Efectos ecológicos de la erosión en laderas derivadas de la minería del carbón a cielo abierto

Resumo

Opencast coal-mining is considered one of the most dramatic human disturbances in terrestrial ecosystems. Mining reclamation represents a complex task on account of the drastic alterations caused by this activity and of the multiple factors involved in the recovery of the disturbed ecosystems. Reclaimed lands from opencast mining are particularly vulnerable to the effects of accelerated soil erosion processes, especially when these processes lead to the formation of rill networks. One of the effects caused by soil erosion is the reduction of water availability for plants, which can noticeably constrain the development of vegetation and hence reclamation success in water-limited environments. The objective of this dissertation is to analyse the effects of soil erosion in the availability and distribution of soil moisture, as well as the consequences of these effects in vegetation dynamics, and the development of soil biological processes in reclaimed slopes derived from opencast coal-mining in a Mediterranean-Continental environment. Firstly, a regional scale study was carried out to identify the pattern and factors controlling vegetation succession in reclaimed slopes of "the Teruel coalfield" (central-eastern Spain). This work revealed a complex pattern characterised by the development of multiple successional trajectories. Rill erosion processes (triggered in general by run-on fluxes coming from the top of reclaimed slopes and up-slope mining structures) represent at this regional scale a significant driving force for restored vegetation dynamics, leading to a very sparse and simple community essentially constituted by a few individuals of Medicago sativa (a sown perennial legume). This erosion-related trend was analysed deeply in a series of specific studies carried out in five reclaimed slopes located in the "Utrillas field site". In spite of having very similar reclamation treatments and initial features, these slopes have developed rill erosion processes of different intensity (from 0 to 70 t ha-1 year-1) owing to the presence of overland flow contributing areas of different size up-slope. The objectives of these works are the analysis of: (a) the temporal dynamics of generation and (b) the spatial dynamics of circulation of runoff and sediment fluxes, (c) the availability and spatial distribution of soil moisture, (d) the dynamics and structure of vegetation, and (e) the development of the physical structure and biological functionality of the soil. The results obtained highlight the role of rill erosion processes in the development of reclaimed ecosystems by diminishing the availability of water for vegetation. In this way, the circulation of surface flows is conditioned by the development of rill networks, thus maximizing the loss of water resources at the slope scale by surface runoff. Additionally, these channelling networks organise the spatial distribution of soil moisture, since they concentrate water resources around rill beds (areas where vegetation development is prevented by the mechanical disturbance associated to the concentrated runoff flows). As a result, the availability of water resources for plants is drastically reduced through a non-linear erosion-related function. Consequently, vegetation performance and dynamics are affected by the increase of drought stress and the inhibition of natural plant colonisation. All these effects together are reflected in ecosystem structure at the slope scale by an exponential decline of species richness and vegetation biomass, showing a drastic simplification of plant communities related to soil erosion rates. The most simplified state is represented by the sparse and poor communities dominated by M. sativa plants, in which case their spatial organisation is ruled by the distribution of soil moisture and mechanical disturbance derived from rill networks. In addition to the identified effects on vegetation, other variables concerning the development of both soil physical structure (soil aggregates stability) and soil biological functionality (the activity of soil microorganisms and a set of soil hydrolases basic for the nutrient cycling) also showed exponential decreases associated to soil erosion rates. A critical threshold operating in the studied reclaimed ecosystems has been determined on the basis of the non-linear nature of the erosion-vegetation relationships identified: (a) vegetation in reclaimed slopes with under 30% vegetation cover and rill erosion rates over 20 t ha-1 year-1 regress to the low diversity and sparse plant communities dominated by M. sativa; (b) when vegetation cover is over 50% and rill erosion rates remain below 5 t ha-1 year-1, vegetation dynamics of these constructed slopes is not conditioned by soil erosion processes and ecosystem dynamics rely on other factors and processes.