Rethinking restoration indicators and end-points for post-mining landscapes in light of novel ecosystems

Degraded post-mining landscapes exhibit unique biotic and abiotic components and processes relative to pre-disturbance natural ecosystems. Yet the concept of pre-disturbance reference natural ecosystems and their associated soil quality indicators (SQIs) (e.g., pH, soil organic carbon) are prominently used for assessing restoration of post-mining landscapes. Limited reviews exist on the validity, limitations, opportunities and knowledge gaps associated with the application of the concept and SQIs on post-mining landscapes. Hence, evidence was examined to highlight constraints, opportunities and future research directions pertaining to the concept and SQIs. First, as novel, hybrid or designer ecosystems, severely degraded post-mining landscapes lack reference natural ecosystems. The framing of restoration is multi-dimensional, and dependent on spatial and temporal scales. Therefore, short-term data on SQIs often measured at point scale cannot adequately account for the multi-dimensionality and scales. Moreover, evidence linking SQIs to ecosystem functions, goods, values, services, and benefits on degraded post-mining landscapes remains weak. Potential redundancy exists among SQIs, because soil properties exhibit spatial and temporal correlation. The universality of SQIs remains unconfirmed, because data validating the measurement protocols and interpretation of SQIs across various biomes are scarce. A framework is presented proposing: (1) a shift from the concept of reference natural ecosystems to novel and designer ecosystems in restoration ecology, (2) the development of the next generation of hierarchical or ecosystem cascade indicators, and end-points addressing the multi-dimensionality and scale issues, and (3) a decision matrix for integrating novel, hybrid and designer ecosystems. The potential applications of novel tools such as drones, laser-based cameras, genomics, and big data analytics are highlighted. Such novel tools could unravel the complex linkages among biotic and abiotic components, and ecosystem function and services, which are currently difficult to investigate using conventional techniques. Finally, ten tentative hypotheses are presented on the restoration of degraded post-mining landscapes.