The role of fire in the coevolution of vegetation, soil and landscapes in south eastern Australia

Abstract

Fire is an important process in the earth system, with biological, ecological, hydrological and geomorphological consequences varying from negligible to severe. The short-term effect of fire on earth system processes had been studied in detail, however, its role in the coevolution of soil and vegetation within the critical zone has never been addressed. In South Eastern (SE) Australia, local studies have shown that post fire runoff and erosion rates increase with aridity (the ratio between potential evapotranspiration and precipitation). The systematic variation in forest type, fire frequency and post fire response make SE Australian uplands an excellent natural laboratory to study the role of fire in coevolution of the critical zone. The aim of this study was to explore the role of fire in coevolution and to identify the key mechanisms, processes and feedbacks involved. Observations in which drier forests burn more frequently and yield more post-fire runoff and erosion, were used to hypothesize that in SE Australian uplands, fire has a critical role in the coevolution of the critical zone, and that its contribution increases systematically with aridity.

Three different methods were used to address the presented aim. The first method focused on the long-term fingerprints of coevolution, soil depth and landform. By considering the observed climate-related differences in forest type, fire frequency and erosion rates, I hypothesised that soil depth and hillslope gradient are north-south asymmetric, and that the magnitude of that asymmetry varies systematically with climate. I addressed these hypotheses by analysing data from soil depth measurements and topographic analysis of digital elevation models. Results showed that soil depth decreased non-linearly with aridity, and that south facing hillslopes were on average steeper and their soils deeper than those facing north. Indices of asymmetry in soil depth (SAI) and hillslope gradient (HAI) expressed a humped-type relationship with aridity, with a peak close to the water-energy limit boundary, pointing to the key role of climate and possibly fire in controlling differential hillslope-scale coevolution across pedomorphic and geomorphic timescales.

In the second method, I used a new numerical model in order to: (i) test the hypothesis that fire related processes and feedbacks are critical to explain observed patterns and magnitude of differences in system states across the landscape, and that their effect increases with aridity; and (if the hypothesis was supported), (ii) evaluate the role of fire related mechanisms in the coevolution process. The model was formulated and parameterised to express processes typical to SE Australian systems, and was evaluated with literature and empirical data. Simulations with stochastic fire controlled by soil moisture deficit replicated the observed pattern and magnitude of difference in system states. The net effect of fire on soil depth increased non-linearly with aridity when results from these simulations were compared to those without fire (i.e., coevolution only controlled by climate). Analysis of simulations designed to isolate the key processes affected by fire indicated that model outputs are sensitive to fire frequency and the effect of individual fires on infiltration capacity (Ic), and less so to the effect of fire on forest cover. Using model simulations, a fire-related eco-hydro-geomorphic feedback was identified in which a long-term increase in post-fire erosion might contribute to more frequent fires and more erosion.

The aim of the third approach was to evaluate and quantify, using intensive field measurements, the way in which contemporary vegetation and soil depth affect the partitioning of rainfall and solar radiation, and to estimate the implications of this on processes in the coevolution of the critical zone. Sub-canopy microclimate (and open reference sites) was measured at sites across an aridity gradient, and the effect of partitioning of rainfall and solar radiation on coevolution was addressed by analysing soil moisture and temperature data, which are central to several processes in coevolution: productivity, flammability and weathering. Results showed that throughfall decrease and net shortwave radiation under the canopy increase with aridity due to the lower rainfall and higher canopy openness (respectively). On wet (dry) sites, the closed (open) canopy and the deep (shallow) soils partition water and energy in a way that resulted in wetter (drier) soils throughout the year, pointing to lower (higher) flammability and higher (lower) productivity. Mean annual soil water stores decreased non-linearly with aridity, and were more than 5 times higher on wet sites, despite annual rainfall only differing by a factor of ~2. Soil weathering is affected by soil moisture, and the results indicates that the differences between the system states may be amplified by weathering rate differences. The results points to a coevolutionary feedback between weathering, productivity, erosion and fire, which is controlled by the partitioning of water and energy across the vegetation and soil.

Overall, results show that fire can play a significant role in the coevolution of soil, vegetation and landscapes in SE Australia. This work is the first to show the importance of fire related eco-hydro-geomorphic feedbacks in coevolution, controlled by soil moisture. Fire was found to operate within feedback loops between its effects on system properties and consequential changes in fire frequency. Two feedback loops were identified: between fire frequency and erosion, and between soil development and fire frequency. By its effect on infiltration capacity and the corresponding reduction in soil depth in direr forests, fire was found to exaggerate the effect of climate on coevolution, and helps to explain the extreme differences in observed system states across an aridity gradient.