The Tasmanian legacy of man and fire

The vegetation of Tasmania is complex and much of it is in a state of disclimax. At the time of European settlement, the proportion of non-forest open vegetation was 370/0, about 5% of this at high altitudes. In the present interglacial climate, in regions ofhigh rainfall, where rainforest dominance might be expected, approximately 45% carries sedgeland, grassland, shrub communities and wet sclerophyll forest. Similarly, drier areas carry extensive grassland, sedgeland and heath, instead of dry eucalypt forest. This complexity of distribution and disclimax can be attributed to fire disturbance. Fire not only produces a successional mosaic but, through Ecological Drift, causes extinction of communities. This level of displacement appears to demand a timespan of human-induced fire sufficiently long to affect soil fertility. A palaeontological record of the last five glacial cycles has been analysed from the Darwin Crater in western Tasmania and compared with that from the Chatham Rise, New Zealand. These show parallel behaviour in the proportions of forest and non-forest communities in the earlier cycles. However, the Tasmanian record shows a marked divergence during the Last Glacial cycle, with a twofold increase in open vegetation relative to closed forest. Eucalypt forest increases relative to rainforest, and charcoal increases relative to woody vegetation. These changes occur through a variety of climates, including full glacial and optimum interglacial, but are not apparent in the New Zealand core, making it difficult to attribute them to a climatic cause. In the Tasmanian vegetation, they can be explained by an increase in fire frequency, due to human activity. Since their onset occurs in isotopic oxygen stage 4 and continues in the differing climates thereafter, it may be inferred that the 14C dates of the earliest evidence of occupation by the Aborigines are gross underestimates. A date of about 70 000 yrs BP is more likely. It appears that, when using 14C methodology, such underestimation for dates beyond30 000 yrs BP is to be expected in palaeosamples from western Tasmania

Vegetation of the Central Plateau

As might be expected in an upland region, the distribution of plant communities is determined largely by low temperatures, in particular by the degree and frequency of frosts below -5°C, glazing storms, and ice-bearing winds. The most significant single index which is readily available is probably the mean summer temperature. A mean temperature of 10°C for the warmest month has been accepted as the indicator of the tree line (Daubenmire 1954). Most of the Central Plateau above 1040m is thus in this category

Nutrient stocks in Tasmanian vegetation and approximate losses due to fire

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 of 180 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

Index of chromosome numbers of Tasmanian spermatophytes

A preliminary index of chromosome numbers for spermatophytes indigenous to Tasmania is presented, with reference citations. This compilation includes reports for 166 species provenanced from Tasmanian material, many previously published, and also 55 unpublished results (mostly from the work of WDJ). Selected reports of species indigenous to Tasmania but derived elsewhere (usually material from New Zealand or the Australian mainland) are also included as a basis for further study. In total, chromosome numbers are listed for 400 species (409 taxa, including subspecies and varieties) out of a total estimated spermatophyte flora of 1568 described indigenous species. Ten of the 108 families have been examined in some detail (>45% of species), but the four most speciose families (62511568 species; Orchidaceae, Asteraceae, Poaceae and Cyperaceae) have a total of only four Tasmanian reports, despite highly variable chromosome complements. Five other speciose families (Apiaceae, Brassicaceae, Juncaceae, Rhamnaceae and Scrophulariaceae) have no Tasmanian reports despite demonstrating cytological variability at generic, inter- and intra-specific levels. Further cytological investigation of these taxa is essential for a clearer understanding of the Tasmanian flora

Influence of Seabed Morphology and Substrate Composition On Mass-Transport Flow Processes and Pathways: Insights From the Magdalena Fan, Offshore Colombia

Although the effects of interactions between turbidity currents and the seabed have been widely studied, the roles of substrate and bathymetry on the emplacement of mass-transport complexes (MTCs) remain poorly constrained. This study investigates the effect of bathymetric variability and substrate heterogeneity on the distribution, morphology, and internal characteristics of nine MTCs imaged within a 3D seismic volume in the southern Magdalena Fan, offshore Colombia. The MTCs overlie substrate units composed mainly of channel–levee-complex sets, with subsidiary deposits of MTCs. MTC dispersal was influenced by tectonic relief, associated with a thin-skinned, deep-water fold-and-thrust belt, and by depositional relief, associated with the underlying channel–levee-complex sets; it was the former that exerted the first-order control on the location of mass-transport pathways. Channel–levee-complex sets channelized, diverted, or blocked mass flows, with the style of response largely controlled by their orientation with respect to the direction of the incoming flow and by the height of the levees with respect to flow thickness. MTC erosion can be relatively deep above channel-fill deposits, whereas more subtle erosional morphologies are observed above adjacent levee units. In the largest MTC, the distribution of the seismic facies is well imaged, being influenced by the underlying bathymetry, with internal horizontal contraction occurring updip of bathymetric highs, erosion and bypass predominating above higher gradient slopes, and increased disaggregation characterizing the margins. Hence, bathymetric irregularities and substrate heterogeneity together influence the pathways, geometries, and internal characteristics of MTCs, which could in turn influence flow rheology, runout distances, the presence and continuity of underlying reservoirs, and the capacity of MTCs to act as either hydrocarbon seals or reservoirs

Influence of Seabed Morphology and Substrate Composition On Mass-Transport Flow Processes and Pathways: Insights From the Magdalena Fan, Offshore Colombia

Although the effects of interactions between turbidity currents and the seabed have been widely studied, the roles of substrate and bathymetry on the emplacement of mass-transport complexes (MTCs) remain poorly constrained. This study investigates the effect of bathymetric variability and substrate heterogeneity on the distribution, morphology, and internal characteristics of nine MTCs imaged within a 3D seismic volume in the southern Magdalena Fan, offshore Colombia. The MTCs overlie substrate units composed mainly of channel–levee-complex sets, with subsidiary deposits of MTCs. MTC dispersal was influenced by tectonic relief, associated with a thin-skinned, deep-water fold-and-thrust belt, and by depositional relief, associated with the underlying channel–levee-complex sets; it was the former that exerted the first-order control on the location of mass-transport pathways. Channel–levee-complex sets channelized, diverted, or blocked mass flows, with the style of response largely controlled by their orientation with respect to the direction of the incoming flow and by the height of the levees with respect to flow thickness. MTC erosion can be relatively deep above channel-fill deposits, whereas more subtle erosional morphologies are observed above adjacent levee units. In the largest MTC, the distribution of the seismic facies is well imaged, being influenced by the underlying bathymetry, with internal horizontal contraction occurring updip of bathymetric highs, erosion and bypass predominating above higher gradient slopes, and increased disaggregation characterizing the margins. Hence, bathymetric irregularities and substrate heterogeneity together influence the pathways, geometries, and internal characteristics of MTCs, which could in turn influence flow rheology, runout distances, the presence and continuity of underlying reservoirs, and the capacity of MTCs to act as either hydrocarbon seals or reservoirs