tasmania : an analysis of correlations with local climate, microtopography and vegetation

Limited data are available pertaining to the peat soils on south west Tasmanian mountains or on the variability of peat soils over the lowland to alpine environmental gradient. This thesis describes organic soils on Mt. Sprent, a 'typical' mountain, in south west Tasmania and analyses their relationships with climate, vegetation and topography. Climate data were collected using data loggers which recorded maximum and minimum air and surface temperatures, and relative humidity over a 30 month period. Rain gauges were located close to the data loggers. The climatic equipment was at four sites, at different altitudes ranging from 509 m to 1059 m. Vegetation data were collected along altitudinal and topographic gradients, using a cover/abundance measure for 25 m 2 quadrats. The altitudinal data were collected for every 10 m increase in altitude, while the topographic data were collected along a grid system of transects laid out at four sites on the mountain. Soils data were collected along the same topographic transects as the vegetation data. Soil depths were determined in the field, while physical properties were determined in the field and the laboratory. Other environmental data were collected along the altitudinal and the topographic gradients. A total of 34 water table wells were located on the mountain, at each of the four topographic study sites and at the climatic stations. Peat soils in south west Tasmania are shallow, with an average depth of 30 cm. Three types of peat were recognised: fibrous, intermediate and muck. These vary in their moisture content, organic content, degree of humification and depth. Where more than one peat type was found in a profile, shallow fibrous peats overlaid intermediate which in turn overlaid deeper muck peats. Peat depth, moisture content and organic content decrease with altitude inferring climatic influences on the processes of peat formation and decay. Rainfall and relative humidity were found to be more than adequate to support peat accumulation. The temperatures at the base of the mountain were higher than those reported in the literature for optimum peat formation. However, these temperatures were offset by very high rainfall and relative humidity values. By using evaporation data from a nearby village (12 km to the north east) and solar radiation values for the summit, evaporation rates for the mountain during summer were estimated. When compared to rainfall for the same period there was a moisture deficit at each station for February. Vegetation varies with altitude and along topographic gradients. Buttongrass moorland, alpine heathland and alpine sedgeland occur in an altitudinal sequence. The deepest soils are found under the lowland buttongrass moorland vegetation, and the shallowest soils occur under the alpine vegetation. The four vegetation groups coincided with the four soil groups. The amount of organic matter in the surface horizon was significantly related to vegetation type at two of the four sites, while soil depth related to vegetation type at three sites. Slope is an important correlate of peat depth at two of the four sites, while rock cover is important at three sites. The mean and modal water table depths are correlated with plant community distributions and the pH of the surface and lower soil horizons. Factors affecting peat formation viz, depth and physical properties are interrelated to such an extent that it is difficult to determine the affects of a single factor. On the mountain, deeper peats occur at lower altitudes, in waterlogged conditions, and under buttongrass moorland vegetation. Fibrous peats are found under moorland, woody and alpine vegetation types in relatively well-drained areas. Reddish-brown fibrous peats occur under woody vegetation while buttongrass tends to produce black fibrous peats. Muck peats are found in areas of impeded drainage. Higher temperatures experienced at lower altitudes may be offset to some extent by higher rainfall. A decrease in peat depth with altitude infers that climate affects the process of peat formation by affecting peat accumulation rates. A precipitation deficit during the summer months may be responsible for the shallow nature of the peats. Alternatively fire history and the relatively low productivity of the vegetation may also account for the shallow soils.