The most important aspect of landfill engineering is to create voids secure enough to contain and manage leachate and gas emissions. To this end, construction usually consists of four components: an impermeable basal seal, the landfill cap, landfill gas and leachate control systems and an extraction system. The landfill capping material is the most significant factor in habitat creation due to the highly impervious substrates used, usually compacted clay. The Dimmer landfill site that hosts Carymoor is capped using the locally abundant Jurassic Blue Lias Clay. This creates an impermeable, inert, neutral “subsoil”, initially devoid of organic material, and very prone to waterlogging. However, this presents the restorationist with a tabula rasa; the clay subsoil can be amended or overlain with other substrates to alter immediate edaphic conditions or create “perched” habitat communities.
Ecological restorationists working on landfill have to contend with a suite of problems not encountered in the majority of habitat creation schemes. The following are the key features of landfill sites that restorationists have to contend with:
The use of subsoil is probably the most important aspect of habitat creation on landfill sites. Subsoils generally have low fertility, a factor that is crucial in the development of many of Britain’s most diverse habitats. For tree planting, subsoils have a number of disadvantages such as the lack of structure and organic content, and proneness to compaction leading to summer drought and winter waterlogging. However, low fertility and stressed conditions of subsoils can produce mosaics of species-rich vegetation, and the sparse, discontinuous, patchy swards provide excellent habitat for some of Britain’s more uncommon invertebrates.
There is considerable support within the conservation fraternity for the utilisation of natural colonisation wherever feasible. Natural colonisation is considered desirable as it allows for continuity of local genetic integrity. Also natural regeneration allows for natural reassembly of communities, and is cheap. However, at landfill sites, relying on natural colonisation for revegetation is rarely a complete option as newly capped surfaces usually require rapid revegetation in order to stabilise surfacing substrates to prevent erosion, and to slow up surface water run-off. Planting and seeding on the other hand, produces rapid revegetation, establishes poor colonisers, and combats the worries about erosion and water run-off. However, in opposition to the above, species’ introductions reduce the “naturalness” of the succession process.
The peripheral areas of the site are not landfilled, and not surprisingly this is where the most of the mature habitats are to be found. These include species-rich and rough grassland ponds; and planted scrub/woodland. Virtually all of these habitats were created as mitigation for the destruction of the landscape because of the ever growing need for landfill sites. The major categories of habitat at Carymoor are as follows:
Because of the primary succession qualities of capped landfill – i.e. capping material is usually inert clay, with no stratification or organic layer, unsown ground is slow to revegetate. This means that the field-layer can remain discontinuous and patchy for many years, in particular providing an important resource for some specialist invertebrates.
Most of the roadside banks and verges are managed as rough grassland. Though fairly species poor in terms of flora, rough grassland is a valuable habitat for many invertebrate (grasshoppers, beetles, butterflies etc) and vertebrate species (voles, shrews, snakes etc). It is possible to manage the rough grassland with a cut of 1-3 years to prevent succession to scrub. The periodicity of cutting rotation depends on the plant species-richness of the grassland, the need for a ‘tidy’ appearance, and requirement for tussock formation.
Management for meadows can grade in with that of rough grassland, however specific meadow grasslands have been created in the experimental field system requiring a yearly hay cut in July. Hay meadows are particularly valuable for floristic diversity and pollinator forage.
This habitat utilises both saplings and natural regeneration of scrub. Scrub is often underestimated for its wildlife potential. Species-rich scrub provides habitat for a plethora of invertebrate species, and particularly forage and roosting/nesting areas for birds e.g. nightingale at Carymoor.
This habitat includes both high forest and coppice; clearly habitat creation looking to the long term. Not commonly attempted on the capped surface due to concerns that tap root penetration may breach the integrity of the capping seal and that deep roots may make the trees susceptible to poisoning (due to the presence of methane and anaerobic conditions) and death once they become mature. Thus, woodland plantations are usually located at the periphery of the site.
Modern landfill sites have ditches, ponds, pools and tanks to contain leachate. In addition, other wetland areas may include surface water run-off pools, open streams and settling ponds. These areas provide habitat for important species: birds such as reed bunting, reed/sedge warbler, water rail; herpetofauna such as great-crested newt and grass snake.