Assessment of value of brownfield sites
IntroductionBrownfield land includes various forms of previously developed land such as former industrial sites of various kinds, including in Essex old sand and chalk quarries, vegetated foundations, land contaminated by e.g. heavy metals or hydrocarbons, silt or pulverised fly ash lagoons, areas developed on PFA or Lytag spoil, disused railway sidings, etc. Brownfield sites can develop to support a variety of habitats of high nature conservation value. They can contain as many rare invertebrates as can be found in ancient woodland and support more species of conservation concern than any other habitat. They are the new lowland heaths and flower-rich meadows, and they may support vegetation communities rare in the modern countryside.
Brownfield sites important for nature conservation contain significant areas of characteristic habitat mosaics, which have been retained over an extended time period and which may be known to support regionally or nationally important assemblages of invertebrate species.
The lack of deliberate management, in particular the lack of any form of regular cutting, is crucial to their importance since it provides a continuity of forage and vegetative resources used by various invertebrate species. The sporadic disturbance, small scale fires and rabbit grazing typical of many sites has more in common with the historical management of heathland than grasslands, and is crucial to their value.
As well as reptiles and amphibians, some of the most important species found on Essex post-industrial land include invertebrate populations of many Red Data Book, Nationally Scarce, Essex Red Data species, and a number included in the UK Biodiversity Action Plan, such as the Shrill Carder Bee, the Brown-banded Carder Bee, the picture winged fly Dorycera graminum, the solitary wasps Cerceris quinquefasciata and C. quadricincta and the ground beetle Anisodactylus poeciloides. In a county in which the majority of open ground low nutrient status mineral soil habitats have now been destroyed, post industrial/extraction sites probably harbour more rare/scarce plant species than any other habitat in Essex. They are also often important for their lichen communities and species.
Whilst in theory post-industrial land of high nature conservation value can be re-created, the best sites are usually over 10 years old and may be up to 50 years old or more, with the development of habitats unpredictable and strongly related to the substrates and contamination present in different areas and subsequent disturbance.
The habitatsHabitats found on brownfield sites of high nature conservation value are often complex in vegetation communities, structural habitats, topography and hydrology. Typically they comprise mosaics of bare ground with early pioneer communities on nutrient poor substrates, more established open grasslands, usually dominated by fine-leaved grasses with many herbs, areas of bare ground, scrub and patches of other habitats such as heathland, swamp, ephemeral pools and inundation grasslands. These habitats can often persist for decades without active management or intervention because of a combination of substrate, summer drought stress, occasional disturbance and rabbit grazing.
The history of brownfield land in EssexMuch of the remarkable invertebrate biodiversity to be found in the Thames Gateway and around Colchester and Brightlingsea can historically be considered to be a fauna of Thames Terrace grasslands, but today there is a strong association with the habitat mosaic that develops on poor, drought-stressed substrates such as found in many abandoned sand and chalk quarries, post-industrial land, silt lagoons and fuel ash lagoons. The Essex side of the Thames has a series of south-facing escarpments between Purfleet in the west and Southend to the east, with various exposures of chalk, Thanet sands, Thames terrace sands and gravels and London clay. The Purfleet-Grays area also has a long history of chalk extraction, with old leases dating back to the sixteenth century and modern times have seen much more extensive extraction of chalk and sand resulting in many abandoned exposures of different ages. It seems likely that there is a very long history of ecological continuity, contrary to the view conveniently expressed by consultancies working for developers, who also describe high quality habitats up to 50 years old as relatively recent in nature, dynamic in nature and since they are essentially man made, claim that they can be replicated through habitat creation and management – despite the fact that no two brown field sites are ever the same, with the development of habitats unpredictable and strongly related to the substrates and contamination present in different areas and subsequent disturbance.
Key features of ecology Post-industrial sites of high nature conservation value are often a complex mosaic, structurally diverse and with very flower-rich grasslands as well as sparsely vegetated areas and bare ground. This is usually because they have developed on nutrient-poor and sometimes contaminated substrates, and are subject to sporadic disturbance. These habitats may support very large and diverse invertebrate assemblages and important reptile populations, vastly more so than the modern agricultural countryside and many semi-natural habitats. In fact they have much more in common with the historic wildlife-rich countryside than the intensively farmed modern version. Although sparsely vegetated ground and early successional habitats are a key feature, it is a misconception that these habitats are the main or only important component of the sites. Brownfield sites take time to develop important wildlife assemblages or contain areas that have been abandoned for many years. Essex is one of the driest counties in Britain, and in the south and east of the county in particular, the dry climate and high summer sunshine levels mean that early successional habitats on nutrient poor substrates persist for long periods of time, even without management, with sites remaining open for as much as 50 years or more. This is an important issue: lack of mowing or cutting means that the forage resources, herbaceous stems, fruit heads, drought and mineral-stressed plants that many invertebrate species depend on is left in situ from one year to the next.
Eventually of course, left alone, these habitats will become shaded out by scrub and gradually develop into secondary woodland, but this is equally true of most semi-natural habitats so valued by all conservationists and naturalists. No one questions the importance of ‘semi-natural’ priority habitats such as lowland heathland, lowland meadows, lowland calcareous grassland or the Breckland of East Anglia and yet the wildlife importance of high-value brownfield land remains under enormous threat. It frequently has a lot in common with these semi-natural habitats. Lowland heath for example is characterized by a range of dwarf shrub and/or acidic grassland vegetation occurring on dry, sandy, nutrient poor soils. This cultural landscape was created by forest clearance in Neolithic times and traditionally maintained as part of the working landscape by a combination of small-scale excavation of sands and gravels, turf and peat cutting, the gathering of gorse or furze for fuel and low level grazing. The nationally important region of lowland calcareous grassland on Salisbury Plain is also maintained by a combination of grazing, rotational cutting and, most importantly, disturbance by the MOD – there are important lessons to be learnt here in how we manage these habitats! The nutrient poor soils, low level grazing (e.g. by rabbits) and sporadic disturbance of the habitat are frequently typical of what can be found on high quality brownfield wildlife sites.
Of particular value to ground nesting invertebrates such as mining bees and digger wasps is a friable sandy substrate or comparable substitute such as pulverized fly ash (PFA), but some species prefer hard compacted substrates and even tarmac and concrete gradually become vegetated and support vegetation that becomes valuable to wildlife. A varied topography within a site is also of high value in its provision of a varied hydrology and shelter.
In a county in which the majority of open ground low nutrient status mineral soil habitats have now been destroyed, abandoned chalk quarries, gravel workings, spreads of clinker, rubble and fuel ash deposits provide the majority of the sites for the Southern Marsh, Early Marsh, Common Spotted, Heath Spotted and Man Orchids, and Marsh Helleborine. Similarly the nationally scarce Cudweeds, Broad-leaved, Red-tipped and Narrow-leaved are now only found on brown-field sites. The chalk quarries in south Essex and Arkesden Chalk pit in North Essex provide the only sites in the county for several species once widespread in other habitats. The same story applies to scarce bryophyte species. Old gravel workings and the floors and cliffs of chalk quarries provide unique habitats for a wide range of species.
When it comes to acquiring land for industrial or residential development, land currently used as arable, planted woodland, and species poor secondary grassland sites are in general of lesser importance for nature conservation than the majority of brown field sites. Former arable in particular, is generally heavy with nutrients, residual herbicides, and fungicides.
Brown field sites are most valuable when kept as open habitat, rather than being planted with trees or seeded with grass mixtures, and ideally a proportion of the surface needs to be scoured at intervals of a few years to refresh the surface and remove scrub which develops into eventually woodland unless kept in check.
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