Monitoring land degradation & ecosystem resilience across Australian water-limited ecosystems

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Degradation of dryland ecosystems has been of interest to ecologists for many decades, and has been reported on every populated continent. Rain-use efficiency (RUE), which describes the relationship between annual above-ground net primary productivity (ANPP) and annual precipitation (P), is a commonly used measure of ecosystem function across water-limited arid and semi-arid ecosystems. The goal of this thesis was to improve our understanding of spatial and temporal RUE relationships across Australian water-limited ecosystem in order to monitor land degradation and ecosystem resilience. A remote sensing approach was taken, as it is the only practical method that allows for spatially and temporally comprehensive assessment of RUE relationships at a continental scale. The first step was to assess spatial RUE variability in relation to spatial variability in precipitation (P) and potential evapotranspiration (PET), as water availability is primarily determined by hydro-meteorological conditions that encompass both water supply (P) and atmospheric evaporative demand, or PET. The results showed that water-limited ecosystems did not adhere to a well-defined spatial ANPP-rainfall relationship due to strong impacts of PET on RUE. Therefore, a new index that normalised RUE by PET was developed and tested - “effective RUE” (eRUE). The eRUE relationship (i.e. the regression between ANPP and the quotient of precipitation and PET) resulted in a spatially well-defined ANPP-water model compared to RUE (which does not consider the effect of PET). Also, during extreme dry years ecosystems showed stronger convergence to a common maximum ANPP-water relationship when the effects of both P and PET were included. This driest-years spatial eRUE relationship (i.e. cross-site eRUE_{dry}) defines theoretical water-limitation boundary conditions. Thus, while critically low rainfall can lead to vegetation water stress and contribute to ANPP losses, increasing PET caused by future climate change is likely to exacerbate drought-induced impacts on ecosystem structure and function, including the frequency of drought-induced mortality events. Vegetation type was also considered as a contributing factor to spatial RUE and eRUE variability. The results showed that vegetation types exhibited significant differences in eRUE (and RUE). Furthermore, these differences were also expressed during the driest years, suggesting that each vegetation type exhibits a unique spatial eRUE relationship during periods of severe water limitation. As such, if cross-site eRUE_{dry} is to be used as a theoretical drought resilience threshold, it should be defined by vegetation type-specific cross-site eRUE_{dry} relationships. Ecosystem function trends were assessed as indicators of land degradation. First, ANPP interannual variability was assessed in relation to interannual P variability, which revealed differences in sensitivity among vegetation types. Tussock grasslands, chenopod shrublands and agricultural lands were identified as the most sensitive to interannual P variability, suggesting that these vegetation types may be most sensitive to future climate change. The residuals trend (RESTREND) method was used to assess ecosystem function trends that were independent from climate trends. Sites with negative ecosystem function trends were observed across the study area, and represent potential sites of land degradation. Open woodlands, mulga shrublands, chenopod shrublands, hummock grasslands, and agricultural lands were identified as widely affected. This thesis has contributed to our understanding of spatial and temporal RUE relationships within the context of P, PET and vegetation type variability. At the continental scale ANPP spatial variability was strongly affected by P and PET. This led to the development of the eRUE metric, which was also applied during the driest years. The cross-site eRUE_{dry} represents theoretical water limitation boundary conditions that encompass water supply and atmospheric evaporative demand. Vegetation type was found to play a significant role in spatial eRUE relationships, suggesting that each vegetation type is likely to have a unique drought resilience threshold. The analysis did not reveal strong effects of PET trends on ANPP trends, perhaps indicating that negative effects of PET may be limited to drought periods. Finally, the possible presence of land degradation processes was identified across several vegetation types.
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