St Austell Clay Pits SAC Conservation Objectives Supplementary Advice
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European Site Conservation Objectives: Supplementary advice on conserving and restoring site features St Austell Clay Pits Special Area of Conservation (SAC) Site Code: UK0030282 St Austell Clay Pits SAC - Baal Pit (photo credit D. Callaghan 2011) Date of Publication: 11 February 2019 Page 1 of 13 About this document This document provides Natural England’s supplementary advice about the European Site Conservation Objectives relating to St Austell Clay Pits SAC. This advice should therefore be read together with the SAC Conservation Objectives available here You should use the Conservation Objectives, this Supplementary Advice and any case-specific advice given by Natural England when developing, proposing or assessing an activity, plan or project that may affect this site. This Supplementary Advice to the Conservation Objectives presents attributes which are ecological characteristics of the designated species and habitats within a site. The listed attributes are considered to be those that best describe the site’s ecological integrity and which, if safeguarded, will enable achievement of the Conservation Objectives. Each attribute has a target which is either quantified or qualitative depending on the available evidence. The target identifies as far as possible the desired state to be achieved for the attribute. The tables provided below bring together the findings of the best available scientific evidence relating to the site’s qualifying features, which may be updated or supplemented in further publications from Natural England and other sources. The local evidence used in preparing this supplementary advice has been cited. The references to the national evidence used are available on request. Where evidence and references have not been indicated, Natural England has applied ecological knowledge and expert judgement. You may decide to use other additional sources of information. In many cases, the attribute targets shown in the tables indicate whether the current objective is to ‘maintain’ or ‘restore’ the attribute. This is based on the best available information, including that gathered during monitoring of the feature’s current condition. As new information on feature condition becomes available, this will be added so that the advice remains up to date. The targets given for each attribute do not represent thresholds to assess the significance of any given impact in Habitats Regulations Assessments. You will need to assess this on a case-by-case basis using the most current information available. Some, but not all, of these attributes can also be used for regular monitoring of the actual condition of the designated features. The attributes selected for monitoring the features, and the standards used to assess their condition, are listed in separate monitoring documents, which will be available from Natural England. These tables do not give advice about SSSI features or other legally protected species which may also be present within the European Site. If you have any comments or queries about this Supplementary Advice document please contact your local Natural England adviser or email [email protected] Page 2 of 13 About this site European Site information Name of European Site St Austell Clay Pits Special Area of Conservation (SAC) Location Cornwall The designated boundary of this site can be viewed here on the Site Map MAGIC website Designation Date 01/04/2005 Qualifying Features See section below Designation Area 0.61 ha Designation Changes N/A Details of the feature condition assessments made at this site can be Feature Condition Status found using Natural England’s Designated Sites System Names of component Sites of Special Scientific St Austell Clay Pits SSSI Interest (SSSIs) Relationship with other European or International N/A Site designations Site background and geography St Austell Clay Pits SAC is comprised of three sub-sites, all located in mid-Cornwall within the china clay district to the north of St Austell, and within an approximate 4 km radius of each other. The sub-sites; Baal Pit (Carclaze, north of St Austell), Prosper Pit (south of Roche) and Trethosa Pit (south of Treviscoe), are located within china clay workings, consisting of pits, spoil tips and granitic debris with sparse vegetation cover. They are underlain by the Upper Carboniferous St Austell Granite, which forms part of the Cornubian batholith. Zones within the granite, altered by kaolinisation (a process of decomposition), have been extensively worked for china clay. Decomposition of the white feldspar component of the granite produces kaolinite, the main constituent of china clay. China Clay was first discovered in Cornwall at Tregonning Hill, near Helston in 1746. By 1748 larger and more pure deposits had been found in the St Austell area, and by the early 19th Century the St Austell china clay deposits had emerged as the largest in the World. China clay continues to be extracted in the St Austell district. The site supports important populations of the internationally rare liverwort, western rustwort Marsupella profunda. In the St Austell Clay Pits SAC, Marsupella profunda is generally found growing on firm clay substrates and soft or crumbling granite boulders. Marsupella profunda is a pioneer species, the largest populations being found on surfaces showing the early stages of colonisation by filamentous algae, other bryophyte species and vascular plants. The SAC lies within the Hensbarrow National Character Area (NCA Profile 154) Page 3 of 13 About the qualifying features of the SAC The following section gives you additional, site-specific information about this SAC’s qualifying features. These are the natural habitats and/or species for which this SAC has been designated. Qualifying Species: • S1390 Western rustwort Marsupella profunda The SAC is comprised of three sub-sites, all within the china clay district to the north of St Austell. The site supports important populations of Marsupella profunda western rustwort, an internationally rare liverwort, endemic to western Europe. It has a restricted, strongly oceanic distribution, and is known only from a small number of sites in Portugal, the Azores, Madeira and the UK where it is known only from Cornwall. Surveys and research have shown that a large proportion of the global population of Marsupella profunda occurs in Cornwall, within former china clay workings in west Cornwall near St Just and Helston, and in the working china clay district of St Austell in mid Cornwall. Marsupella profunda is a specialist of open clay substrates and kaolinised granitic material (boulders, rock outcrops) which are bare or in the early stages of colonisation by filamentous algae and other small bryophytes. Marsupella profunda often grows intermixed with the similar species, Marsupella sprucei, and/or with Marsupella emarginata. Marsupella profunda is a ‘mobile’ colonist species, which functions as a typical ‘metapopulation’. This species is limited by the duration of suitable habitat. The maintenance of the metapopulation relies on a balance between local extinctions due to vegetation succession, and its ability to colonize new sites. The critical requirement of a metapopulation is the continual availability of suitable habitat. A metapopulation is the overall population within a general area, consisting of a number of, sub- populations which may be temporary in nature, with a balance between local extinctions and colonisation of new sites. Marsupella profunda is a poor competitor which is soon shaded out as other plants become established. It freely produces wind-blown spores, a typical reproductive strategy of a colonist. Factors influencing the success of colonisation are spore production, dispersal distance, rates of establishment and survival, and most importantly, the availability, proximity and spatial distribution of habitats suitable for colonisation. Page 4 of 13 Table 1: Supplementary Advice for Qualifying Features: S1390. Marsupella profunda; Western rustwort * Attributes Targets Supporting and Explanatory Notes Sources of site- based evidence (where available) Population Population Restore the abundance of the This will ensure there is a viable population of the feature which is being CALLAGHAN D. 2011 (of the abundance population to a level which is maintained at or increased to a level that contributes as appropriate to its feature) above the 2003 baseline Favourable Conservation Status across its natural range in the UK. Due to HOLYOAK D. T. 1998 population cover the dynamic nature of population change, the target-value given for the population size or presence of this feature is considered to be the HOLYOAK D. T. 1999 minimum standard for conservation/restoration measures to achieve. HOLYOAK D. T. 2003 This minimum-value may be revised where there is evidence to show that a population’s size or presence has significantly changed as a result of HOLYOAK D. T. 2006 natural factors or management measures and has been stable at or above a new level over a considerable period (generally at least 10 years). The HOLYOAK D. T. values given here may also be updated in future to reflect any strategic 2007a objectives which may be set at a national level for this feature. HOLYOAK D. T. 2009 Given the likely fluctuations in numbers over time, any impact- assessments should focus on the current size of the site’s population, as HOLYOAK D. T. 2010 derived from the latest known or estimated level established using the best available data. This advice accords with the obligation to avoid HOLYOAK D. T. 2010 deterioration of the site or significant disturbance of the species for which the site is designated, and seeks to avoid plans or projects that may affect PORLEY R. 2007b the site giving rise to the risk of deterioration. Similarly, where there is evidence to show that a feature has historically been more abundant than the stated minimum target and its current level, the ongoing capacity of the site to accommodate the feature at such higher levels in future should also be taken into account in any assessment. Unless otherwise stated, the population size or presence will be that measured using standard methods, such as peak mean counts or breeding surveys.