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:
One of the curious features of alpine plants in Ireland is that many of them do not remain perched high up on the mountains but descend down to sea level and into the countryside, sometimes well away from their alma mater. In fact, even when on the mountains they prefer reasonable to dizzy heights. Praeger looked at their distribution in relation to elevation in Ireland and found that the number of species increased steadily as one travelled towards the summit with a maximum number of 19 species at 300 m above sea level. Thereafter their numbers decreased equally steadily.9 Praeger also noted that 16 of the 42 so-called Watsonian ‘highland’ plants (including arctic-alpines and alpines) occur down to sea level. In fact, most Irish summits have a paltry flora, generally almost devoid of alpines and arctic-alpines. These are happier growing at lower levels, in more favourable habitats where, for instance, there are outcrops of mineral-rich rocks, and the ground is free from tall vegetation.
A few of the alpines do not give in to this ‘erratic’ behaviour but, as Hart observed, they are not numerous: ‘Of those more thoroughly alpine plants which never descended to sea level in these mountains (or elsewhere in Ireland), only three are ever met with on the exposed summits or outer ridges.’8 They were alpine clubmoss, dwarf willow and stiff sedge. As to the species that will occur very high up, Praeger listed eight that persisted above 914 m on the Macgillycuddy’s Reeks, Co. Kerry.3 The species, with their maximum recorded altitudes in Ireland were: starry saxifrage (1,036 m), alpine hair-grass (1,027 m), stiff sedge (1,006 m), green spleenwort (960 m), mountain sorrel (960 m), roseroot (960 m), alpine meadow-grass (945 m) and dwarf willow (930 m). These, together with the alpine clubmoss, could be taken as the hard core of the Irish alpine and arctic-alpine flora.
Juniper is common on some mountains, where the subspecies Juniperus communis subsp. nana is mainly found on siliceous rocks, always growing prostrate.
Most Irish summits – generally covered by thin peat or just bare soil and stones – make drab spots for flowers, whether or not of the alpine kind. One wishing to find ‘summit’ flowers might well have to abseil down along the north-facing cliffs and scarps in order to peruse the nooks and crannies and the rock overhangs. Praeger – himself a fearless cliffhanger – listed 13 of these core ‘summit’ plants, based on an analysis of the flora of the seven highest Irish peaks. They were tormentil, heath bedstraw, bilberry, heather, sheep’s sorrel, crowberry, heath rush, great wood-rush, hare’s-tail cottongrass, tufted hairgrass, sweet vernal-grass, sheep’s fescue and fir clubmoss.
Four maritime plants, which could be considered ‘sea-loving’, are frequently encountered growing on mountains at high altitudes in the company of alpine species. The first, thrift, a common coastal species, occurs on most of the highest mountains in Ireland. Hart encountered it at or close to the summits of the Mayo and Galway mountains – on Nephin and Mweelrea, Co. Mayo, both at 805 m, Croagh Patrick, Co. Mayo, at 675 m, and in the Twelve Bens, Co. Galway, up to 686 m. At Corraun, Achill Island, Co. Mayo, thrift grows abundantly on siliceous rocks at sea level, but then appears only above 366 m. It also grows at the summit of Carrauntoohil, Co. Kerry, at 1,039 m. Thrift adopts different shapes according to environmental stress. In its mountain habitat, subjected to grazing or particularly dry conditions it contracts into a dense cushion with short leaves and stems. Without stress, especially flourishing at sea level, it takes on a more straggly, less defensive aspect.
The sea plantain grows on the highest Galway and Mayo mountains up to 792 m. A curiously stunted variety, with broad leaves and a stem 3.8 cm high, was found by Hart at 701 m on the Askaheeraun ridge, Mweelrea, and on the summit of Ben Creggan (693 m), Co. Mayo. In the Twelve Bens the species ascends to 518 m. On Achill Island, Co. Mayo, it occurs continuously from the summit of Slievemore (671 m) to the sea-drenched cliffs below. This plantain also grows inland at low altitude levels, breaking with convention when it occurs around the shores of Lough Derg – some 40 km from the coast. It is also on the limestone pavements around Loughs Corrib, Mask, Carra, and Conn, Co. Mayo.
There are also outposts of maritime plants, including the two already mentioned and the following species, at the Killarney lakes, Co. Kerry, and around the shore of Lough Neagh in Northern Ireland. The white-flowered sea campion, normally found growing on sea cliffs, islands and coastal shingle, occurs on most of the Galway and Mayo mountains – on Croagh Patrick at 764 m, Mweelrea and Benlettery, Connemara, at 580 m and Birreencorragh, Co. Mayo, at 610 m. The common scurvygrass, common in dry salt marshes and coastal areas is also found in its subspecific form Cochlearia officinalis subsp. alpina in the Galty Mountains, Co. Tipperary, as well as joining the other three species already mentioned on the Binevenagh cliffs, Co. Derry. Finally, Hart also mentions a fifth species, the English stonecrop, as a member of the group which displays such aberrant distribution. A plant of rocky places, very frequently near the sea and rarely inland, this stonecrop is found on the Cork and Kerry mountains up to 801 m while in Wicklow it occurs up to 594 m and in Donegal up to 259 m.
Sea campion in its more natural lowland and coastal habitat, Little Saltee Island, Co. Wexford.
A plausible explanation for the mountain ascent of these maritime plants was put forward over 100 years ago by the Scottish naturalist Buchanan White who argued that they were part of the late glacial flora that was forced to migrate ahead of the expanding woodland cover during the Atlantic period. As the upper parts of the mountains were generally the only tree-free areas this is where the plants retreated to and survived until today.10 Work carried out by Watts on the flora of the Quaternary period in Ireland has shown that several species – likely candidates for migration up the mountain slopes – could be identified from the interglacial deposits at Kilbeg, Co. Waterford.11 The records included alpine meadow-rue (fruits), alpine clubmoss (spores), juniper (pollen, seed, needle), lesser clubmoss (micro and megaspores) and the sea plantain (pollen). Once established on mountain tops these, and many other species, found themselves safe from the re-advancing glaciers when another cold spell clamped down on the Irish landscape.
Principal locations for alpine and arctic-alpine species
The Wicklow Mountains and uplands
The Wicklow Mountains and uplands are the most extensive area of granite in Ireland and greater than any in Britain. The region is of considerable natural beauty and topographical diversity, where glacier-scoured river valleys and glens clothed in broadleaved woods host enchanting lakes; where large tracts of blanket bog are presided over by granite summits and dominated by Lugnaquillia Mountain (925 m); where there is a swaddling coverage of coniferous plantations, and where cliffs and corries are frequent. The vegetation found there is characteristic of the upland and mountain habitats of many other sections of Ireland.
During the winter of 1901–2 George H. Pethybridge, a young English plant physiologist, teamed up with Praeger to start a survey of the vegetation lying south of Dublin, including the Wicklow uplands.12 According to White this was the start of the modern investigations of Irish vegetation.13 Pethybridge & Praeger distinguished four zones: (i) the lowest, or the littoral zone which is of little or no interest to botanists; (ii) farmland, that merged into (iii) hill pasture at about 275 m (with gorse to begin with and western gorse at higher altitudes) before giving way to (iv) heather moorland which developed at around 380 m and continued upwards to the highest – flora-poor – summits.
The dry heath of the hill pasture, dominated by western gorse, heather and bell heather, is СКАЧАТЬ