Vegetation

Plant cover on landfills contributes to its landscape and assists in the reduction of leachate discharge through evapotranspiration. The latter function is particularly important if the landfill is not capped with an impermeable layer to control infiltration. Other benefits provided by the vegetation cover include visual improvement of the site, creation of wildlife habitat, and the sequestration of greenhouse gases. The species chosen for revegetation purpose depends on the afteruse of the site, climatic conditions, nursery stock availability, and hardiness of the species.

Despite the tremendous efforts and investment devoted to site engineering, the inclusion of a soil cover does not guarantee the successful establishment of vegetation. The depth and quality of the soil layer affect revegetation as a thicker soil cover is required for woody species which have deeper root systems.

Poor vegetation performance is a common feature of many old landfills. In the US, a nationwide survey conducted in the early 1980s showed that the major cause for plant failure was the high concentration of landfill gas in the root zone. Negative correlation was found between landfill gas concentration and plant coverage or tree growth in municipal landfills because tree growth was hampered by high landfill gas content, and to a certain extent by high soil temperature and drought. In addition, root development, and hence plant growth at landfills was also adversely affected by pedoclimatic conditions such as high underground temperature, drought, soil acidity, and contamination by leachate. To counteract these problems, the planting of species tolerant to the above adverse conditions is recommended. This is why earlier studies on landfill revegetation focused on the adaptability of plant species to landfill gas. Leguminous trees are better than nonlegumes in their tolerance to high landfill gas that prevailed in old landfills.

Rehabilitation is traditionally initiated by hydroseed-ing with grass species and/or planting of tree species for erosion control and esthetic improvement. Landscaping and artificial revegetation are the initial rehabilitation works, irrespective of the afteruse of the site, as this accelerates ecological development. The site is revege-tated preferentially using grasses which grow fast and provide good immediate ground cover to control erosion and reduce visual impact. Grass swards also survive better than trees on landfills with gas influence, a feature which is attributed to their shallow rooting depth. Tree planting is less popular especially on the top platform of a landfill, because of the negative effects of tree growth on landfills. Following initial revegetation, the rehabilitated site is left for secondary succession to take place.

Grassland can be a versatile habitat option for closed landfills as it can be established on a wide range of soil types. While pasture or arable grassland is more demanding on soil quality and requires greater fertilizer input, low-maintenance grassland can be established on infertile soils. A seed mix of more species and the inclusion of wildflowers can increase the species richness of the vegetated sites. Open grasslands are good habitats for many animal species (e.g., butterflies), but others prefer scattered scrubs and trees for shelter. Planting trees had not usually been recommended as it was believed that tree roots would perforate and crack by drying out the landfill cap. In addition, tree growth on landfill soil may be difficult because of poor soil quality. However, as woodlands have the greatest conservation value, it seems desirable to plant trees to form woodlands which have the benefits of increasing forest resources, habitat connectivity, wildlife biodiversity, and landscape integration.

Vegetation is an integral part of the landfill ecosystem, and flora composition of vegetated sites differs with respect to landfill technology (i.e., gas and leachate control), hydrometeorological conditions, as well as the quality and depth of the soil cover. Vegetation composition is also directly affected by the species planted, survival of the planted species, replanting/enrichment planting, natural invasion of other species, and the seed bank in the soil cover material. A suitable species will very much adapt to and survive in the landfill conditions, at least for a certain period of time, and facilitate the growth of late-successional species. With differential site availability, species availability, and species performance, rehabilitation can be directed by using different soil and planting strategies to achieve successional intervention. A good choice of species for revegetation could enhance the sustainability of ecosystem development. Nitrogen-fixers and those pioneer species usually outcompete other species in the first 10-20 years of ecosystem development after rehabilitation. Nitrogen-fixing trees such as the tropical species of Acacia confusa, A. auriculiformis, A. mangium, Albizia lebbeck, Casuarina equisetifolia, Leucaena leucocephala and temperate species such as Alnus glutinosa are widely used for planting on closed landfills. These species assist in nitrogen accumulation in the landfill soil and are very important in the successional development of the soil cover. Therefore, enrichment planting of late-succes-sional species is sometimes necessary at a later stage of development to enhance plant density and maintain species-rich vegetation in secondary forests on closed landfills.

The establishment of woodland communities is the result of gradual ecological development, which cannot be achieved simply by tree planting. The success and speed of succession rely on the availability of appropriate seeds with the proper dispersal mechanism and the presence of effective animal dispersers, and species with the appropriate ecological characteristics. The seed bank, in the cover soil, supplies the species for the early vegetative colonization, which resembles the floristic composition of the areas where the soil is obtained. Soil seed densities decline with landfill age, a trend similar to the course of old-field succession. Young landfills have more r-selected species, which tend to produce more seeds, whereas older sites have more ^-selected species, which produce fewer seeds but a higher population of perennials. Some woody plants that are more adaptive can invade gaps and establish slowly. Trees that are either early-successional species or leguminous species should be planted in greater proportion to accelerate succession in landfills, preserve the biodiversity of local flora, and provide more favorable habitats for wildlife conservation. Planting more native or exotic species has been debated; natives, though not necessarily fast growing, are adaptive to local environmental conditions, and provide indigenous characters that are not found in artificial revegetation, but whether natives or exotics are better choices depends on their adaptation to landfills conditions and the quality of soil for revegetation.

Postclosure landfills can be a good refuge for rare species including wild orchids, and are important to the conservation of native flora. Older sites are better developed in terms of soil quality and vegetation coverage. Ecosystem development on closed landfills can be rapid and is accelerated by artificial planting and good management practices.

faunal assemblages on closed landfills, but rehabilitated landfills are potential sites for faunal colonization because they attract insects and herpetofauna and have an important role to play in wildlife conservation.

Open grasslands developed on abandoned landfills are an important insect habitat, and some closed landfills which have been converted into woodlands or grasslands provide valuable habitat for butterflies, especially those species which are declining in population and distribution. However, butterfly community composition and structure have stronger links with vegetation that are either a source of nectar or host plants for larvae, and do not necessarily reflect the successional development of closed landfills.

Closed landfills could also be colonized by amphibians and reptiles within a few years after revegetation, and herpetofaunal diversity and abundance increase with time after closure. Constructed wetlands, though not a conventional option for habitat creation on landfills, provide refuges for amphibians and reptiles. An example of this are ponds that have been designed and constructed on a landfill in Cheshire, England, specifically for great crested newts that were originally present on the site before landfilling.

Birds play a very vital role in the secondary succession on landfills as seed dispersers. It has been reported that birds introduced 20 new plant species to a landfill annually via endozoochory. This increases the floral diversity and contributes to vegetation development. However, only species that produce fleshy fruits will be spread by frugivorous species. It is generally advocated that more fleshy-fruited natives should be planted to attract birds, and even small mammals such as bats for full restoration of the ecological function of landfill as a wildlife habitat.

The reestablishment of faunal communities is closely related to that of vegetation. Closed landfills are potential refuges for uncommon and rare species, and it is suggested that planting of more natives can aid in the creation of a more favorable habitat for ecological diversity. Rehabilitated landfills may not be as ecologically diverse as natural areas, but their conservation values should not be overlooked, as they can be good wildlife habitat and connecting links to enhance remnant fragmented areas. Sites with relatively high biodiversity and rare-species records should be designated conservation areas, especially for those which are not suitable for other alternative development.

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