Effects of Land Management on the Location and Status of Populations of Spiranthes Romanzoffiana in Scotland: a Desk Review and Evaluation of the Available Evidence
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Effects of land management on the location and status of populations of Spiranthes romanzoffiana in Scotland: a desk review and evaluation of the available evidence Dr. Scott McG. Wilson MICFor Consultant Forester and Forest Ecologist Report to Plantlife Scotland – June 2005 Executive Summary Irish lady’s-tresses (Spiranthes romanzoffiana) is one of the rarest members of the Orchidaceae in Europe, occurring only at scattered locations on the western seaboards of Scotland and Ireland, and at one location in south-western England. Otherwise the species occurs widely in many parts of North America in wetland habitats. The species was first recorded in Ireland in 1810 and in Scotland in 1921, with an accumulation of records in both countries until the 1990’s. Survey work conducted in both Ireland and Scotland during the past 10 years has indicated that the species is now absent from many sites at which it has previously been recorded. For that reason it is the subject of a Species Action Plan in both the UK and Ireland and has been included within Plantlife Scotland’s “Back from the Brink” recovery programme. In Scotland Spiranthes romanzoffiana occurs mainly on a range of wet pasture and wetland sites on lands subject to extensive pastoral agriculture in the crofting districts. A variety of vegetational contexts are involved, including Molinia grassland, mesotrophic mires and Juncus pastures. A set of broad land management prescriptions for the species has been developed based on empirical observation, including seasonal inundation, ground disturbance by livestock, extensive winter grazing by cattle and restriction of summer grazing (especially by sheep). However the relationship between these practices and Spiranthes romanzoffiana longevity is rather tentative. The current study sought to explore the possibility that historical land management information for sites where Spiranthes romanzoffiana has persisted might shed light on the optimum land management practices to be recommended under future agri- environmental schemes. A range of sources and techniques were employed in the work, including discussions with relevant specialists, incidental site visits and review of published and unpublished documents held by libraries and by the National Archives of Scotland, Edinburgh. Studies using land management records and estate archives have proved valuable in illuminating past woodland management in Scotland, but their application to other habitats has been much more limited to date. This work produced some further evidence in support of the existing management prescriptions for Spiranthes romanzoffiana based on recent observations. Historical records provided a chronological overview of changes in land management practices in the crofting areas concerned over the past two centuries, including a reduction in arable and potato cultivation, an increase in sheep stocking at the expense of cattle and a reversion of considerable amounts of former agricultural land to semi-natural vegetation under extensive grazing systems. While this historical information could at times be given a local flavour, there was insufficient evidence available to inform detailed ecological reconstruction of individual sites with Spiranthes romanzoffiana. Similarly, it was found to be impractical to relate the present status of Spiranthes romanzoffiana populations to land management prior to that of recent years, as data of sufficient spatial resolution do not exist. While the desk review may not be exhaustive it is not indicated that extension of the work using similar methods would be useful. Future work on Spiranthes romanzoffiana in Scotland should employ field studies, preferably under controlled experimental conditions, to investigate the physiological and reproductive responses of this species to different patterns of herbivory. Further work on the reproductive system and life-cycle of the species is also recommended. Contents 1 Introduction ..........................................................................................................2 2 Objectives ............................................................................................................4 3 Methods ...............................................................................................................5 4 Results .................................................................................................................9 5 Conclusions........................................................................................................13 6 Recommendations for further work ....................................................................14 7 References.........................................................................................................15 8 Acknowledgements ............................................................................................17 Front cover image © Andrew Scobie Back cover image © D. Long / Plantlife 1 Introduction 1.1 Distribution Spiranthes romanzoffiana presents a conundrum in that its major ecological distribution is in North America, with a small number of European populations along the western seaboards of Scotland and Ireland. This is termed an “amphi-Atlantic” distribution and is displayed by a number of other scarce plants in the British Isles. While a number of hypotheses have been advanced for this distribution pattern, including fragmentation by continental drift and introduction to Europe by animal or human vectors, the most likely appears to be wind transport of the very light seeds. Although the species was not recorded in Ireland until 1810 or Scotland until 1921, it is considered to be native in both territories. Its occurrence prior to these dates is unknown – it may have been present and gone unrecorded or it may have been absent. Early records in Scotland included the Isle of Coll (1921) and the Isle of Colonsay (1931). These were added to in the post-war years by occasional records on the Isles of Colonsay, Coll, Tiree, Mull, Barra and Vatersay and on the mainland in Ardnamurchan, Morvern and Kintyre. A total of some 70 locations were recorded. In the mid-1990’s a more systematic survey was commissioned by Scottish Natural Heritage which relocated the species at only some 11 of these sites and at a further seven sites where the species had not been recorded before, although in nearby areas. A similar reduction in the number of recorded sites has been observed in Ireland. This evidence prompted the preparation of Species Action Plans (SAPs) for Spiranthes romanzoffiana in both the United Kingdom and the Republic of Ireland. Plantlife Scotland are the lead partner for Spiranthes romanzoffiana in the UK, carrying out a programme of work under a funding arrangement with Scottish Natural Heritage. 1.2 Ecological site requirements The habitat requirements of Spiranthes romanzoffiana in North America are understood to be natural wetlands such as muskeg forest and beaver meadows, which will be subject to grazing by natural herbivore species such as elk and deer. The sites where the species has been recorded in Europe display a wide variety of conditions within a broadly wetland habitat category. These include lake margins, peat bogs, rush pastures and wet grasslands dominated by Molinia caerulea. Spiranthes romanzoffiana appears to require good top-lighting and at least moderate soil fertility. A preference for seasonal inundation has been suggested, but the species does not favour permanently aquatic conditions. The topographical position of most populations in Scotland is transitional between coastal “machair” ecosystems with a shell-sand influence and largely ombrotrophic blanket mire systems further inland. Most such sites have in the past been subject to some form of agricultural system. 2 1.3 Reproduction and life-cycle Spiranthes romanzoffiana has not been observed to set seed in Scotland and attempts to collect viable seed have invariably failed. It is assumed to reproduce entirely by asexual means, although the mechanism of spread has not been established as yet. Some form of physical disruption of the roots of the plant may be essential in permitting vegetative spread and it is proposed that cattle trampling can achieve this. Recent genetic studies have suggested that the populations in Scotland may be able to achieve some level of out-crossing, or have done so in the past by unknown means. Spiranthes romanzoffiana plants go through a complex life-cycle of flowering spikes, vegetative florets and “underground” phases. In the latter case trophic support by a fungal associate is likely to be involved, and this may also serve to synchronise the physiology of plants forming a single site population. In the “underground” phase the plant cannot be recorded by botanical survey, and this may lead to previously recorded populations failing to be relocated in a particular year. This has potentially serious consequences for the reliability of the hypothesis that the species is declining in Scotland and Ireland. It may indeed remain heavily under-recorded. Work on Barra has suggested that continued survey and plant marking over a period of several years may be required if population status is to be assessed with confidence. Flowering spikes are produced in the summer season and that is the easiest time to conduct surveys. Herbivory in summer by ungulates and slugs may remove flowering spikes. 1.4 Land management prescriptions Current conservation management for rare plant species is based upon the development