ALBERT

Description: Few investigations link post-fire changes to sediment sources and erosion processes with sediment yield response at the catchment scale. This linkage is essential if downstream impacts on sediment transport after fire are to be understood in the context of fire effects across different forest environments. In this study, we quantify changing source contributions to fine sediment (〈63 µm) exported from a eucalypt forest catchment (136 ha) burnt by wildfire. The study catchment is one of a pair of research catchments located in the East Kiewa River valley in south-eastern Australia that have been the subject of a research program investigating wildfire effects on runoff, erosion and catchment sediment/nutrient exports. This previous research provided the opportunity to couple insights gained from a range of measurement techniques with the application of fallout radionuclides 137 Cs and 210 Pb ex to trace sediment sources. It was found that hillslope surface erosion dominated exports throughout the 3.5 year post-fire measurement period. During this time there was a pronounced decline in the proportional surface contribution from close to 100% in the first six months to 58% in the fourth year after fire. Over the study period, hillslope surface sources accounted for 93% of the fine sediment yield from the burnt catchment. The largest decline in the hillslope contribution occurred between the first and second years after fire, which corresponded with the previously reported large decline in sediment yield, breakdown of water repellency in burnt soils, substantial reduction in hillslope erodibility, and rapid surface vegetation recovery. Coupling the information on sediment sources with hillslope process measurements indicated that only a small proportion of slopes contributed sediment to the catchment outlet, with material derived from near-channel areas dominating the post-fire catchment sediment yield response. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Critical transitions in ecosystem states are often sudden and unpredictable. Consequently, there is a concerted effort to identify measurable early warning signals (EWS) for these important events. Aquatic ecosystems provide an opportunity to observe critical transitions due to their high sensitivity and rapid response times. Using palaeoecological techniques we can measure properties of time series data to determine if critical transitions are preceded by any measurable ecosystem metrics, i.e. identify EWS. Using a suite of palaeoenvironmental data spanning the last 2,400 years (diatoms, pollen, geochemistry and charcoal influx) we assess whether a critical transition in diatom community structure was preceded by measurable EWS. Lake Vera, in the temperate rainforest of western Tasmania, Australia, has a diatom community dominated by Discostella stelligera and undergoes an abrupt compositional shift at ca. 820 cal yr BP that is concomitant with increased fire disturbance of the local vegetation. This shift is manifest as a transition from less oligotrophic acidic diatom flora ( Achnanthidium minutissimum , Brachysira styriaca and Fragilaria capucina ) to more oligotrophic acidic taxa ( Frustulia elongatissima , Eunotia diodon and Gomphonema multiforme ). We observe a marked increase in compositional variance and rate-of-change prior to this critical transition, revealing these metrics are useful EWS in this system. Interestingly, vegetation remains complacent to fire disturbance until after the shift in the diatom community. Disturbance taxa invade and the vegetation system experiences an increase in both compositional variance and rate-of-change. These trends imply an approaching critical transition in the vegetation and the probable collapse of the local rainforest system.