Soil Preferences

To illustrate the soil preferences of many of the species on Skye, Birks (1973) grouped the vascular flora into categories, on the basis of field observations of species abundance in relation to soil type and parent rock. He emphasised that the observations were restricted to Skye, as some species, such as Sibbaldia procumbens, Equisetum hyemale, Melica nutans, and Vaccinium vitis-idaea, show soil preferences on Skye which differed from those elsewhere in Scotland.

Calcifuges

These are species that are largely restricted on Skye to iron or humus podsols, alpine rankers, acidic peats (wet or dry), and soils with raw humus, generally with a soil pH of less than 4.5.

Calciphiles

These are species that are exclusive to, or strongly selective for, soils developed on limestone, with a pH greater than 6.0. This list corresponds closely to McVean & Ratcliffe’s (1962) group of “exacting calcicoles” that characterises soils of high exchangeable calcium content (>300 milligrams/100 grams soil) and high pH (>6.0). Several of these species appear to have a specific need for calcium, and will not grow on other base rich substrata (Ferreira (1963)).

Basiphiles

These are species that are confined to base-rich soils, generally brown earths with a pH range of 5.0-6.0, developed on limestone, basalt, or gabbro. Many species are included here that are characteristic of communities

Indifferents

This group included species that grow on a wide range of soil and rock types, and that appear to be indifferent to soil acidity and calcium status. Some of the species only occur sparingly on soils at one other end of the soil basicity series, and their optimum soil conditions are denoted in the table by “A” (acid soils).

Species that avoid the poorest soils

These are species that have a wide edaphic tolerance on Skye, but which avoid the poorest soils, especially acid peats and mor humus of low pH (<4.5). Many of the species included in this group have their optimum occurrence in basiphilous vegetation.

Phytogeography

In Birks (1973) the floristic elements adopted were similar in part to those used by Matthews (1937, 1950) to characterise the British Flora, and in part to those used by Hultén (1950) to characterise the Scandinavian flora, he distinguished the following floristic elements which contained ferns.

ATLANTIC ELEMENT

his floristic element consists of species with a western distribution in Europe that are confined to the strictly Atlantic coastal region extending from western Norway to Portugal and south-west Spain. There may be a few scattered occurrences to the east and/or south of this region. This large element can conveniently subdivided into smaller groups on the basis of latitudinal distribution within the Atlantic zone.

Southern Atlantic

This group contains species found mainly in southern and western parts of the British Isles, western France, the Iberian Peninsula, and often extending to Macaronesia and the Tropics. Very few of the species in this group are known in Scandinavia.

Widespread Atlantic

This group consists of species that are widely distributed throughout the Atlantic zone of Europe, extending from western Norway and the Faeroes to the Iberian Peninsula, and Macaronesia. Several species occurring on Skye have such a distributional pattern in Europe, and they occur in a range of habitats on the Island.

SUB-ATLANTIC ELEMENT

This large floristic element consists of species that are widely distributed in the Atlantic province of Europe, but unlike the Atlantic Element, this group is well represented in parts of central Europe and frequently occurs in southern Fennoscandia. Some of the species in this element are found quite widely around the Mediterranean, and a few reach Asia Minor.

Widespread Sub-Atlantic. This large group of species occurs widely throughout Western Europe. It is difficult in some cases to decide whether to assign a species that occurs rather rarely in southern Scandinavia to this group on to the Southern Sub-Atlantic group.  An (s) in the following list indicates those species classed in this group that have southern affinities, and which are rather rare and local in southern Scandinavia.

CONTINENTAL ELEMENT

this floristic element consists of species with a rather eastern distribution in Europe, with their headquarters in central Europe, and often extending eastwards into Russia and Asia. A sharp distinction between the Continental and the Sub-Atlantic Elements is rather difficult to define, and some of the species in these elements may appear to other phytogeographers to be misplaced. The Continental Element is divided into groups on the basis of latitudinal distribution with mainland Europe.

Southern Continental

This small group of species is found mainly in central and southern Europe, extending into the Mediterranean region, and even into north Africa and south-west Asia. The species in this group do not extend further north than southern Scandinavia. None of the species in this group play a prominent role in the vegetational cover on Skye, being rather rare or local species restricted either to dry, south facing limestone slopes or basic rock outcrops.

Northern Continental.

This group contains members of the Skye flora with distributions centred mainly on northern and central Europe and often extending north to the Arctic Circle. In southern Europe they are largely found sub-alpine situations. The group is well represented on Skye in a range of habitats.

NORTHERN-MONTANE

This element consists of species with distributions centred on northern Europe where they occur in the woodland or the sub-alpine zones. They also occur in central and southern Europe, but they are restricted there to sub-alpine or montane habitats. They are generally absent from the lowlands of central Europe.

ARCTIC-ALPINE

This element consists of species with distributions centred on arctic and sub-arctic areas of northern Europe, but unlike the Arctic-Subarctic element, the species also occur at high altitudes in central Europe. It is well represented in the modern Skye flora.

WIDESPREAD SPECIES

This is the largest floristic group within the present Skye flora. The species within this group have widespread and nearly continuous distributions in Europe, and they belong to the General European, European, and Northern Hemisphere Elements distinguished by Matthews (1937). Many of the species in this group have present ranges that encompass several floristic provinces, and are more or less cosmopolitan in their world distribution. Although the group is of limited phytogeographical interest, many of the species within it are ecologically important and form a prominent component of the present vegetation of Skye, i.e. Pteridium aquilinum.