The vulnerability of peatlands in the Australian Alps

Abstract

Peatlands are coupled to earth's wet climates (Whinam et al. 2003). They are formed when inundated plant material decomposes slowly relative to production, causing partially decayed organic matter to accumulate as soil. A high water table allows non-vascular Sphagnum moss species to prevail in many peatlands. Peatlands promote acidic soil conditions, produce decay-resistant biomass, reduce surface runoff and have an exceptionally high water holding capacity, features which stimulate further peat development. Lowering of the peatland water table can accelerate decomposition and cause a shift away from Sphagnum to shrubs or grass. Hence climatic drying or disturbance causing drainage can compromise organic soil accumulation. Hydrological disturbance from activities such as cattle or horse grazing can also damage peatlands through compaction of peat, increased drainage and runoff and soil erosion. Drying also increases the impact of fire on peatlands. Fire won't burn waterlogged peats because high moisture content retards peat combustion. However, drying peats become vulnerable to smouldering ground fires, which consume the organic soil layer. Deep peat horizons are often a legacy of wetter climatic periods, when peat accumulation was rapid. The positive feedbacks that help maintain subalpine moss cushion peatlands can be summarised as follows: saturation promotes Sphagnum growth and peat formation and protects from fire; upper peat layers provide good physical and chemical conditions for Sphagnum growth; Sphagnum produces resistant organic matter and chemical conditions that promote peat accumulation, and also confers fire protection; the high water storage capacity of Sphagnum and resulting peat soil slow runoff, promoting a high water table and saturation. There is an opposite set of feedbacks that make it less likely for vegetation on mineral soils to convert to peatland: less organic matter reduces water retention during drought; this leads to loss of organic matter through oxidation, fire and erosion; mineral soils and drier conditions are more conducive to shrub and grassland development, less conducive to Sphagnum growth; vascular plants are less conducive to peat formation than Sphagnum, and drive higher fire frequency/severity; this leads to further loss of organic matter and promotion of mineral soils.