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Home , Caithness, Ceann Iar, Durness, Machair, Sound of Harris, Tidal island

DE THA ? TOWARDS A MACHAIR DEFINITION

Stewart Angus

Coastal Ecologist, Advisory Services, Scottish Natural Heritage, 10 Henderson Road, , , UK. IV1 1AU [email protected]

ABSTRACT

Machair is listed in Annex I of the EC Habitats Directive but is not defined in the Directive or its associated documentation. The only published definition of machair systems (Ritchie 1976) involves six main criteria, yet does not permit the separation of machair and non-machair systems in all parts of Scotland, which has two thirds of the EU and world extent of the habitat.

The work employs the Ritchie criteria in conjunction with GIS-based National Vegetation Classification maps of Scottish systems. A first attempt had addressed only the machair , but retained ahigh level of uncertainty. Adding the landward transition (and thus the wider machair system) to the criteria greatly improved the ability to distinguish between machair and non-machair systems. Once the machair system has been identified, the machair grassland can usually be reliably separated from the other components of the system, thus clearly identifying and separating machair grassland and machair systems for the first time. The methodology of the landscape-vegetation model is described and reviewed, and examples given of ‘problem’ sites and issues. Although the method cannot be reliably applied to all parts of all sites, it takes core definition as far as is possible using currently available information and accepts the premise that all such classifications can never be totally comprehensive.

This is an expanded version of the paper that appeared in the Proceedings of Littoral 2004.

1. INTRODUCTION

The term ‘machair’ has been applied by naturalists to particular dune grasslands since 1926, although the term was not adopted by scientists until the 1940s. Derived from the Gaelic mach, meaning ‘’, it has been used widely in place-names in both and Scotland, even in parts of both countries that have never supported machair habitat, e.g. Machair Aonghuis, the Angus Plain (E Scotland). In Scotland, some Gaels tend to use ‘machair’ as a general term for the whole dune system, including the dune ridge, while others restrict its use to the extensive flat grasslands inland of the dune ridge. In Ireland, the word has been used only in place-names, and the habitat’s existence there was only recently confirmed (Akeroyd & Curtis 1980). There is a very strong correlation between the distribution of machair in Ireland and the current distribution of native Gaelic speakers; the same would be true of Scotland if one were to add the ( and ), where Gaelic was never spoken (Angus 1993). It may be that there are important cultural components of machair, but there are presently no means of identifying these (Angus 1994).

Biologists and geographers had used the term fairly loosely until the 1970s, when Ritchie (1976) published what remains the only serious attempt to define machair. Even Ritchie’s six-paragraph definition does not enable all Scottish systems to be allocated to machair or non-machair dune grasslands, with systems on the northern mainland, the Northern Isles and some of the Inner posing particular problems. This became an issue with the listing of machair on Annex I of the Habitats Directive in 1992 (Council Directive 92/43/EEC). The interpretation manual (European Commission DG Environment 1996, updated 1999) described rather than defined the habitat, but clearly restricted its coverage to the plain rather than the wider system. The terms ‘machair grassland’ for the Annex I habitat and ‘machair system’ for the wider sediment unit have been adopted to distinguish the two usages (Curtis 1991a, Angus 1994). The former term was originated by Dickinson & Randall (1979) in a more descriptive context.

 An inventory of all dune systems on the of Ireland was published in 1991, identifying all machair systems using the criterion “systems in which the bulk of the area consists of a flat to gently-sloping from which ridges are absent or very restricted and with a consequent restriction in the area of Ammophila” (Curtis 1991b). The Irish approach would include many Scottish systems with acid grassland that would be excluded by the Ritchie definition. The definition of Ritchie (1976) is reviewed here, with suggestions made for refinement of the biological section with reference to the landform context.

2. RITCHIE CRITERIA

Ritchie (1976) employed a six-paragraph definition; the seventh, added here, is substantially based on subsequent work by Ritchie (1979). 1. a base of blown which has a significant percentage of shell-derived materials 2. lime-rich with pH values normally greater than 7.0 3. a level or low-angle smooth surface at a mature stage of geomorphological evolution 4. a sandy grassland type vegetation with long dune grasses and other key dune species having been eliminated. Core plants are red fescue Festuca rubra, common bird’s-foot-trefoil Lotus corniculatus, white clover Trifolium repens, yarrow Achillea millefolium, lady’s bedstraw Galium verum, ribwort plantain Plantago lanceolata, eyebright Euphrasia officinalis, daisy Bellis perennis and the moss Rhytidiadelphus squarrosus 5. biotic interference such as is caused by heavy grazing, sporadic cultivation, trampling and sometimes artificial drainage should be a detectable influence within the recent historical period 6. an oceanic location with a moist, cool climatic regime 7. Machair flood or are at least marshy in winter Ritchie’s first two criteria are directly related, and have been amalgamated here for ease of discussion. ‘Significant percentage’ is not further defined; this percentage will clearly vary according to the relative contributions of mineral sand (often derived from reworked fluvioglacial deposits) and biogenic input, but in ecological terms the percentage is the important variable. Although frequently referred to as ‘shell sand’, the carbonate component is not restricted to molluscan fragments, and may include fragmented sea urchin tests, crustacean exoskeletons and even Foraminifera tests. The only nationwide Scottish dataset of carbonate content is that supplied by the Countryside Commission for Scotland (CCS) reports (Mather & Ritchie 1977 and references therein) but most have a single sample, sometimes without precise location details. A fixed threshold could be misleading, in that shell content is know to vary considerably within single systems: a transect study of calcium carbonate content at Drimsdale, , revealed that carbonate level climbed slightly landwards from the beach, then decreased gradually towards the inland margins of the machair, where it quickly declined. The danger of utilising a single sample to categorise a site is illustrated by the carbonate figures for Sandside Beach in , which ranged from 7.4% on the beach to 41.5% in the . At , also in Caithness, the figures ranged from 5.1% at the caravan site to 21.9% on the southern beach. The figures may also vary within a particular level of the system, as at Bay in Caithness, where beach sand had a shell content of 30.1% in the east and 63.8% in the west (Mather & Ritchie 1977). Shell sand is known to have been used extensively for land improvement in ‘reseeding’ schemes in the 1960s-1980s (Angus & Elliott 1992); though this was aimed mainly at acid peatlands, shell sand has at times been spread on the ‘blackland’ of some machair townships (W. Ritchie, pers. comm.), which might still influence shell content, pH and vegetation in the blackland involved.

Although machair grasslands generally conform to the third criterion (Fig.1), some machair systems have steeper slopes, and some east Scottish mainland systems that would not qualify under other criteria would qualify here. There is an additional problem in that some dune systems were landscaped during World War II and this work is poorly documented. Ritchie (1979) emphasised that the non-dune was the distinctive landscape element of machair; he suggested a typical machair:dune ratio of 7:1, but this would not apply to some systems widely acknowledged to be machair such as the Monach Isles (Fig.2), where a large area is occupied by extensive dunes [probably] lying over an earlier machair plain, with a machair:dune ratio of approximately 1.7:1.

 Fig.1. Grogarry, South Uist. A ‘text-book’ Fig.2. Ceann Iar, Monach Isles, showing extensive machair, with an extensive, cultivated plain, secondary dune cover over assumed machair terminating landwards (foreground) in a grassland. network of lochs.

Ritchie’s botanical paragraph is accurate but limited, and this aspect is explored in further detail below. The��������� fifth feature is probably equally applicable to many non-machair systems, including some acid systems.

Scotland has been completely mapped for a range of bioclimate attributes, and all the known machair areas are in the ‘hyperoceanic’ sector, as is the entire western and northern Scottish . Eastern dune systems from Loch Fleet southwards are ‘euoceanic’ (Birse 1971). Almost all dune systems in Scotland have a mean wind speed exceeding the movement threshold for dry sand of 4.4m/s (15.84km/hr); only the dune systems of the , small parts of the Solway Firth and very small areas of the western mainland are more sheltered (Birse & Robertson 1970). Although these maps are extremely valuable, and possibly provide a theoretical maximum geographical extent for machair within their hyper-oceanic sector, they were substantially derived by interpolation from a limited number of stations and it would be unwise to apply the climate criterion as a distinguishing characteristic at a more local level.

Ritchie (1979) emphasised the importance of the role of water in machair systems. The larger machair grasslands are often of flat appearance over extensive areas, but there is often a slight downward (sometimes upward) slope inland from the dunes to the blackland (a terrestrial transition where there is reduced input of blown sand on peaty soils). This gradient is reflected in the high water table and considerable expansion of the area of standing water in winter. The reference to flooding is descriptive rather than definitive, as many non-machair dune systems have seasonal standing water. Ritchie (1979) further stipulated that where the machair plain gives way landwards to a marsh or loch, there is active sand deposition in that marsh or loch, thus gradually extending the machair grassland inland (Fig. 7).

Even combining all seven criteria and employing an element of judgement proved inadequate as a means of allocating the more difficult systems to machair or other types of dune grassland.

3. SAND DUNE VEGETATION SURVEY OF SCOTLAND

Scottish Natural Heritage commissioned a complete survey of the vegetation of coastal blown sand deposits in Scotland, mapping National Vegetation Classification (NVC) sub-communities at a scale of 1:10000 (Dargie 2000). The maps were subsequently digitised to enable them to be used in a Geographic Information System (ESRI ArcView). Dargie identified a wide range of ‘new’ vegetation types that did not conform to the descriptions in the NVC volumes covering mires (M) and swamps (S) (Rodwell 1991, 1995). Though the coastal volume (Rodwell 2000) used versions of Dargie’s maps, it is disappointing that his new sand dune (SD) vegetation categories were not addressed. As yet there are no protocols for

 the formal adoption of new NVC types.

Scottish Natural Heritage used the vegetation maps in conjunction with the Ritchie criteria in an initial attempt to identify all machair grasslands. Ritchie’s vegetation paragraph was replaced by a list of NVC categories, mainly sub-communities of the Festuca rubra-Galium verum fixed dune community (SD8) (Table 1). SD17 Potentilla anserina - Carex nigra dune slack, Agrostis stolonifera sub-community was later added, as it tends to occur on seasonally flooded areas within the machair grassland that are clearly integral parts of the machair grassland habitat rather than distinct .

Table 1: Sand Dune Vegetation Survey of Scotland vegetation types of machair grassland

SD8a Festuca rubra-Galium verum fixed dune community, typical subcommunity SD8c Tortula ruralis ssp. ruraliformis sub-community SD8d Festuca rubra - Galium verum fixed dune grassland, Ranunculus acris - Bellis perennis sub-community SD8e Festuca rubra - Galium verum grassland, Prunella vulgaris sub-community SD8x Centaurea nigra – Daucus carota sub-community (Dargie type, not in published NVC) MG11 inundation grassland SD17 Potentilla anserina - Carex nigra dune slack, Agrostis stolonifera ­sub-community (added later) Arable and fallow

This analysis produced the first published map of machair distribution in Scotland (Angus 1999), but this included a large number of sites in a ‘possible machair’ category, which is understandable but unsatisfactory. Although the machair plain may be distinctive in landscape terms, it is not sufficiently distinct in botanical terms. The list of vegetation types characterising machair grassland was later amended to include SD17.

Dargie (2000) applied Detrended Correspondence Analysis to his own data for Scottish dune systems, and derived a classification which placed most of the and in ‘core machair’ and the eastern Outer Hebrides, western , the Northern Isles, the northern mainland and small parts of and in ‘outer machair’ (Fig. 3). This latter category included two localities not hitherto regarded as machair: NE (Fraserburgh and Inverallochy-Peterhead) and NE Caithness (Sinclair’s Bay and ); two of these systems are reviewed below in respect of machair status.

Fig. 3. Biogeographical regions within Scottish dune and machair vegetation (Dargie 2000).

10 4. VEGETATION IN MACHAIR SYSTEMS & LANDSCAPE SETTINGS

Although the attempt to identify machair grasslands by addressing only the grasslands had clearly fallen short of objectives, it was noted that there seemed to be a correlation between vegetation and the landward transitions identified in a descriptive treatment of machair landscape (Angus 2001), and that this vegetation was more distinctive than that of the machair grassland. Used in conjunction with earlier work, this additional approach concentrates on the boundary zone between machair grassland and adjacent habitats. Although unusual vegetation types had been noted as long ago as 1983 (Pitkin et al 1983), and vegetation zonation had been noted by previous workers (Randall 1976, Randall & Dickinson 1979, Kent et al 1994), the distinctive nature of the combination in a landscape context had not.

Ever since the machair grassland was distinguished from the wider system, it has been accepted that it exists only as an integral part of that wider system, so approaching the identification of the Annex I habitat from a viewpoint within the system - but outside the habitat - is not as anomalous as it might first appear. By identifying machair systems, it should then be possible to identify the machair grasslands within these.

Where dunes are present, the mobile dune vegetation (SD5 Leymus arenarius mobile dune and SD6 Ammophila arenaria mobile dune) are no different from their counterparts in other parts of Scotland, but the landward margin of the dunes, and where there has been secondary deposition of sandhills over a machair plain, as on the Monach Isles, the typical semi-fixed dune vegetation seems to have a greater tendency to be replaced by a provisional new type, the Ammophila arenaria-Festuca rubra semi-fixed dune,Galium verum sub-community (SD7x) (Dargie 2000).

Fig.4 (a-d in text denotes top to bottom). The range of machair landscapes, greatly simplified. The third and fourth diagrams (c and d) represent a breach in the dune front and saline incursion (from Angus 2001)

Where machair grassland gives way inland to a slope (Fig 4a), there is a distinctive sequence of vegetation: the machair grassland SD8 types tend to be replaced upslope with SD8z Festuca rubra - Galium verum fixed dune, provisional new Thymus praecox sub-community (Dargie 2000). Though still primarily calcareous in character, this vegetation features a number of acidophilous species, reflecting thinner sandy soils and leaching of the shell content therein. With increased leaching and/or thinner soils, this gives way inland and upslope to the CG10b Carex pulicaris-Carex panicea subcommunity. The classic example of this sequence is the island of Pabbay in the of Harris (Figs 5-6). More

11 gradual upward inland slopes are very characteristic of , where the blackland is the main area for growing hay and for the creation of improved grassland.

Fig.5. Pabbay, Sound of Harris. Influence of blown sand can be seen to diminish from the beach (left) towards the summit.

Fig. 6. NVC map of blown sand area on the island of Pabbay, Sound of Harris. Semi-fixed dune is extensive (khaki), with two main areas of SD8 machair grassland (bright green), with an uphill transition to climbing machair SD8z and ultimately to CG10b on the uphill margin (both pale green). Mobile dune is pale yellow. Mires are shown in pale blue; these are wet areas within a semi-fixed dune sector rather than the transitional mires described below. Grid lines at 1km intervals convey scale.

12 Fig. 7. , Sound of Harris. Superficially this site shows a clear transition from western dune through increasingly wet machair grassland to a fresh-water loch, Loch Bhruist, which displays classic blown sand influence on its western margin. Minor variations in surface morphology are reflected in a highly complex mosaic of vegetation types, where wet machair grassland vegetation (SD8e) occurs in mosaics with semi-fixed dune (SD7x) and fen Mxbd (see text).

The typical transition to terrestrial vegetation involves an increasingly wet machair grassland to marsh (mire, M) and ultimately swamp (S) and open water, a situation that pertains in much of South Uist (Fig. 4b). This vegetation is occasionally referred to in the literature as ‘dune slack’ but true (structural) slacks between dune ridges are very rare in machair systems, although vegetation closely resembling dune slacks elsewhere (e.g. Potentilla anserina - Carex nigra dune slack, Agrostis stolonifera sub-community, SD17) is sometimes found in depressions on the machair grassland.

Fig. 8. Classic downslope sequence from semi-fixed dune ridge through machair grassland to inland loch at Loch Hallan, South Uist. Bright green is uncultivated machair grassland SD8, red is arable and red hatching is fallow machair grassland (two types of hatching denote types of fallow). Blue cross-hatching shows SD17 which generally occupies depressions in the machair grassland. These tend to be replaced inland by MG11 inundation grassland (blue-green colour, regarded as a type of machair grassland) then by mire (M, light blue) and ultimately swamp (S, purple) which in turn is replaced by true aquatic vegetation types.

13 The truly distinctive nature of the transitional vegetation is indicated by Dargie’s detailed results from the Western Isles (Dargie 1998): he recorded �����������������������������������������������������������������������864.5ha of M (mire) vegetation on sandy soils, of which 762.7ha (88.2%) was assigned to a new NVC community, and 812ha (93.9%) to unpublished sub-communities. For swamp, two new NVC sub-communities together contributed 73.9ha to the overall extent of 177.4 ha (41.7%). The provisional new mire community was named the Carex nigra rich fen (Mx), with five sub-communities, of which one, theCarex nigra – Carex diandra fen (Mxcd) was confined nationally to known machair sites in North and South Uist and is thus diagnostic of machair, but it covers only 31.7ha on these and thus has no value in assisting the classification of sites outwith the core machair areas. The other sub-communities were: Carex nigra – Prunella vulgaris – Molinia caerulea fen (Mxbd), Carex nigra – Menyanthes trifoliata fen (MxMt), Carex nigra – Triglochin maritimum mire (MxTm) and Carex nigra – Potentilla palustris fen (MxPp). The unusual nature of this vegetation was first noted by Pitkin et al (1983) in the Uists and a similar type has been described from Irish machair systems (Crawford et al. 1998).

Fig. 9. , a off North Uist, showing zonation from western dune ridge (right) through machair grassland to inundation grassland MG11 to Carex nigra – Triglochin maritimum mire (MxTm) with some Scirpus lacustris ssp. tabernaemontani swamp (S20) and Eleocharis palustris swamp (S19), which pass ultimately (beyond the left of the picture) to saltmarsh and sandflat – see Fig. 10.

The two new Western Isles swamp sub-communities were Phragmites australis – Calliergon cordifolium tall herb fen (SxTHF) (tall herb fen) and Hippurus vulgaris swamp (SHv). A third sub-community, Berula erecta swamp (S19x) was identified from ditches in Tiree and (Dargie 2000).

Individual lochs on the machair may straddle a wide range of substrates, from highly acid inland to shell sand on machair grassland. Water may enter the machair system with a pH as low as 4.5 and leave with a pH as high as 8.1 (Ritchie 1974). Superimposed on this range of variation there may also be a salinity gradient. Much of the machair is low-lying and where there is a gap in the dune ridge, saline water may flood in at high , creating brackish lochs, which form a distinct priority Annex I habitat within the machair systems (Fig. 4c, Fig. 11). The vegetation around these water bodies reflects their varied water chemistry, and saline tend to have saltmarsh rather than marsh at their margins, for example around Loch Bee in South Uist. Saltmarsh is also characteristic of the extreme end of this variation, where the land is so low and the dune breach so large that there are tidal sand flats separating the machair grassland from inland habitats, as seen at and Vallay in North Uist (Fig. 4d, Fig. 10). The saltmarsh vegetation is of a type particularly associated with NW Scotland (but by no means confined to sandy substrates) (Adam 1978) and similar vegetation has been noted in NW Ireland (Adam 1987). The classification of Scottish saltmarsh vegetation is such that even the authors of the NVC admit to problems with SM13 and SM16, the two main communities found in Scotland (Rodwell 2000), and it may be that an inventory survey analogous to the Sand Dune Vegetation Survey of Scotland could identify particular characteristics of these machair saltings.

Certain vegetation types are substantially absent from machair: upland (U) or heath (H) vegetation on blown sand or Carex arenaria-Festuca ovina-Agrostis capillaris dune grassland (SD12) should not be significant components of the vegetation.

14 Fig. 10. The tidal island of Vallay, off North Uist, displaying the area shown in Fig. 9. The wettest areas visible in the photograph are shown in purple (swamp), with saltmarsh (SM) vegetation shown as royal blue adjacent to the intertidal flats that separate the island from North Uist. Otherwise key as Fig. 8.

Fig. 11. Loch Bee, South Uist. Loch Bee Machair lies in the distance. Loch Bee itself is the large water body in the middle distance; it is connected to the sea by sluice gates on the far right, and inland (foreground) becomes part of an intricate drainage network. Loch Bee is also connected to the sea on coast by an artificial .

Although the mire and swamp vegetation are characteristic of machair, with rare exceptions they are not confined to it. It is more the sequence of vegetation types in the landscape, combined with the other Ritchie criteria, that allow machair systems to be identified more confidently.

5. CASE STUDIES

The landscape-vegetation model was applied to a number of ‘problem’ sites as a test of its value, including two of the four sites within Dargie’s mainland ‘Outer Machair’ zone. The landward transition at Fraserburgh in NE Scotland is substantially altered to cemetery, golf course and improved grassland. Such transitions as remain include MG11 and there is significant SD8 plain. This site could thus be regarded as machair even though it looks ‘wrong’ (Figs 12, 13).

Much of the southern transition of Sinclair’s Bay in Caithness has been converted to improved grassland, but in the north the landward transition involves MG11, and there is SD8 vegetation that could be regarded as machair grassland (Fig. 14). Much of the semi-fixed dune is SD7x, so the site qualifies as machair.

15 Fig. 12. Fraserburgh system, NE Scotland: NVC categories. Bright green denotes calcareous fixed dune SD8, khaki semi-fixed dune SD7, yellow stippling mobile dune SD5, SD6, blue-green inundation grassland MG11, blue saltmarsh (SM), red arable and brown SD9. Dark grey is improved grassland MG7, as is light grey (within polygons) MG6.

Fig.13. Fraserburgh system, NE Scotland, looking west over golf course towards Fraserburgh.

Fig. 14. Sinclair’s bay system, Caithness. Key as for Fraserburgh.

16 Fig.15. Sinclair’s Bay, Caithness, looking inland (north-west) from dune ridge

Fig. 16. Sandwood Bay system, NW . Key as for Fraserburgh. Unshaded areas within site denote open water or bare sand. Most of the pale green is CG13 rather CG10. Note very limited extent of SD8 (bright green).

Fig.17. Part of the Sandwood system, NW Sutherland.

17 A third site that has often been described as machair is Sandwood in NW Sutherland (Figs 16, 17). The landward transition is made up primarily of CG10a, not one of the indicators of machair systems; there is no SD7x in the semi-fixed dune, and the only SD8 present is SD8b. With no indicators of machair systems, this site is regarded as non-machair.

Dargie (2000) classes NE Oronsay (adjacent to , Inner Hebrides) as machair; though SD7x and SD8z are present, the landward transition is almost entirely acid, including not only SD12 but also heath communities (H) and acid blanket (M16) on sand, so this site would not qualify as machair using the landscape-vegetation methodology. The southern section on Oronsay, however, is clearly machair.

6. PROBLEMS

The terrestrial transitions are distinctive on most machair systems, but cannot be used where such a transition is absent, as on a narrow , narrow island, or . It has to be accepted that the more difficult sites must remain unresolved with regard to their machair status.

Dune systems with extensive plains have been prime candidates for development for hundreds of years, primarily for agriculture, but golf courses and airports have also featured. This development masks and often completely eclipses the landward transition, and this poses a particular problem in Orkney, where much or all of the terrestrial transition in some systems is occupied by improved grassland. Where such conversion has taken place, it could be argued that such systems no longer support machair grassland, in terms of the Ritchie criteria and Annex I, even if they once did. However, omitting them completely from the analysis would deny their potential for restoration to machair as part of the UK Machair Habitat Action Plan (Angus & Dargie 2002). At best, it might be possible use some criteria to class these as ‘historic machair’, but without supporting evidence from the landscape-vegetation model.

The allocation of individual mapped NVC polygons to machair grassland within machair systems is not without its difficulties. There are many polygons with intermediate vegetation, and within these the machair grassland type may be mixed with types which are clearly not machair: if they are of equal importance in the polygon, to which habitat should the polygon be allocated? There are other Annex I habitats within machair systems, and extent is a critical aspect of monitoring and reporting.

Fig.18. Traigh na Berie, Lewis. Most of the area in the foreground is climbing machair SD8z despite being undulating machair grassland.

18 Some polygons are ‘mosaics’, containing three or more different vegetation types, sometimes more than five, all with a different level of representation within the mosaic. The problems of intermediates are multiplied, with the added complication that the vegetation of the polygon may be so complex that it may be impossible for a conservation officer to relate the map to the reality, in terms of both vegetation types and polygon boundaries. There is the added problem of the qualifying threshold: in extreme cases, a polygon with 15% machair grassland vegetation may actually appear more machair-like than one with 30% machair grassland vegetation, except to a highly experienced vegetation surveyor.

The allocation of individual polygons in Lewis raised a further interesting issue, where vegetation does not comply with the ‘rules’. At Traigh na Berie, in SW Lewis, climbing machair SD8z was initially omitted from the map of machair grassland extent, but local officers suggested that some of the climbing machair area was in fact machair grassland. Fieldwork confirmed their suggestion: there was an extensive area of confirmed SD8z vegetation on otherwise typical machair grassland (Fig. 18) – the sand seemed fairly deep, free from adjacent acidic influence, and the land was generally flat – but acidophiles were nevertheless frequent in the vegetation.

7. IDENTIFYING EXTENT OF MACHAIR GRASSLAND

Once a system has been confirmed as a machair system, then most individual polygons can be assigned to machair grassland or other habitat. Polygons containing only vegetation types listed in Table 1 are straightforward, and the great majority are in this category. A significant number of polygons, however, have intermediate or mixed (‘mosaic’) NVC types, so that the polygon is shared by machair grassland and ‘other’. Initially a 40% threshold for machair grassland types was set, but it became apparent that below this level the relationship was not necessarily linear, so that some polygons with a low machair grassland component (e.g. 10-15% types listed in Table 1) might superficially more closely resemble the classic machair grassland habitat than polygons with 25-30% compliance.

Examination of SD17 vegetation in the field suggested that these areas, which occupy large areas within the machair grassland expanse (often as intermediates or mosaics), should be incorporated within machair grassland as they are not sufficiently wet in summer to justify separation as distinct habitats. SD17 has thus been added toTable 1.

As machair grassland is by its nature an expansive habitat, it makes sense to incoporate within it the small mapped areas of distinct habitat such as bare and mobile sand (often denoting a transient blowout feature). Although other features may be permament, such as localised swamp and rock outcrops, these were removed from the selection for machair grassland only where they are extensive or on the margin of significant machair grassland.

Fig. 19. Vegetation of Camas Uig, machair system, Fig. 20. Machair grassland polygons at Camas Uig. western Lewis. Key as before.

19 Figures 19 and 20 illustrate the allocation of machair system polygons to machair grassland. This could be improved further by ‘ground-truthing’, especially where small outliers or exclusions are involved. For example, the climbing machair in the NE of the site may be re-classified as machair grassland after a field visit, while the small outlier around the swamp might be disregarded.

8. CONCLUSION

The application of the above criteria greatly improves, but does not solve, the problem of machair identification. Machair is an exceptionally complex habitat, and is arguably no more than an extreme form of dune grassland on shell-rich . If so, that extremity is reflected in a correspondingly extremely high conservation value, and it is more constructive to work with a problematic definition of such an Annex I habitat than to deny the existence of the habitat.

This landscape model for the transition zone between unambiguous machair grassland and adjacent habitats supplements Ritchie’s definition and is designed to be used in conjunction with it. This landscape-vegetation model probably takes machair definition as far as it can be taken using the present information base, so that even with the best data, a certain amount of ‘black art’, using instinctive judgement based on experience is the best solution to the most difficult cases. It may be argued that the landscape-vegetation model places undue emphasis on ecological aspects of machair, but it was developed with the aim of meeting the requirements of the Habitats Directive and a Habitat Action Plan, and is in any case designed to be used in conjunction with other criteria rather than on its own.

The best opportunity for advancing this work is probably remote sensing (see Angus & Hansom, this volume), where a wide range of empirical criteria can be applied to unified datasets.

ACKNOWLEDGEMENTS

I thank Professor William Ritchie, Dr Tom Dargie, Professor Martin Kent, Dr John Baxter and Professor Robin Pakeman for their comments on the early stages of this work. Further helpful advice was received from James Hansom (University of ), Mike Shewry (SNH), George Lees (SNH), Alistair Rennie (SNH) and Professor Des Thompson (SNH). The GIS component, which was so important in the testing of this work, was ably assisted by SNH’s GIS staff: Lachlan Renwick, Kenny Monteath, Claire Seymour, Lee Douglas and Philippa Vigano.

REFERENCES

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