Collins New Naturalist Library. David Cabot

Чтение книги онлайн.

      Western Connemara, Co. Galway, the location of rare plant assemblages.

      Devoy has discussed five possible land bridge connections between Ireland and Britain, including the Continental shelf, across which flora and fauna could have moved into Ireland when the country was released from the grip of ice during the late Ice Age.⁸⁶ Devoy considered that the route from the south, across the Continental shelf, along which the southern Lusitanian species would have travelled, would have been problematical. An interconnected series of channels and troughs off the south coast of Ireland led west and southwest into canyons lying at a greater depth than 100 m below today’s sea level. The movement of meltwaters and sediments over this area at the time even when sea levels were much lower than today would have created adverse conditions (pools, channels, rivers, etc.) for species sensitive to water, and made it very difficult, if not impossible, for plants to move dry-shod from Portugal, northern Spain and western France into Ireland. Thus the southern entry route for the so-called Lusitanian and other Mediterranean species now present in the Burren and in Connemara seems to be ruled out. Entry into Ireland of these species across the other four land bridges traversing what is now the Irish Sea is not supported by any historic or present day evidence. Survival of these species, many of which were already present in Ireland, in ice-free areas or refugia (off the west coast) thus appears to provide the most plausible explanation to account for the curiosities of Clare and Connemara.

      The oceanic flora

      The lowlier plants – ferns, mosses, liverworts and lichens – reproducing by minute, wind-blown spores, have less difficulty in crossing expanses of sea, and the mild, oceanic climate of Ireland has been particularly favourable to their colonisation. Especially in the extremely humid west, the profusion and luxuriance of these plants is a striking and important feature of the vegetation. In many of the western woods, communities of fìlmy-ferns, mosses and liverworts cover the rocky woodland floor and lower tree trunks, while lichens are most conspicuous on the upper trunks and branches. Many of these species have an extremely restricted European and even world distribution, confined to the far western seaboard and the Atlantic Isles of the Azores, Madeira and the Canaries (Macaronesia).

      The most famous is the Killarney fern, much the largest and finest of our three fìlmy-ferns, once locally abundant along the rocky streams and in the lower corries of the southwestern mountains, but reduced to rarity by indiscriminate collecting. The moss-like Tunbridge and Wilson’s fìlmy-ferns are in amazing quantity in many rocky woodlands and shady block screes, while the hay-scented buckler-fern, with its distinctive crinkly fronds, grows large and in unusual quantity. Visitors from Britain are struck by the general abundance in western Ireland of the royal fern, often to be seen in great dense patches on peaty ground. The fern collectors made less impression on it here than in western Britain, where it was once also abundant in places. The Irish spleenwort is a very rare and beautiful fern of exposed dry rocks and banks in southern Ireland and is unknown in Britain, but another Mediterranean-Atlantic fern of similar habitats, the lanceolate spleenwort, is more frequent in western Britain than Ireland. The delicate maidenhair fern, a widespread species in warmer parts of the world, grows luxuriantly on limestone, especially in crevices of the Burren pavements. Also of interest are the liverworts Cephalozia hibernica, Lejeunea flava, L. hibernica and Radula holtii, found in Ireland but not in Britain (see Appendix 1).

      The profusion of ferns, mosses and liverworts, including many oceanic species, extends up the mountains. In the extreme west, the shady corries and slopes facing between north and east have communities of leafy liverworts amongst or below heather or other dwarf shrubs and on rock ledges. These are virtually identical to liverwort carpets in similar situations on the equally wet mountains of the western Scottish Highlands. They contain species notable for their highly discontinuous world distribution in humid mountain regions as far apart as southwest Norway, British Columbia, Alaska, the Himalayas and Yünnan (e.g. Mastigophora woodsii, Herbertus aduncus and Pleurozia purpurea). The abundance of the woolly fringe-moss on stable block screes, bog hummocks and peat hags, and its dominance on many high mountain tops, is another feature of the oceanic climate.

      Many plant introductions from warmer regions have flourished in the mild Irish climate. The luxuriant and colourful hedges of fuchsia are one of the most distinctive features of the west, while the invasion of woodland by rhododendron has created a conservation problem. The southwest, with lowest incidence of frost, has well-established growths of escallonia, New Zealand flax, giant rhubarb, the hedge veronica Hebe elliptica and the cabbage palm tree.

3

       Mountains and Uplands

      Nothing more sharply exemplifies relativity than a mountain. A 100 metre-high hill in a flat landscape assumes mountainous proportions, but a normal arctic-alpine plant, casting a cold eye for a frosty north-facing cliff, would pass it by. Compared with their cousins in Wales, Northern England and Scotland, Irish mountains are not only generally lower but also cover a smaller proportion of the landscape and, as a consequence, offer fewer opportunities for a rich mountain, alpine or arctic-alpine flora and fauna.

      Raven & Walters¹ define a mountain as land over 2,000 ft or 610 m – in practice any height above 600 m is generally accepted as mountain land – which puts only about 0.3% or some 240 km² of Ireland in the ‘mountain’ category with approximately 190 peaks penetrating the 600 m limit. ‘Upland’ is a more difficult issue and is taken to include all land between 300 and 600 m and as such would embrace some 4,100 km² of Ireland. Together, mountains and uplands occupy 5% of the country’s surface. Most elevations are located in the coastal counties with the notable exception of the Galty Mountains rising up from the south Tipperary lowlands to reach 919 m.

      The great Irish botanist Nathaniel Colgan was the first person to point out that of the 67 species comprising the so-called Watsonian ‘highland’ group of plants found in Britain (named after the British botanist H.C.Watson), only 42 occurred in Ireland.² However, as Praeger has said, Watson’s definition of highland plants – ‘species chiefly seen about mountains’ – does not fit well in Ireland, where many of these plants are found in more places than just the mountains. Sixteen of them occur as far down as sea level.³ In the 1950s Raven & Walters provided a much more rigorous list of 150 ‘mountain’ species recorded in Britain and Ireland.¹ The vast majority of these fall into the ‘arctic-alpine’ category, i.e. plants that occur both in the Arctic and on some or all of the main European mountain ranges. If one ignores the taxonomically complicated and controversial dandelion-like hawkweeds (Hieraciums) and the real dandelions (Taraxacums), only 58 species or 39% of the Raven & Walters list are found in Ireland.

      Despite the impoverished representation of the ‘highland’ and ‘mountain’ plants in Ireland, Colgan made some interesting discoveries in the Mayo and Galway Highlands. ‘It may sound like a paradox to say that the botanical survey of an Irish mountain region derives a peculiar zest from the very poverty of our flora in alpine species. Yet the assertion may be made with perfect truthfulness. That the rapture of discovery varies directly with the rarity of the object sought for, that the value of the thing attained is measured by the labour of attainment – these are time-honoured truisms in every system of proverbial philosophy; and their essential truth is daily borne in upon the mind of the botanist who devotes himself СКАЧАТЬ