The Vegetation of Irish Machair

Abstract

Machair is a highly specialised and complex sand dune habitat that is confined globally to the north west coasts of Ireland and Scotland. It comprises a flat or gently undulating sandy plain that develops in an oceanic location with a cool, moist climate. Machair systems are highly calcareous, the sediments usually containing a high percentage of shell fragments and having pH values in excess of 7. The vegetation is herbaceous, with low frequency of typical sand-binding species such as Ammophila arenaria. Irish machair is designated as a priority habitat under the EU Habitats Directive and as such Ireland has special responsibility for its conservation. The quality and extent of machair in Ireland is increasingly under threat from human pressures such as housing developments, recreation and changes in agricultural practice. This paper describes the vegetation of Irish machair and examines how climatic, edaphic, geographic and anthropogenic factors not only influence the distribution and formation of machair, but the plant communities present. A comparison between the vegetation of Irish and Scottish machair reveals a high degree of similarity, with some variation that can be explained by different land-use and/or management practices. Karen Gaynor (e-mail: karen. gaynor@environ.ie), Research Branch, National Parks and Wildlife Service, 7 Ely Place, Dublin 2. INTRODUCTION Machair is a distinct geomorphological and ecological habitat that forms as a result of a unique combination of climatic, geographic, edaphic and anthropogenic factors. Irish machair is designated as a priority habitat under the EU Habitats Directive (Commission of the European Communities 1992) (code: 21 AO). It is described therein as a 'complex habitat comprised of a sandy coastal plain resulting partially from grazing and/or rotational cultivation, in an oceanic location with a cool moist climate'. The system is highly calcareous, the sediment containing high shell fragment contents and pH values in excess of 7. The vegetation is herbaceous, with low frequency of sand-binding species. Machair is unique in that it is considered a natural landscape that is the product of cultural activities. The features of a typical machair system are presented in Fig. 1. In most cases, a small escarpment or dune ridge fronts the system. The true machair plain represents that area where wind erosion has eroded the original dune system down to a level just above the water table, where the wet consistency of the sand prevents further erosion (Pl. I). In general, the degree of flatness depends on the age of the system, as well as the underlying topography, geology, outcropping of local rocks and historical management. The machair plain is often terminated on the landward side by a lake or associated marsh/fen. There are a number of exceptions to this typical sequence. Flat machair like surfaces may form over rock platforms by the sea, bypassing the dune building phase. Where the wind is particularly strong and the direction suitable, large quantities of sand can even be deposited on the seaward face of hills inland to form hilly machair (e.g. Garter Hill, Co. Mayo) (Bassett and Curtis 1985). Scottish machair was first identified as a distinct habitat in the 1940s (Wilmott 1945; Darling 1947; MacLeod 1949) and has since been extensively described (Ranwell 1974a, 1974b; 1977; 1981; Dargie 1993; 2000; Angus 2004). Although Tansley (1939) described machair vegetation from Dog's Bay, County Galway, the habitat was not formally recognised in Ireland until 1980, when the first account of Irish machair on the Mullet Peninsula was published (Akeroyd and Curtis 1980). The criteria used to define Scottish machair were tested on a number of sites in the west of Ireland, revealing a close similarity between Irish machair sites and those found in Scotland. Further research subsequently supported this assessment (Bassett 1983; Bassett and Curtis 1985; Crawford et al. 1996). The most recent inventory of Irish sandy systems lists a total of 50 machair sites (Curtis 1991a). Following on from the work of Ritchie (1976), Curtis (1991b) used the following five criteria to define Irish machair: 1. A mature coastal sandy plain, with a more or less level surface. 2. A significant proportion of shell fragments in the sand producing a lime-rich soil (pH > 7.0). Biology and Environment: Proceedings of the Royal Irish Academy, Vol. 106B, No. 3, 311-321 (2006). ? Royal Irish Academy 311 This content downloaded from 157.55.39.225 on Thu, 09 Jun 2016 05:16:57 UTC All use subject to http://about.jstor.org/terms Biology and Environment 4-Mobile Dune-^ ^-Machair Foredune Strandline : Hummocky machair