Nutrient stocks in Tasmanian vegetation and approximate losses due to fire

Abstract

Previously published data on the mass and concentration of major nutrients in Tasmanian vegetation types are presented in common units. The losses of nutrients in intensely hot wild-fires occurring once or twice a century are calculated and used to estimate losses in more frequent but less intense fires. Potential losses from consumed foliage and litter and from erosion and leaching of ash post-fire are estimated. Losses of phosphorus as a percentage ofthe mass ofthat element in the above-ground biomass and litter in rainforest or wet and dry eucalypt forest range from 75-85%; losses in scrub, heath and sedgeland range from 35-45%. Losses of 18% in potassium and 30% in calcium in sedgeland fires are particularly serious impediments to successional processes in western regions. Data are provided on the input of nutrients into the precipitation and its attenuation with distance from the coast. In the absence of studies of nutrient balance in Tasmania, the available data from Vicroria have been analysed to provide some information which is probably applicable to eastern Tasmania. In addition, the study provides the input-output concentration difference for a number of Tasmanian watersheds and a table showing the outflow concentrations from a selection of catchments displaying a range of geology, precipitation input (P) and evaporation (E). In Tasmania, the large differences in PIE ratios between western and eastern catchments has a major nutritional effect. The input of sodium in the Yolande catchment in the west is 1270 ppm, but the difference in input-output concentration is only 0.55 ppm, whereas the Lisdillon catchment in the east has a sodium input of180 ppm, but a concentration difference of 20 ppm, indicating the effect of high evapotranspiration on the residence time and availability of nutrients. In precipitations of >2000 mm in western Tasmania, the large input of nutrients is balanced by equally large outflows from both rainforest and sedgeland. Thus, any additional mineralised nutrients following a fire will probably be lost by erosion, leaching or run-off. The data suggest that hot fire should not be used to regenerate cut-over forests in environments with high PIE ratios. The relatively high predominance of disclimax fire-tolerant vegetation such as sedgeland and scrub in western Tasmania is due to the combination of a long period of Aboriginal burning during the Late Pleistocene and the low occurrence of clay-forming minerals in the geological exposures. In the eastern half, the topographical capping of dolerite has done much to supply an erosional mix of clay-forming minerals, so that the occupation of this region by Aborigines during the Holocene had a lower impact.