Revegetation on Tropical Steep Slopes after Mining and Infrastructure Projects: Challenges and Solutions

Highlights

What are the main findings?

• Five frequently interacting abiotic constraints challenge steep slope revegetation
• Spontaneous vegetation establishment in natural steep slope habitats is a long-term process
• Specific plant functional groups show adaptations for establishing in steep slope environments

What is the implication of the main finding?

• Different planting strategies, e.g., hydroseeding and geotextiles, enhance vegetation cover
• Combining soil conditioning, planting strategies, and carefully selected species enhances steep slope rehabilitation.

Abstract

The revegetation of steep slopes after mining and infrastructure projects is not an easy task. To enhance the effectiveness of revegetation projects, the present study aimed to review (i) specific challenges of steep slope revegetation, (ii) ecological succession patterns in similar environments, (iii) soil conditioning and revegetation strategies to enhance vegetation cover, (iv) the importance of microorganisms to enhance steep slope revegetation, and (v) the functional plant traits necessary to establish on steep slopes. In general, steep slopes are characterized by high bulk densities, potentially toxic elements, and low water and nutrient availability. Additionally, high temperature and elevated radiation constrain the soil conditioning and vegetation cover establishment. Lessons from ecological succession in natural steep slope habitats show that steep slope revegetation is a long-term process. Planting strategies, including hydroseeding and geotextiles, may enhance the implementation of vegetation cover. Different plant functional groups show adaptations necessary for establishment in steep slope environments, and mixtures of species containing different functional groups can promote diverse and resilient plant communities. Promising species may be retrieved from local rupestrian ecosystems, as these floras are adapted to shallow, oligotrophic soils. Further research on combining methods of soil conditioning with individual planting and/or seeding strategies of carefully selected species is necessary to enhance steep slope revegetation and rehabilitation, contributing to slope stability, erosion reduction, and carbon fixation in the long term.

1. Introduction

Economic development, even when sustainable, such as the transition to support a low-carbon future, depends on infrastructure construction [1,2] and mineral extraction [3,4,5]. These activities are frequently accompanied by excavations that result in the formation of exposed slopes, spatial impacts, and the amount of overburden that needs to be replaced [6]. The majority of these slopes are steep or extremely steep, with inclinations greater than 45°, creating extensive and unsightly scars in the landscape [7]. In addition, slope failure in mine pits or along transport infrastructure causes loss of property and lives or injuries, as well as disruptions in economic activities [7,8]. Therefore, the stability of steep slopes is a major concern, and revegetation by means of soil bioengineering techniques is considered the most viable alternative to increase the overall safety of slopes [9,10,11].

Vegetation cover reduces the soil erodibility of steep slopes and downstream sediment loads [12] by increasing infiltration capacity and soil water storage [13] and decreasing soil desiccation [14]. Furthermore, the greening of steep slopes can trigger environmental rehabilitation through ecological succession, reducing the impacts of mining or infrastructure projects on biodiversity and ecosystem functions and services in the long term, especially when native species are used [15]. However, the steep slope hampers the revegetation process due to environmental conditions differing largely from areas planted for agricultural purposes, parks, or gardens [16,17]. Major challenges include the improvement of soil conditions (due to the presence of potentially toxic elements, compaction, low nutritional status, and reduced biological activity), the selection of fast-growing native plant species adapted to environmental constraints similar to those found on steep slopes, and the fixation of seeds on inclined surfaces. Thus, understanding the relationships between physical, chemical, and biological soil properties and spontaneous vegetation on steep slopes is crucial to overcoming these challenges and enhancing vegetation cover [18].

Here we review the challenges of steep slope revegetation, considering the breadth of the available natural and engineering solutions. To integrate ecology and engineering science, we discern the constraints challenging steep slope revegetation. Then, we look closely at ecological succession patterns in natural environments showing similar characteristics as artificial steep slopes and try to functionally characterize species found spontaneously establishing on steep slopes. With these lessons in mind, we revised technical and nature-based solutions to enhance steep slope revegetation, highlighting the physiological and ecological characteristics of plant species that enable the successful establishment and greening of these environments. We expect that our findings can guide the search in regional floras for promising candidate species that promote steep slope revegetation and their rehabilitation in the long term.

2. Challenges of Steep Slope Revegetation

A set of inherent environmental characteristics of steep slopes make their revegetation challenging. A detailed understanding of these characteristics is necessary to enhance revegetation practices. First, the inclination of mineland benches per se reduces the chances of fixing seeds and additional revegetation inputs such as fertilizers or organic composts [19,20]. Seeds and fertilizers tend to accumulate in the lower portions of the slope [21] and generate spatially heterogeneous vegetation patterns, leading to an inability to retain superficial water runoff [22,23]. This increases the risk of soil erosion [24] and highlights the importance of soil protection measures [21].

Second, difficulties in fixing fertilizers on steep slopes aggravate nutrient availability, as substrates outcropping on steep slopes are generally unweathered bedrocks without fertile topsoil layers and with low contents of organic matter [25,26]. Low nutrient availability is magnified by the low abundance and diversity of microorganisms, including plant mutualists, such as nitrogen-fixing bacteria or mycorrhizae, which can facilitate plant establishment [27,28]. This makes steep slope ecosystems unable to sustain vegetation cover [29]. The fastest way to increase nutrient availability in these soils is by chemical fertilization [30]. However, inclination and low vegetation cover at the beginning of revegetation activities challenge nutrient incorporation in biomass [31]. An alternative to managing steep slope soil fertility is organic fertilization, which is positively correlated with microbial activity and diversity, but it requires effective incorporation into the slope substrate to prevent its rapid displacement [32,33,34].

Third, the lack of vegetation cover increases the radiation intensity on steep slope surfaces, increasing the soil temperature and water evaporation, especially in tropical regions. High temperature and water deficiency can impair germination and prevent seedling emergence [35], as seed germination, specifically that of native species, requires a specific range of soil temperatures and osmotic potentials [36,37]. Thus, the colonization of steep slopes is often restricted to undesired alien invasive species with germination at elevated temperatures [38]. In many revegetation projects worldwide, fast-growing, nonnative, commercial grass-legume mixtures are frequently applied for steep slope revegetation, and these mixtures are able to cover degraded slopes rapidly [39]. Nitrogen-fixing legumes are expected to function as green fertilizers, improving soil nitrogen content [40], while C4 grasses increase soil organic matter through their high photosynthetic activity combined with high water-use efficiency [41]. The advantages of these seed mixtures are their availability at local markets and low costs [42] and their good coverage of steep slopes after seeding. Nevertheless, if seeded in high densities, these mixtures may reduce the establishment and survival of native species desired for long-term slope rehabilitation [43].

Fourth, depending on the mined ore and the geological context, benches and steep slopes from mine pits may contain potentially toxic heavy metals [44,45,46]. This requires selecting metal-tolerant plant species [47] or hyperaccumulators [48,49] for revegetation. Removing plants enriched in heavy metals may contribute to phytoremediation and phytomining [50,51,52], but the viability of such activities on steep slopes is usually limited.

Fifth, unweathered bedrocks suddenly outcropping on the surface generally show high bulk density and penetration resistance, which reduces root growth, soil moisture content and overall vegetation establishment [53,54]. As the soil density and mechanical resistance to penetration increase, the abundance of water-storing pores decreases [30,55], favoring superficial water runoff [56]. Additionally, steep slopes reduce the possibilities for mechanical aeration, such as subsoiling or deep plowing, commonly applied to overcome compaction in agricultural soils. Therefore, where other solutions are unavailable, root growth, e.g., by compaction-tolerant plants, is expected to increase porosity, hydraulic conductivity and, consequently, water storage capacity on steep slopes [57,58].

Abiotic constraints (inclination, low nutrient and water statuses, high radiation, high bulk density and the eventual presence of potentially toxic elements) make steep slopes the most challenging environments for revegetation. Revegetation of these habitats requires selecting species with specific physiological mechanisms and growth strategies that allow their germination and establishment [7]. Therefore, approaches focusing on species’ functional characteristics represent promising tools for doing so [59]. Furthermore, implementation techniques that enable the fixation of seeds and further revegetation inputs on steep slopes should be developed while minimizing operational risks.

3. Ecological Succession in Natural Steep Slope Ecosystems

Ecological succession describes the temporal sequence of distinct communities colonizing naturally or anthropogenically disturbed habitats. A habitat can be disturbed either naturally or artificially, and disturbance is the removal of biomass through, for example, natural tree falls, logging, as well as all kinds of earthworks. After such a disturbance, chance [60], differences in species life histories [61,62], environmental filtering or changing interaction patterns result in recurrent or nonrecurrent changes in plant communities over time [63]. Generally, natural succession starts with the colonization of a few pioneer species, and the diversity and complexity of the stands increase with time [64,65]. In contrast to restoration models, e.g., by natural regeneration [66], human rehabilitation activities after disturbances manipulate succession paths to recover original species composition, soil fertility, and site stability [67]. Thus, natural succession is considered a guide for ecosystem rehabilitation [68,69,70], despite underutilized lessons in practice [71].

Depending on soil type, climate, and other factors, landslides after heavy rainfall or earthquakes may show inclinations, soil compaction, and fertility similar to those of artificial steep slope ecosystems because landslides lead to sudden outcrops of bedrock parent material on the surface, delaying or even hampering plant colonization and ecological succession [72]. There are a few well-studied cases of delayed succession worldwide. For example, after a landslide in Lombardy, northern Italy, ecological succession achieved ecological maturity only 15 years after slope stabilization [73]. On a landslide chronosequence in New Zealand, vegetation establishment was even slower: mosses, grasses, herbs, ferns, and small shrubs were observed to colonize within short periods, but vegetation cover greater than 50% was achieved only after more than 50 years [74]. In another example, in Puerto Rico, the upper parts of landslide slopes showed less plant density and diversity than the lower parts [72], most likely due to differences in organic matter accumulation. These findings show that unassisted vegetation establishment and ecological succession after landslides is a slow process that depends on soil characteristics, especially on the steeper, upper parts of the slope. Delays in vegetation development furthermore create opportunities for the invasion of undesired alien species [75].

Therefore, revegetation and rehabilitation activities should analyze the functional traits that enable the spontaneous establishment of some species in these environments (Box 1) [76] to accelerate ecological succession on artificial steep slopes. Prospecting native species with similar ecological traits increase functional redundancy, resilience, and the success rate of revegetation [77].

Box 1. Small seeds enable spontaneous colonization of ruderal or invasive species on steep mining slopes in Carajás.

The Carajás National Forest in southeastern Amazon, Pará State, Brazil, is home to the Carajás Synclinorium, which features 18 billion tons of high-grade iron ore, with Fe contents well above the international trading standards [78]. The iron deposits are exploited in open pit mines characterized by steep cut slopes reaching heights up to 15 m and inclinations up to 85°.

Species detected during field inspections of mining cut slopes at this site are small-seeded, wind-dispersed, ruderal species, including some sedge species, lilac tasselflower (Emilia sonchifolia (L.) DC., Asteraceae) and exotic molasses grass (Melinis minutiflora P. Beauv., Poaceae) (Figure 1). These species produce large amounts of small seeds, and their invasive potential has been described for disturbed areas [79]. Additionally, the silver fern (Pityrogramma calomelanos (L.) Link, Pteridaceae), bird-dispersed, small-seeded alien Singapore cherry (Muntingia calabura L., Muntingiaceae) and pokeweed (Phytolacca thyrsiflora Fenzl. ex J.A. Schmidt, Phytolaccaceae) were also found frequently.

Similar patterns were found in other tropical steep slopes [80], highlighting the colonization abilities of invasive or ruderal weeds and ferns [81] on steep slopes. Thus, large amounts of small seeds, which are dispersed by wind, seem crucial for successful steep slope colonization. Therefore, native flora should be prospected for species with these or similar traits to find promising candidates for steep slope revegetation, as slope colonization by natives may impede or reduce the spreading of alien invasive species such as molasses grass or Singapore cherry.

Figure 1. Plant species that have spontaneously colonized iron mining cut slopes in the Carajás National Forest [81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102].

Figure 1. Plant species that have spontaneously colonized iron mining cut slopes in the Carajás National Forest [81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102].

4. Soil Conditioning and Implementation Strategies

As natural steep slope revegetation is a slow process [103], partly because these areas are colonized spontaneously by only a few species (see Box 1), active revegetation is needed to accelerate this process and launch steep slope rehabilitation [104]. However, active revegetation by sowing or seedling planting is challenged by difficulties in fixing plant propagules on surfaces with extreme inclinations, reduced nutrient and water availability, and soil compaction. This section presents techniques to enhance soil quality and support plant propagules in steep slope environments (Figure 2).

Reducing slope angles: The inclination of steep slopes prevents mechanical soil management and aeration using agricultural machinery [105]. Furthermore, loosened soil material becomes prone to soil erosion, eventually causing environmental disasters [106]. Reducing the slope angle may thus be an alternative to overcoming revegetation challenges [107,108] by facilitating seeding, planting and soil aeration, increasing water infiltration, and reducing operational risks. However, the reduction in slope angles increases land consumption, the amount of overburden that needs to be deposited in other sites and project costs. For mining activities, the reduction in slope angles may be achieved by filling mine pits with waste or tailings after complete exploitation of the ore deposit. However, mountainous relief may impede straightforward logistics that enable the backfilling of caves as mining activities advance [109].

Soil aeration: In natural environments, soil aeration is achieved by bioturbation, i.e., the reworking of soils and sediments by animals or plants, including burrowing, ingestion, and subsequent defecation of soil particles. Bioturbating animals such as certain tunneling dung beetles [110] or anecic earthworms [111] have positive long-term impacts on the structure of compacted soils when soil conditions such as acidity, nutrient and food availability and moisture content are favorable. The colonization of steep slopes by these invertebrates may occur with advances in rehabilitation, but further research may outline the potential of steep slope inoculation with these organisms and their eventual effects on soil porosity.

Cover soils: The natural development of soil through bedrock weathering is a slow process, and soil that evolves on steep slopes is likely to be eroded before plants can establish. Natural or synthetic cover soils may be sprayed as an adhesive slurry on steep slopes, forming a thin continuous layer on the bare slope surface and facilitating root development and water infiltration [112]. Such soil spraying mixed with seeds and fertilizers represents a modified hydroseeding technique. A special form of cover soil is so-called topsoil, i.e., the upper soil layers containing microorganisms, nutrients, organic matter, and seeds. Topsoils originating from logging areas positively influenced soil fertility, microbial activity, and vegetation cover in postmining landscapes [113,114]. Nevertheless, its use is limited, as topsoil depends on local availability, quality, and treatment (during removal, transport, storage, and application), highlighting the demand for suitable alternatives [115] and in situ soil reconstruction [103,116].

Soil amendments: These are intended to improve the chemical and physical soil conditions and enhance early seedling performance [117]. While chemical fertilizers and biosolids increase the availability of micro- and macronutrients, lime and organic components are applied mostly to improve soil structure, moisture and cation exchange capacity [42]. Additionally, organic and inorganic amendments can reduce bulk density and the bioavailability of potentially toxic elements [118,119]. Water superabsorbent polymers (SAPs), also referred to as hydrogels or superabsorbers, are synthetic compounds that can absorb up to 400 times their weight in water [120,121] and retain moisture and nutrients [122], especially where drought events are intense and frequent [123]. When isolated or combined with fertilizers, SAPs enhance early survival and growth in a wide range of plant species [124,125,126], with positive effects on germination rates and plant establishment [127,128]. Thus, SAPs may be applied for mineland revegetation activities [107], e.g., by incorporation into seed coatings [129] or in planting containers.

Irrigation: Artificial irrigation, particularly drip irrigation, has a large potential to overcome temporal water scarcity, reduce drought stress and enhance vegetation development in systems limited by soil volume, such as steep slopes [130]. Experiments demonstrate that cost-effective revegetation can be achieved when low-irrigation systems are combined with the hydroseeding of native species [131]. However, irrigation may reduce slope stability significantly [132] and increase the risks of liquefaction-induced landslides [133,134,135], especially in combination with extreme rainfall events. Thus, irrigation projects require intense monitoring activities to facilitate safeguards and reduce the risk of landslide hazards [134,136].

Planting containers: A crucial step for successful revegetation (provided that a suitable substrate is available or can be developed for plant growth) is the introduction of plant propagules that can be established on a slope. This can be achieved by planting nursery-raised seedlings or plants rescued from logging areas. Seedlings can be specifically conditioned to the expected environmental conditions of revegetation sites, providing a quicker recovery of the desired vegetation structure and ecological succession than seeding strategies [137]. In addition, larger planting containers improve seedling survival and growth [138,139] because larger pits reduce damage resulting from water scarcity and solar radiation [140] and prevent topsoil/substrate desiccation and degradation [53]. However, the creation of large planting pits and seedling plantations is labor intensive and increases the risk exposure of operators and thus is not always feasible.

Hydroseeding: Seeding is less expensive than planting but can be difficult to achieve on steep slopes, as seeds, especially larger ones, are easily washed away. Thus, hydroseeding, i.e., the spraying of a slurry containing seeds, mulch, fertilizers and other inputs on the slope’s surface, may represent an attractive strategy for the revegetation of bare slopes due to a high degree of mechanization, high efficiency and reduction in operational risks [141], especially when native species are used [112]. The hydroseeding slurry varies across projects and contains soil amendments and other materials that increase water retention and organic matter accumulation and trigger soil development. The composition of the hydroseeding slurries determines the success of vegetation recovery and its effects on the slope’s physical and chemical soil properties [142]. However, the success of hydroseeding depends mostly on microsite suitability [143,144], and established vegetation cannot always persist in subsequent years [145].

Air-borne seeding methods: The use of remotely piloted aircrafts for monitoring and inspections is becoming prominent in industry [146,147]. With the availability of this technique, drones, small airplanes or helicopters are frequently used for seeding activities [148], presenting a viable alternative to hydroseeding.

Geotextiles: i.e., mats made from dry organic fibers, have been widely used to fix revegetation inputs, protect the soil surface, control erosion and homogenize vegetation development on steep slopes [149,150]. They can be made from either organic synthetic fibers (e.g., polypropylene, polyester, polyethylene) or natural fibers (e.g., jute, coir, sisal), with different designs according to functional needs [151]. By retaining soil moisture for a longer period, geotextiles are expected to provide more suitable conditions for seed germination, seedling growth, and overall vegetation development [152,153], although context-specific failures have been reported [154]. Temporary materials, such as geosynthetic biodegradable biopolymers are currently tested; the results are promising, but more effort should be addressed [155,156,157].

5. The Importance of Microorganisms to Enhance Steep Slope Revegetation

Although many microorganisms have beneficial effects on soil quality and vegetation development, two groups able to favor plant development in inhospitable environments, such as steep slopes, are noteworthy. First, symbiotic rhizobacteria fix atmospheric nitrogen and provide nitrogen compounds to their hosts in exchange for photoassimilates [158]. In addition to nitrogen capture, this association promotes an increase in aboveground biomass and species diversity, which are key for the coexistence of different legume and nonlegume species [159]. Although rhizobia are abundant in the soil of many ecosystems [160], rhizobia capable of efficiently nodulating a given legume host are not always present in the soil, so the selection and inoculation of efficient strains may become necessary [161].

Second, the mycelia of arbuscular mycorrhizal fungi (AMF) increase the root’s absorption surface for water and nutrients, delivering soil nutrients to their plant hosts in return for carbon [27]. Some species increase the soil moisture content in the root zone [162], enhancing plant survival in periods of water shortage [163]. Furthermore, some AMF species are of interest due to their high tolerance to contaminated environments, such as species of the Glomeraceae family [164] that show high resistance to elevated availability of Zn, Cu, Cd, and Pb [165,166].

The promotion of this tripartite symbiosis among plants, rhizobia, and AMF can favor the establishment and development of plants in oligotrophic or nutrient-depleted environments such as steep slopes [167]. Furthermore, topsoil spraying or the use of inoculated seedlings can provide benefits for revegetation [168,169]. Most research on microbial inoculants is related to crops of commercial interest [170], and the development of inoculants for native plant species, e.g., using trap cultures [169], should be investigated for the improvement of steep slope revegetation/rehabilitation activities.

6. Promising Plants for Steep Slope Revegetation

A first step in selecting species is to consider that environmental legislation from many countries requires the prioritization of native species in rehabilitation projects, especially within conservation units [15]. Native species reintroduced to steep slopes during revegetation activities should provide erosion control and support vegetation development toward full ecological rehabilitation [171]. As denser vegetation and root systems provide greater physical soil protection, easily propagated and fast-growing species are desired [81,172]. So-called nurse plants, able to facilitate the arrival and establishment of additional plant species [173], may be required to trigger ecological succession, establish trophic networks and achieve dynamic, process-based rehabilitation goals in the long term [15].

Additionally, reintroduced plant species must be adapted to the environmental constraints presented by steep slopes (Figure 3) [174]. Plant establishment on uncovered steep slopes is limited by high radiation and elevated temperatures in tropical or subtropical regions, which reduces water availability, especially when precipitation is not distributed homogeneously throughout the year. Therefore, species should be drought-tolerant [175] and able to develop with little or no soil, e.g., species from shallow-soil ecosystems [15,176] or that grow between or on rocks, such as some orchids, ferns, liverwort or lichens (so-called lithophytes) [177]. Foliar water uptake from fog or erratic rain pulses can support successful plant establishment on steep slopes and provide higher photosynthetic rates and better thermotolerance than that observed in species without this adaptation [178,179]. Alternatively, soil compaction-tolerant species, such as nonforest [180], early successional [181] and/or finely or fibrously rooted species [182], can be prospected for this purpose. Furthermore, the ideal species is adapted to acidic, oligotrophic soils with high concentrations of potentially toxic elements.

Thus, promising species should couple high photosynthetic activity with high water-use efficiency and desiccation tolerance, similar to C4 or CAM plants [183]. Furthermore, small plants with sclerophyllous leaves and photosynthetic tissues covered by trichomes are more promising for target environments than species without these adaptations [184]. Plants adapted to oligotrophic ecosystems should be preferred over nitrophilic species, and species able to increase nutrient and/or water uptake by acidic root exudates [185] or interactions with spontaneously establishing or inoculated microorganisms such as rhizobia or mycorrhiza are highly welcome [186]. Finally, small-seeded species or species with seeds featuring adhesive structures, attributes generally related to better dispersal [187], should facilitate fixation on steep slopes, especially of self-seeding generations that should attach to the slope surface without the application of fixing components such as those applied during hydroseeding procedures (Figure 3).

Plant functional groups, i.e., groups of plant species defined by physical, phylogenetic or phenological characteristics, differ in their potential to overcome revegetation challenges and link to long-term rehabilitation targets (Figure 4). Light-demanding, fast-growing native pioneer forest species, commonly observed during initial successional stages and frequently applied in restoration projects [188], are adapted to high temperatures and radiation and generally produce elevated numbers of small, wind-dispersed seeds that are more easily fixed on steep slopes [189,190]. Furthermore, some pioneer species are able to deal with and recover degraded and compacted soils [191,192] and initiate ecological succession toward self-perpetuating mature ecosystems. However, drop interception for erosion control is more effective near the soil surface, so herbs and shrubs have greater potential than pioneer tree species to reduce the impacts of drops [14,81].

Plant species from ecosystems characterized by water shortages and/or nutrient limitations are expected to perform better on steep slopes than species from less severe habitats [193,194]. Nonforest ecosystems, including rupestrian vegetation, are generally better adapted to high radiation, elevated temperatures and high iron concentrations and are furthermore able to develop with minimal amounts of soil [195,196]. Ephemeral herbs from such ecosystems are characterized by short life cycles during favorable seasons and produce large amounts of small, wind-dispersed seeds, some of which have elevated germination even in mining substrates [197]. High biomass accumulation during short life cycles increases soil organic matter and initiates nutrient cycling. Perennial, desiccation-tolerant life forms from these environments can guarantee year-around vegetation cover, thus reducing superficial soil desiccation, increasing soil water storage by vertical root distribution and triggering successional advances of steep slopes [14]. Species with lightweight fruits, e.g., pappus from the Asteraceae family, can fix more easily on small fissures on steep slopes. Additionally, desiccation-tolerant Fabaceae species able to nodulate and fix atmospheric N from severe environments are expected to positively influence the soil quality of steep slopes [198].

Grasses develop a dense, multilayered cover that can effectively intercept rain, and an elevated number of stems represents a mechanical barrier to runoff and sediment losses [199,200] due to high carbon fixation capacity and high water-use efficiency [41]. Additionally, strong fibrous, superficial root systems (Figure 5) prevent erosion by holding loose soil on the slope surface. These root systems can also adapt to poor soil conditions and form mulches after drying [201].

Fern species represent a promising alternative plant group that is able to form dense vegetation cover on natural or artificial slopes in tropical or subtropical climates [202]. Characteristics generally attributed to pioneer species such as small propagules (microscopic spores), colonization of disturbed habitats due to extensive rhizome growth [203] and adaptations to low contents of soil nutrients and moisture observed in some fern species [204] enable spontaneous colonization on steep slopes. Preliminary tests highlighted that elevated fern cover is able to reduce runoff volume and sediment loss [81], although large differences between single species were observed [205]. Different fern species can be found in all ecosystems [206], and their prospect and inclusion in steep slope revegetation projects can benefit the establishment of plant communities [207] and moderate the physical environmental constraints (e.g., temperature) of the slopes to facilitate invertebrate colonization [208].

Although herb and shrub species are the primary targets for steep slope revegetation activities, other life forms can also be considered [209]. Lianas, vines and other life forms with scandic habits emit branches with growth ability both upward and downward. In many species, branches can adhere to vertical faces by tendrils or air roots, developing a heat- and drought-resistant curtain [210]. Many vines and lianas are pioneer species that develop under full sunlight conditions [211]. Thus, planting or seeding of lianas at the top or at the bottom of steep slopes prevents the development of roots on water- and nutrient-depleted, compacted, rocky, shotcrete, unstable rubble-stone or even overhanging slopes [212]. Such vertical greening technologies require only small amounts of soil [213] and have been successfully applied in an abandoned quarry in eastern Zhejiang Province, China [211].

Finally, the Cactaceae family, which includes approximately 130 genera and 2000 species [214], is characterized by adaptation to arid and semiarid climates in tropical and subtropical regions [215]. Cactus Pear (Opuntia ficus-indica L.) was used to revegetate degraded soils in northern Ethiopia and successfully improved fertility and physical soil properties compared to those in adjacent open areas [216]. Cacti also play a major role in erosion control on steep slopes, particularly when established in hedges, by reducing surface crusting and superficial runoff [217].

7. Conclusions

In order to enhance steep slope revegetation, we mapped five interacting abiotic challenges for steep slope revegetation, i.e., inclination, low nutrient and water statuses, high radiation, high bulk density, and the eventual presence of potentially toxic elements. These environmental constraints, also found in natural steep slope habitats, delay vegetation establishment and ecological succession. Thus, artificial steep slope revegetation should thus aim for compliance with long-term goals related to rehabilitation and ecosystem restoration, instead of solely focusing on short-term greening.

Neither a single plant species nor a single plant functional group has been identified as a jack of all trades for steep slope revegetation. However, small, wind-dispersed seeds, ideally with adhesive structures, and species with rapid growth ability and desiccation tolerance were recognized as being suitable for steep slope revegetation. Promising candidate species with these adaptations may be retrieved from regional nonforest ecosystems with shallow soil and nutrient and water limitations, and seed collection strategies, propagation protocols, and implementation techniques should be developed.

A set of promising approaches to overcome environmental constraints and enhance revegetation of steep slopes is available, but thus far, no single solution has shown overall success. Thus, further research on combining methods of soil conditioning with individual planting strategies of carefully selected species is required to overcome the challenges of steep slope revegetation and rehabilitation, contributing to slope stability, resilient vegetation cover, and carbon fixation in the long term.