Collins New Naturalist Library. David Cabot
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Название: Collins New Naturalist Library
Автор: David Cabot
Издательство: HarperCollins
Жанр: Природа и животные
isbn: 9780007400423
isbn:
Wherever drainage is poor in the uplands or on the mountains, acid peat bog develops. This is one of the three main vegetation types typical of Irish high ground, the others being grassland and heath. On the exposed mountain summits, a more montane community is often present. Within the mountain environment there are many habitats hosting different groups of liverworts, mosses, ferns and flowering plants. Boulder screes, gullies, streams, vertical cliff faces, ridges and even snow fields that persist for several months each year provide specialist niches for the 58 species that are characteristic of Irish mountains and uplands.
Average annual rainfall 1951–1980. From Rohan5.
The occurrence of calcareous outcrops, pockets of base-rich rocks such as mica schists, or out-flushings of mineral rich waters from deeper below, have dramatic impacts on the mountain flora, allowing many species to flourish profusely in an otherwise acidic environment. The largest limestone outcrop and mountain range in Ireland is Benbulbin, Co. Sligo. Benbulbin ascends vertically in the upper parts to a blanket bog-covered plateau with a maximum altitude of 526 m. On first sight this smothering of peat appears to defy ecological good manners, sitting on top of limestone rock which should, according to conventional rules, be supporting a community of calcicole or lime-loving plants. Peatland communities, made up of more astringent calcifuge or lime-fleeing species are normally found in lime poor habitats. The dramatic cliff walls, hanging over the lowlands below, are where most of Benbulbin’s renowned arctic-alpine flora is to be found.
Burning and grazing are traditional agricultural practices that have moulded and shaped the upland and mountain plant communities for centuries. However, since Ireland joined the European Union in 1973 the number of sheep grazing the mountains has increased dramatically, prompted by generous subsidies and premiums from Brussels and the government. Numbers nearly trebled from 3.3 million in 1980 to 8.9 million in June 1991. The heavy grazing intensity steadily eradicates the dwarf shrubs, such as heather and other ericales, and allows their replacement by grasses and, in dry places, bracken. Under high densities of animals, peat also suffers compaction which alters its oxygenation, leading to a premature death of the vegetation skin. The mechanical trampling leads to the disappearance of peat mosses, Sphagnum spp., whose water absorbency is crucial for the ecology of the bog. Tussock-forming sedges are the most resistant to the sheep’s aggression. The compaction of peat and the loss of Sphagnum, compounded by the removal of vegetation by incessant grazing, leads to a faster runoff of water down the mountain slopes.6 During a limited ecological survey in Co. Mayo, conducted nearly eight years ago, a total of 66 selected blanket bog sites were visited with the objective of identifying the more intact areas for conservation. Twenty-five, or 38%, of the sites, covering some 12,500 ha, contained significant areas that were overgrazed. Several sites among the 25 were completely destroyed. Indeed, it is estimated that in the case of very eroded blanket bog, rock-bare in places, it would take between 5,000 and 10,000 years for just 2.5 cm of soil to be regenerated.6
How the precious alpine and arctic flora has fared under this new regime of unabated encroachment is not known. It is easier to assess the declining populations of moorland breeding birds such as the red grouse, merlin, hen harrier, and even of the Irish hare. Other impacts on upland and mountain flora come from burning, reclamation of moorland through drainage, afforestation and the application of a wide range of chemical fertilisers, often by air.
Erriff Valley, Co. Mayo. Excessive grazing by sheep (A. Walsh).
The arctic-alpine flora
David Webb, doyen of modern Irish botanists, in a critical review of the flora of Ireland in its European context, concluded that there were 16 genuine arctic-alpines in the Irish flora.7 His criteria were that the plant ‘must be fairly widespread in the arctic and sub-arctic regions of Europe and must reappear at high altitudes (at least up to 2,000 m) in the Alps (often also in the Pyrenees). But it must be scarce or absent in the intervening areas for otherwise it becomes merged in the main mass of northern continental species.’ He excluded any species that was found at low altitudes south of about 54°–55°N and any occurring in Central Europe at altitudes below 800 m in the immediate neighbourhood of high mountains. Using these criteria he excluded the following species often described by many different authorities as arctic-alpines: alpine saxifrage, marsh saxifrage, northern rock-cress, stiff sedge, spring sandwort, bearberry, cowberry and spring gentian. While all the these species are arctic they do not fulfil Webb’s other arctic-alpine criteria. According to Webb the spring gentian is a well-known example of incorrect geographic placement. It is often assumed – because of its association with mountain avens in the Burren, Co. Clare, and in the high Alps – to have an identical geographic distribution to mountain avens which is a true arctic-alpine. However, the spring gentian is common in central and southern Germany below 800 m and also present in the karst country of Slovenia. Moreover, its representation in the Arctic is extremely meagre, being confined to a few locations in arctic Russia.
The arctic-alpine species occurring in Ireland, according to Webb’s criteria, are alpine lady’s-mantie, fringed sandwort, hoary whitlowgrass, mountain avens, chickweed willowherb, mountain sorrel, alpine meadow-grass, alpine bistort, roseroot, dwarf willow, alpine saw-wort, yellow saxifrage, purple saxifrage, starry saxifrage, moss campion and alpine meadow-rue. Webb also identified the following seven ‘alpine’ and ‘arctic-sub-arctic’ species, some with reservations: alpine: recurved sandwort; alpine (with reservations): large-flowered butterwort and Irish eyebright; arctic-sub-arctic. Scots lovage and oysterplant; arctic-sub-arctic (marginal): water sedge and alpine saxifrage.
The cloudberry, which has one station in Ireland on the Tyrone side of the Tyrone/Derry county boundary in the Sperrin Mountains, is usually thought to deserve in other texts the appellation ‘arctic-sub-arctic’ but, according to Webb, it is too widespread south of the Baltic, in north Germany and Poland, to qualify for that status. Therefore it is not included in any of the above categories. All the seven ‘marginal’ species listed above are presumed to be arctic in origin, and to have spread southwards in front of the glaciers without going far enough to get up high in the Alps, Pyrenees or Carpathians.
In his seminal paper ‘On the range of flowering plants and ferns on the mountains of Ireland’ the great Irish botanist Henry Chichester Hart considered that 13 species qualified as ‘alpines’ but without giving precise criteria. Hart was perhaps the most intrepid and adventurous of all explorers of Irish mountains with an unrivalled knowledge of Irish mountain flora. His list of alpines is important as a tool for comparing the floras of various mountains throughout Ireland: cloudberry, northern bedstraw, the hawkweed Hieracium anglicum, bearberry, cowberry, juniper, stiff sedge, blue moor-grass, parsley fern, holly fern, green spleenwort, lesser clubmoss and quillwort. None meet the Webb criteria of arctic-alpines.8
Alpine and arctic-alpine species are both rare and thinly spread throughout the country. Concentrations occur principally in the coastal counties, where most mountain ranges are found, with more species in the northern than in the southern parts of the island. The elevation at which they occur increases from the north to south as temperatures are higher in the south and thus more elevation is required to find a suitable cold spot. Of 17 species analysed by Praeger that were common to the Donegal–Derry and Kerry–Cork mountains, the mean lower limit in the north was 168 m while in the south it was СКАЧАТЬ