Observations on the restoration of limestone grassland in quarry environments on Portland.
Background
These comments are made as a result of personal observations of the various restoration schemes on the Island over the last 20 years. The most successful has been in the Admiralty Quarry/Nicodemus Knob area where a large area was infilled in the mid 1970s. This site now has a good variety of native flora including species such as Bird’s-foot Trefoil, Horseshoe Vetch, Carline Thistle and Pyramidal Orchid. Blue butterflies have also colonised the site, which is now one of the only sites where all of the island’s six species can be seen. Silver-studded Blues are particularly numerous, with other butterfly groups represented by species such as Dingy Skipper and Marbled White.
The success of this site appears to be due to a number of factors:
1. The site has a warm microclimate, with southeast facing banks and shelter from the prevailing westerly winds.
2. Topsoil was not introduced to the site, native plants being allowed to colonise the compacted rubble surface left after the infilling was completed.
3. The site does not have any introduced varieties of plants resulting from the use of seed mixtures. This has resulted in varieties of naturally low-growing species that provide suitably warm microclimates for butterflies such as the Silver-studded and Adonis Blues.
4. The large population of Rabbits in the area keeps turf low and discourages invasive species such as Brambles.
Recommendations
In the light of these observations, I suggest the following recommendations for restoring quarry sites:
1. The landform should be created to provide banks with a variety of gradients and aspects. Southeast to southwest facing aspects should predominate where possible.
2. The surface (ideally fine, compacted rubble) should be prepared, leaving flat stones to provide sites for colonisation by invertebrates such as ants and over-wintering insect larvae.
3. Locally collected seeds of limestone species should be broadcast onto the site to encourage rapid colonisation.
4. If available, samples of limestone turf should be placed around the site to provide islands of material from which plants and invertebrates can spread.
5. Native limestone plants can be propagated in advance and planted out as fully-grown plants.
Potential problems
This list summarises some of the problems that have occurred locally in the restoration of quarry sites:
1. Locally derived topsoil should not be used due to the problem of Black Mustard seeds that are prevalent in most soils on Portland. Black Mustard produces such dense stands that other species are prevented from becoming established.
2. Agricultural topsoils should not be used due to their high fertility, encouraging growth of “coarse” species that prevent colonisation of limestone species.
3. Seed mixes should be avoided. Even if the species composition matches that of the local flora the genetic make-up of the plants used will be different from plants obtained locally. Seed mixes often obtain vigorous strains that grow taller than local strains of the same species. These plants may not provide the microclimate required for the development of butterfly larvae, which use surface heat from the soil to raise their body temperature. (See ref. 1)
Bob Ford
References:
1. Andrew Jones “We plough the fields, but what do we scatter?” ; British Wildlife, Vol. 12 No. 4, April 2001; British Wildlife Publishing