They have excited ecologists because they are in effect well-defined islands with distinct soil characteristics in a sea of more normal land. The soil has extreme pH values and low levels of nutrients, making it unsuitable for most plants - but some species can thrive in such situations. The isolated nature of the tips enables scientists to study how colonisation by a particular species occurs in such habitats, and to determine what barriers exist to more widespread colonisation by immigration of new species.
Tony Bradshaw, former Professor of Botany in the Department of Environmental and Evolutionary Biology at Liverpool University, has just completed studies stretching over more than a decade, examining the flora of tips and investigating which species could be introduced artificially.
'Even after 100 years the sites are very open and have a very restricted flora,' he says. 'This appears to be due to the extreme soil characteristics, which limit plant growth, combined with the difficulties that appropriately adapted species have in immigration.'
Even common weeds cannot tolerate the conditions. 'There is little or no competition for species that can colonise these sites,' says Professor Bradshaw. 'If the usual weeds do hop on to the island from the surrounding 'sea' they tend to grow very badly.'
One site Professor Bradshaw and his colleagues, Hilary Ash and Ray Gemmell, focused on was at Darcy Lever, near Bolton, where there is an 80-year-old tip of waste from the Leblanc process for the manufacture of soda ash. The tip comprises mainly calcium hydroxide, calcium sulphate and cinders. It is now surrounded by a landfill site, and is, essentially, a small island of highly alkaline soil.
The most noteworthy plant that has colonised the site without help is a fragrant orchid, Gymnadenia conopsea. 'There is a splendid patch, which people would have spotted first about 15 years ago as just a single plant. Since then it has spread across the heap,' Professor Bradshaw says. Also present naturally is purging flax, which produces a tiny white flower. These are species which would otherwise not be found in the vicinity, because the soil conditions are not suitable.
'Perhaps the most curious is creeping willow, of which there are a few plants. This certainly does not belong there. All these plants look very much like what is found on sand dunes 18 miles away, near Southport. They have small, light seeds which would have been brought on the wind.'
The botanists introduced 36 species on to the Leblanc tip. Of these, 17 became permanently established. The most spectacular successes were yellow wort (Blackstonia perfoliata) and felwort (Gentianella amarella), which, 11 years later, have spread across the heap.
The plants that failed were generally ones that needed a higher level of nutrients from the soil. Buttercup did not get a foothold, despite being fed with fertiliser, and cowslip grew only poorly. Perhaps unsurprisingly, commercial grass species grew only after being pampered with large amounts of fertiliser, and could not persist.
Another site is a 50-year-old tip of blast furnace slag at Kirkless Lane, near Bolton. This is a very stony site, its alkaline soil characterised by high levels of calcium- aluminium silicates, calcium carbonate and calcium hydroxide. Originally this site was colonised principally by poorly-growing grasses, but now supports a healthy growth of yellow rattle, a pretty dwarf annual that stands 6in high and bears a bright yellow flower, yellow wort and felwort, all introduced by the Liverpool team.
At Westwood, near Wigan, is a pulverised fuel ash dump some 20 years old. This has a spectacular naturally occurring population of orchids, especially marsh orchid and fragrant orchid, and marsh helleborine, which almost certainly flew in from Southport. The team introduced eyebright, with small white flowers, which, 15 years on, covers the whole seven- and-a-half acre site.
Professor Bradshaw believes his team's work has implications for the future. New habitats are continually being produced by industrial activity. While these are often left to become areas of wildlife, natural colonisation would occur only slowly, with a restricted number of species.
'Introduction of appropriate native herbaceous species into these industrial waste habitats is not difficult,' he says. However he adds: 'Which species or populations survive is the outcome of a very complex set of ecological interactions including the effects of climate during establishment, and it may never be possible to predict exactly what will happen.'
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