0079-B1

Degraded Opencast Minelands in Tropical Environments: Their Peculiar Features in Revegetation Efforts to Restore Functional Ecosystems

S.O. Agele^[1]

Abstract

Vast expanses of land are left bare, degraded and subjected to accelerated erosion and the threat of desertification as a result of mining activities in Nigeria. The numerous and widespread opencast mines of solid minerals are often abandoned after a time with no attempt at reclamation. These opencast mining activities cause soil disturbance and produce changes in the concentrations of toxic elements, dusts, biogenic gases and microclimatic gradients. These gradients in microclimate have implications for leaf to air vapour pressure differentials that control plant water relations and energy balances. The loss of forest covers and topsoil, deposition of minespoils and habitat gradients are accompanied by loss of biodiversity, accelerated conversion of primary to secondary forests and savanna and invasion by weedy natives and exotic annuals.

There is an urgent need for soil reconstruction and restoration of productive and functional soil-plant systems on these abandoned and degraded minelands. The recuperation of these degraded ecosystems will provide alternative land uses, such as wildlife/amenity parks, pasture/range lands and horticultural plantations. Sustainable land use options and restoration/rehabilitation of these degraded ecosystems will control erosion, halt further degradation and preserve environmental quality.

Introduction

Solid mineral mining (exploitation) is important to the economy of many developing countries. In Nigeria for example, there had been increased interest in the exploitation of these resources and hence the widespread and increasing numbers of open cast mines of solid minerals (limestone, granite, iron ore, gemstones etc) leaving behind vast expanse of bare and degraded land (>10 hectares per mine; Ministry of Environment, Abuja, Nigeria, 1999) at close of mining operation. For example, extensive mining activities are carried out in the Okpela, Itape and Ajaokuta belt in the rainforest zone of Nigeria. The resultant mine spoils pollute the land, degrade the soil and in addition, the vein outcrops of minerals present toxic and stressful growing environment to plants. Mine lands left bare and degraded are further exposed to erosion and threat of desertification. These disturbances produce changes in soil quality and shifts in plant community and structure in addition to microclimatic gradients, confounds the widespread and increasing need for soil and environmental conservation in the tropics. The degraded abandoned mines present different habitats and change in the structure of vegetation such as edge of gallery forest with shade plants, shrubs, savanna ecotypes, rock outcrops and open minespoils. Degraded abandoned mines are non-supportive of plant growth and the shifts in plant community and structure cause gradients in microclimate. There is therefore high irradiance, soil water deficits, high temperatures and vapour pressure deficits, variable substratum depths and toxic levels of some minerals. Soil surface is scrapped coupled with huge deposition of minespoils producing increased soil bulk density and conditions inhibitory to root penetration. High soil strength resists root growth, compact pores inhibits aeration and reduced hydraulic conductivity. Compaction decreases total porosity and pore volume and shifts in pore size distribution towards smaller sized pores (Felton, 1995). Diversity of species involved in the colonization of these habitats must tolerate the prevailing harsh growing conditions. Soil disturbance due to open cast mining activities affect the stability of soil functional processes; it facilitates rapid conversion of tropical rainforests from primary to secondary forests and savanna and invasion of weedy natives and exotic plants. The change in vegetation affects the rates of C and N cycling, ecosystem productivity, microbial community and structure and soil functional processes. The microclimatic and stress factors of degraded habitats strongly affect the survival and competitive advantage of native and alien (invaded) plant species. Consequently, plant species with high tolerance of such elements are at a competitive advantage during initial colonisation and subsequent establishment. There is therefore great need to protect/conserve tracts of vegetation (remnants of primary forests and late successional species). Thus the recruitment of these native components of biodiversity will reduce initial and maintenance costs of reclamation process thereby promoting the feasibility of restoring vegetation by using species important in the existing plant communities. The diversity among colonising species and ecophysiological strategies underlying species adaptations/tolerance to these stress factors need be evaluated. Information is also required on mineral uptake and its growth regulator status within plants growing under these conditions and the effects of these elements on the ultimate health of plants.

This study examined the dynamics of vegetation succession and recovery following physical and chemical degradation due to open cast mining activities in a rainforest belt of Nigeria. It further highlights the recovery of forest in relation to vegetation succession following degradation due to open cast mining activities in a humid tropical environment.

Methodology

Land use practices accelerate environmental pollution, produce physical and chemical soil degradation, loss of forest covers and biodiversity and microclimatic gradients. The vegetation (species diversity, composition and dominance) was assessed from 100m² transects including adjacent undisturbed soil (non-degraded) on top and floor of quarry using 3 x 3m plots/quadrants using methods of Shankar Raman et al. (1998). The structure of the vegetation is characterised by the growth and establishment of natives (remnants of primary forest species and colonizers/the weedy natives)) and invasion by weedy aliens. This was assessed through the examination of species composition and density through the identification of numbers of each component species in the sampled quadrant. Shifts in plant community/structure due to open cast mining activities reflected in the different fractions of richness constituted by natives (remnants of primary forest species) and invasion by weedy aliens. The composition of different species in the vegetation presumably through invasion by colonizers (the weedy natives) and exotic species of grasses (annual/perennial graminoids), shrubs/herbs (broad leafed plants) and remnants of primary forests was examined through the identification of numbers of each component species in the sampled quadrant. Tree species (remnants of primary forests; shrubs/herbs; broad leafed plants) Albizia zygia, Ficus exsperat, Terminalia superba, Ceiba petandra. Weedy (annuals/perennials; herbaceous species and grasses) either natives or exotic colonisers. Chromalaena odorata, Elephant grass, Cynodon spp., Imperata cylindrica

Complicating factors affecting plant success such as water and nutrient stress, soil physicochemical status was assessed by detailed sampling and analysis of the growth substrate (soil). Soil physicochemical properties such as infiltration rate (double ring infitrometer), bulk density (core samples), substratum depth (soil profile characteristics), soil moisture holding capacity and soil tension (tensiomters), soil temperature. Mineral elements concentrations in soil and plant uptake and accumulation (Na, N, P, K, Ca, Mg, Zn, Cd, Bo, Cu and Fe) were determined using atomic absorption and photo-spectrometric devices.

Results and discussion

Open cast mining activities cuminates in anthropogenic disturbance and produced shifts in the structure/richness of the vegetation of the hitherto native species which dominated this ecosystem. Ecosystem disturbance of any form creates conditions which facilitates invasion by exotic plants and persistence of native species. Disturbance damages species richness and facilitates biological invasion (invasion by exotic plants) and/or persistence of native species. The understanding of the effects of disturbance (the regime, intensity and nature) on ecosystem function and plant biodiversity may play useful roles in reclamation efforts and pattern of revegetation after anthropogenic disturbance. Vegetation survey was carried out to identify changes in plant community, structure composition and dominance (diversity) of species (physiotypes) which colonise the different sites along the transects of abandoned/degraded open cast minelands in some part of humid southern Nigeria. The soil and climatic variables and hence plant performance (substrate composition from mine spoil cover, rock outcrops and undisturbed/native soil was also assessed. This is based on the premise that toxic elements and resultant mine spoils could affect the ultimate health of the species in the plant community. Chemical analysis of the mineral composition of plants and the soil substrate could therefore indicate if plant success is related to nutrient uptake or deficiency or the presence of toxic elements. The assessment of the processes of vegetation succession (revegetation dynamics) showed the recovery of the structural attributes of vegetation in relation to the time elapse since abandonment (5,10 over 20 years after abandonment). The recovery of each component of the vegetation (plant community) may be related to sources of propagules and life histories of species. Slower recovery of native species compared to the aggressive weeds/exotics was observed. The observed remnants of primary forests (shrubs/herbs; broad leafed plants) were dominated mainly by tree species such as Albizia zygia, Ficus exsperat, Terminalia superba, Ceiba petandra. The observed changes in the species composition in the vegetation is attributable to through invasion by colonizers (the weedy natives) and exotic species of grasses (annual/perennial graminoids).

The microclimatic gradients and other stress factors affect plant performance; the consequent environmental perturbations resulted in varying degrees of stability of soil functional processes. Ecosystem disturbances affect soil resources (pH, CEC, organic matter, N, P, K, Ca, Mg) and vegetation structure and richness. Changes in plant community, structure or richness and soil resources are therefore possible indicators of ecosystem disturbance. These degraded sites are characterised by different habitats, for example, edge of forest gallery, open minespoils and rock out crops. The accompanied microclimatic factors are high irradiance (1000 -1200 mol/m²/s) with day time light doses typical of these sites are PPFD < 33.5 mol/m2/s (400-700 nm), high temperatures (between 29 - 43^oC), wet to dry season transitions, extreme soil moisture deficits, leaf to air water vapour pressure differences (VPD) and high concentrations of mineral nutrients and variable substratum depths. Soil surface is scrapped coupled with huge deposition of minespoils produce increased soil bulk density and conditions inhibitory to root penetration. High soil strength resists root growth, compact soil pores inhibits aeration and reduced hydraulic conductivity. Compaction decreases total porosity and pore volume and shifts in pore size distribution towards smaller sized pores (Felton, 1995). Soil disturbance due to open cast mining activities affect the stability of soil functional processes; it facilitates rapid conversion of tropical rainforests from primary to secondary forests and savanna and invasion of weedy natives and exotic plants. The change in vegetation affects the rates of C and N cycling, ecosystem productivity, microbial community and structure and soil functional processes. Consequently, plant species with high tolerance of such degradation are at a competitive advantage during initial colonisation and subsequent establishment. There is therefore great need to protect or conserve tracts of vegetation (remnants of primary forests and late successional species). Thus the recruitment of these native components of biodiversity will reduce initial and maintenance costs of reclamation process thereby promoting the feasibility of restoring vegetation by using species important in the existing plant communities. The use of natural successional processes can contribute to the rehabilitation of abandoned and degraded lands. The feasibility of recolonising/reclaming degraded abandoned minelands using tracts of vegetation (remnants of primary forests and late successional species) should be exploited towards the restoration of the ecological balance and productivity of degraded lands as wildlife/amenity parks, forest reserves and horticultural plantations.

Adaptation of plant species to new environments is a major evolutionary phenomenon as plants species involved in primary colonization of eroded and degraded habitats are vigorous competitors that are able to survive physiologically stressful conditions. Such stresses may include high concentrations of potentially toxic elements associated with mineral vein outcrops, dusts and biogenic gases. The success of any plant therefore depends on both the microclimate and influences of stresses due to toxic concentrations of mineral elements, drought, dusts and biogenic gases. The microclimatic and stress factors of degraded habitats strongly affect the survival and competitive advantage of native and alien (invaded) plant species.The understanding of the temporal dynamics of physiological flexibility in the native/exotic species, for example, will be useful in the evaluation of successional changes (diversity/dominance) and reforestation based on selection and use of plant species tolerant/adapted to stress factors inherent in abandoned and degraded habitats. In plant species, the understanding of ecophysiological adaptations elicited by selection and shaping pressures of environmental stresses (Herzog et al. 1999a) is an important component in the biorestoration and management of degraded habitats. The understanding of the strategies underlying specie adaptations/tolerance to the different transects and stress factors in degraded habitats is basic to successional changes and selection of species for reforestation. This understanding is important to the success of biorestoration and management of functional soil-plant systems on the widespread degraded opencast minelands produced by solid mineral mining activities in Nigeria. Specie diversity, adaptation and occupation of sympatric habitats characterised by microclimatic gradients and other stress factors are based on genotypic ecophysiological plasticity (Scarano et al. 1999). Ecophysiological plasticities are multiple stress factor responses in plant species (Haag-Kramer et al. 1992). This expression is elicited/amplified for example in plant species growing in habitats characterised by high light intensity, soil moisture deficits, extreme temperatures and mineral nutrient depletion. Ecophysiological plasticity for example, CAM, provides high water use efficiencies and strong photo-protection. This is through the establishment of high internal CO₂ concentrations for use in photochemical work and photo-energy dissipation through heat emission via the xanthophyll cycle (Gilmore, 1977; Haag-Kramer et al. 1992). The establishment of high internal CO₂ concentrations prevents damage to photosynthetic apparatus under changing environmental conditions (Mattos et al. 1997) and adjusts photochemical efficiency. Factors which determine ecophysiological behaviour of species along different transects of a habitat, for example, adaptation of the photosynthetic apparatus to perform C₃/CAM in plant species should be screened in circumstances of the prevailing light intensities, high vapour pressure deficits and temperatures and/or soil moisture deficits. The criteria/parameters commonly used for the detection and description of the indicators of effects of multiple environmental stresses in plants growing in diverse habitat will be employed. For example, chlorophyll a fluorescence parameters, carbon isotope signatures and recovery of potential quantum yield of photosystem II (Larcher, 1995; Ball et al. 1995) and the correlations of anatomical features with CAM pathway (Guralnick et al. 1986). These studies (under controlled condition) will reveal ecophysiological diversity/plasticity characterised by different modes of photosynthesis among the species which colonise degraded habitats. It will also contribute to improved understanding of the ecophysiological behaviour of species along the different transects of these habitats is relevant in the biorestoration of degraded ecosystems. The information generated will serve as useful component in the efforts and the feasibility of restoring vegetation on soils containing potentially toxic levels of certain elements and bare/degraded lands as a result of mining activities using species important in the existing plant communities. The research results will contribute towards control of erosion and further degradation of abandoned minespoils, ensuring reserve of natural woodland and development of humic topsoil.

Vast expanse of land are left bare, degraded and subjected to accelerated erosion and threat of desertification as a result of mining activities in Nigeria. The numerous and widespread open cast mines of solid minerals are often abandoned after a time with no attempt at reclamation. There is urgent need for soil reconstruction and restoration of productive and functional soil-plant systems on these abandoned and degraded minelands. Sustainable land use options and restoration/rehabilitation of these degraded ecosystems will control erosion, halt further degradation and preserve environmental quality. The recuperation of these degraded ecosystems will provide alternative land uses such as wild life/amenity parks, pasture/range lands and horticultural plantations. It is therefore important to identify packages of sustainable options for the restoration of functional ecosystems on these widespread abandoned and degraded minelands. The success of biorestoration of degraded landscapes for environmental protection may be built on the recruitment of native components of biodiversity for revegetation of soils containing potentially toxic levels of certain elements and bare/degraded lands as a result of mining activities. Successful reclamation efforts may include examination of ecophysiological strategies/adaptation elicited by selection and shaping pressure of the environment in some native dominant species and afforestation plants. Assessment of site factors and amelioration of such factors through organic amendments and appropriate choice of adapted plant species are also important. These efforts will produce balanced ecology involving diversity of species while soil reconstruction will enhance the survival of the less competitive species (Agele, 2002). Such reclamation will control erosion, halt further degradation and ensures sustainable alternative land use options. Such reclamation will control erosion, halt further degradation and ensures sustainable alternative land use options. Information is provided on the feasibility of restoring vegetation on soils containing potentially toxic levels of certain elements and bare/degraded lands as a result of mining activities using species important in the existing plant communities. The research results will contribute towards control of erosion and further degradation of abandoned minespoils, ensuring reserve of natural woodland and development of humic topsoil. The adoption of reclamation options on already degraded abandoned minelands by local communities, NGOs and government agencies is hereby emphasized. Mine owners should strictly comply with reclamation guidelines and abandonment of minelands without reclamation should be penalized

Table 1. Physical properties of surface soil at site of experiment

Table 2. Elemental composition of substrate samples from site of study

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