Assessing landscape structure and pattern fragmentation in semiarid ecosystems using patch-size distributions

Spatial vegetation patterns are recognized as sources of valuable information that can be used to infer the state and functionality of semiarid ecosystems, particularly in the context of both climate and land use change. Recent studies have suggested that the patch‐size distribution of vegetation in drylands can be described using power‐law metrics, and that these scale‐free distributions deviate from power‐law linearity with characteristic scale lengths under the effects of increasing aridity or human disturbance, providing an early sign of desertification. These findings have been questioned by several modeling approaches, which have identified the presence of characteristic scale lengths on the patch‐size distribution of semiarid periodic landscapes. We analyze the relationship between fragmentation of vegetation patterns and their patch‐size distributions in semiarid landscapes showing different degree of periodicity (i.e., banding). Our assessment is based on the study of vegetation patterns derived from remote sensing in a series of semiarid Australian Mulga shrublands subjected to different disturbance levels. We use the patch‐size probability density and cumulative probability distribution functions from both nondirectional and downslope analyses of the vegetation patterns. Our results indicate that the shape of the patch‐size distribution of vegetation changes with the methodology of analysis applied and specific landscape traits, breaking the universal applicability of the power‐law metrics. Characteristic scale lengths are detected in (quasi) periodic banded ecosystems when the methodology of analysis accounts for critical landscape anisotropies, using downslope transects in the direction of flow paths. In addition, a common signal of fragmentation is observed: the largest vegetation patches become increasingly less abundant under the effects of disturbance. This effect also explains deviations from power‐law behavior in disturbed vegetation which originally showed scale‐free patterns. Overall, our results emphasize the complexity of structure assessment in dryland ecosystems, while recognizing the usefulness of the patch‐size distribution of vegetation for monitoring semiarid ecosystems, especially through the cumulative probability distributions, which showed high sensitivity to fragmentation of the vegetation patterns. We suggest that preserving large vegetation patches is a critical task for the maintenance of the ecosystem structure and functionality

Mine closure and ecosystem development - Alcan Gove bauxite mine, Northern Territory, Australia

Closure is a critical phase of a mining operation since this is when the environmental effects of mining and\ud rehabilitation works will be most stringently judged. Since the time scales required for biological community\ud development and soil formation are long in comparison to mining operation life cycles - and to human life spans - it is normally only possible to demonstrate the early trajectories of these processes. However, favourable and predictable closure outcomes can usually be achieved by making early decisions on postmining land use, ensuring close integration of mining with incremental rehabilitation works and adequate monitoring. This assumes that a successful system of rehabilitation is available for the materials and environments involved and takes no account of threats or stressors. This paper defines the stages of early post-rehabilitation ecosystem and soil development on highly weathered soil materials at the Alcan Gove Mine using information derived from studies of selected sites from a 26-year chronosequence of sites and local native forest sites. Specifically, we seek to determine the stage at which a high probability of successful ongoing development may be credibly assumed in the absence of further management inputs of energy and materials and in relationship to potential external threats