Читать книгу Natural History in the Highlands and Islands - F. Darling Fraser - Страница 9

The relation of geology and scenery we have taken for granted; that of geology, climate and vegetation we take almost for granted, and because of that we are too apt, perhaps, to generalize. Our old school geography books informed us that the climate of the British Isles was mild and humid and that the laving waters of the Gulf Stream (which we now call the Atlantic Drift) kept our insular climate an equable one. Though it is generally true that the south-eastern side of Britain is drier than the north-western, it is for the naturalist to inquire more deeply into such generalizations, for he knows that altitude, position and slope in relation to sun, nearness to sea and so on have considerable local effects on climate. At least, he is finding out that he must know local climates quite well if he is to be a good naturalist—for example, Loch Tay is 20 miles long; from east to west there is an increase of an inch per year of rainfall for every mile you go. We should rightly judge, therefore, that the more detailed natural history of Killin would show considerable differences from that of Aberfeldy on the scoe of rainfall alone. When I lived at Dundonnell at the head of Little Loch Broom I found the rainfall to be about 72 inches a year. Seven miles to the south in Strath na Sheallag beyond the Torridonian cones of An Teallach (Plate IIIb), the rainfall in my gauge there measured 100 inches. Seven miles north of Dundonnell lies Ullapool, which receives an average of 48 inches. Both snow and frost are more severe in Strath na Sheallag than in Ullapool. The differences in the vegetational complex of these two areas were marked, and so were those of the animal groupings, though of course it would be wrong to put it all down to the climate. If climates can alter so markedly within a few miles—and that is the rule rather than the exception in the Highlands—so can they alter within a few yards. The ecologist, the man who studies organisms in relation to their environment, is now giving more attention to what are called micro-climates. That gully in the north corrie of Ben Nevis which was mentioned previously as never getting the sun, and where the snow sometimes remains the year round, is an example of a distinct micro-climatic region. What is its annual mean temperature and its extremes? What does its relative-humidity chart look like? How much light gets in there? We do not know. And coming to its living things, what plants are found there? Perhaps the fauna would include a few spiders, some of which creatures have a habit of living in unlikely places on mountains. Again, knowledge remains incomplete. The distinctive natural history and weather data of that micro-climate remain to be discovered and set down, despite the fact that a meteorological station was maintained on the summit of Ben Nevis for twenty years from 1884 to 1903 and hourly records of all kinds taken.

The north side of a tree has a micro-climate quite distinct from that of the south side, and as a result of this there are definite zones of disposition of mosses and lichens. Similarly, the upper canopy of a tree has a different climate from that at its foot. The micro-climates of such a broken-up area as the Highlands are legion and beyond the scope of this book: attention will have to be confined to some of the variations likely to be met in our passage here and there through the hills, glens, lochs and islands.

Let us first of all realize the amount of indentation of the land of the West Highlands, which allows the sea to enter far into the countryside. That in itself, the western ocean being relatively warm for this latitude of 53–59° N., makes for mildness in the immediate neighbourhood of the sea lochs. It must be remembered, however, that altitude far outweighs latitude and distance from the sea in the matter of climate. West Highland hills tend to rise steeply from the sea, and as a great deal of ground lies above the 1,500-foot contour, the over-all temperature is low. The coasts, especially in favoured places, are exceptionally mild, and years may pass without more than 2° F. of frost being recorded. Only the coasts of western Wales and southern England can exceed the mean annual warmth of those of the West Highlands. The mean January temperatures of the West Highland coasts are between 40 and 42° F. compared with 38° and less on the east coast of Scotland. The July isotherms tend to go east and west rather than north and south, but as the west coast of Scotland is reached they take a distinct dip south-westwards and show the West Highlands to have a mean summer temperature of 55–57° F. compared with 56–58° F. on the east coast. If we take the differences between annual mean summer and winter temperatures, there is only 14° F. of difference on the West Highland coast, compared with 20° F. at Dundee and 24° F. in London, Kent and East Anglia. These, of course, are sea-level temperatures. The Highlands show a completely different story as soon as you go uphill, and when you truly go inland into the Monaliadh or Cairngorm regions, conditions are much more extreme. Much work has been done on mountain climate in Britain by Dr. Gordon Manley, President of the Royal Meteorological Society. The twenty years’ work on Ben Nevis is available to me in summary and I give these figures as a comparison with the sea-level conditions which pertain on the coast only a few miles away. The mean temperature of the hottest month, July, was 41.1° F., and of the coldest month, February, 23.8° F., a range of 17.3° F. The extreme records were, Maximum 66° F. (28 June 1902) and Minimum 1° F. (6 January 1894).

FIG. 2a.—January Isotherms (reduced to sea level)

FIG. 2b.—July Isotherms (reduced to sea level)

FIG. 3.—Average annual rainfall in the Highlands Based on a map prepared in the Meteorological Office and reproduced by permission of His Majesty’s Stationery Office Crown copyright reserved

When we come to rainfall we see how easy it is to fall into a trap by generalizing that the west coast is wetter than the east. The beltof very high rainfall is not on the coast but a few miles inland, and even then it does not extend uniformly from north to south of the Highlands. Reference to Bartholomew’s Atlas of Scotland will show the monthly rainfall distribution in fair detail. Coming south from Cape Wrath, the strip of country with a rainfall over 100 inches a year does not start until Latitude 57.30° N. and then goes southwards and slightly south-eastwards to Latitude 56° N. There is a good area all round this strip with a precipitation of 60–100 inches, but the coastal promontories, especially in the north, and the Hebrides, receive only 40–60 inches of rain, a low figure which is not reached elsewhere in the true Highlands until the central area. A very few parts of the eastern Highlands receive as little as 30–40 inches. There is no doubt that Skye and the Inner Isles are partly responsible for the heavy precipitation on the mainland coast to the east of them.

These two climatic factors of temperature and rainfall are of immense importance in determining the vegetation of an area, but there are interrelations of these two which must always be taken into consideration when natural history is being studied. For example, something must be known of the rate of evaporation relative to the precipitation. The ratio between these two in any given situation has a big influence on the types of plants and to a lesser extent on the animals to be found there. The evaporating power of the air is measured by what is called the saturation deficit, which can be calculated from the difference in temperature between the wet and dry bulb thermometers when the temperature of the air is known. The saturation deficit is a measure of the humidity of the air. It is found in practice that there is often a constant variation in the saturation deficit during the day, and where such variation pertains, it may in itself influence the grouping of living things. The evaporation rate tends to be high in summer on the West Highland coast and for some way up the hills, but in the glens and on the high tops the evaporation rate is generally low. The annual average difference between wet and dry bulb readings on Ben Nevis was only 0.7° F., which must be quite the most humid climate in Great Britain.

Sunshine records in the Highlands are highly variable. Broadly speaking, the amount of sunshine is in inverse proportion to the rainfall, so the strip of the Highlands to which allusion has already been made as enduring 100 inches or more of rain enjoys least sun. It is also possible to draw a line bisecting the Outer Hebrides longitudinally, showing an annual total of sunshine of less than 1,200 hours on the west side and 1,200–1,300 hours on the east. I remember very well during several months on North Rona, equidistant 47 miles north of the Butt of Lewis and Cape Wrath, how much oftener it was possible to see Cape Wrath lighthouse quite clear than the north end of Lewis which would be shrouded in cloud. The resident in the Highlands knows well how much more sun there is on the little islands and promontories than on even the general run of coastal areas. The published figures probably do not show the true position because the number of sun gauges is few in the West Highlands. One of the surprises of the West is the local area of high sunshine records for the island of Tiree (Plate 25), the outermost of the Inner Hebrides. The island is low (beneath the waves, as Gaeldom described it in the old days) and has but little cloud-stopping or cloud-gathering power. The soil is mostly of shell sand or loam, so its moisture drains or evaporates quickly. It was not without good reason that in past days Tiree was known as the granary of the Isles. Only the south coast of England equals or exceeds Tiree’s record. Grain needs sunshine to ripen it and fill it, but apart from that agricultural fact the plant ecologists say it is hard to demonstrate the precise effect of differing amounts of sunshine on vegetation. It remains probable, nevertheless, that actual lack of sunshine or a very low summer figure would inhibit the growth of such plants as broom and harebells, and encourage others such as the bryophytes (mosses and liverworts).

Tradition has it that the climate of the Highlands and Islands has deteriorated in living memory and in the fifty to seventy years before that. The meteorologists are always telling us we are wrong in thinking the weather was better in “our young days.” All the same, in the North-West Highlands salt pans were in general use many years ago for evaporating sea water, but it is said the fall in the amount of sunshine—much more than the remission of the salt tax—was responsible for their use being discontinued.

It will be best for us to consider the climatic factor of snow when we come to note its effect in the higher mountainous regions. But let it be said that the Highlands as a whole do not suffer nearly so much snow as the Southern Uplands of Scotland or the Pennine Chain of England. Snow comes earlier and stays later on the tops of the hills because the factor of altitude is concerned, but the Atlantic mildness pervades much of the lower ground. Nearness to the sea is a considerable factor in determining how low the snow will come on a hill face and how long it will stay. This is brought home to anyone living offshore from the West Highland mainland and who has for a view a wide range of peaks stretching from three to twenty miles inland. Snow as a climatic factor can be of much importance in a region long after it has fallen if the catchment area of the snow is large. The Cairngorm hills provide an excellent example: their greatest accumulation of snow is at the end of April or even in early May, whereafter there is a steady melting which is not complete until August. It is in the dry month of June that the snow held on the spacious tops and plateaux of this region can maintain the water even in rapid-running rivers and affect the fish life down in the Spey and Dee Valleys. Obviously, snow has a great effect on plant and animal life in areas where it lies long, but we are here dealing with the general climate of the Highlands and must not be drawn away into discussion of local and micro-climates.

The shore line of the West Highland mainland is particularly free from snow. I have known years when the snow has not lain for more than a couple of hours in a whole winter—and then, like as not, it has been at the end of April or in the first week of May. That first week or so of May is regularly a wintry period, and is so well known in the North that it is called the Gab o’ May. Taken all in all, it is remarkable how little is the effect of snow on vegetation in the Highlands except on the summits and in those places where it drifts and packs. Its influence on the behaviour of animals may be profound, but of that more later.

Frost has a distribution in the Highlands somewhat like that of snow, except for the peculiar conditions which will produce spring or autumn frosts on the floor of a glen and not at a few hundred feet up the hillsides. That is a phenomenon, of course, which is known all too well in the fruit-growing districts of England. The shore line of islands off the western seaboard may, as has been said, register no more than 2° F. of frost all winter, but occasional bad years such as the early 1940’s may show up to 14° F. of frost. I remember seeing very thin pancake ice forming on the flat-calm sea of the Anchorage of Tanera in January 1941. Tanera is well out to the Minch and the sea is of fairly high salinity there.

Wind is a climatic factor of great variability and of exceedingly great importance in Highland natural history and the topography of the area. It would be entirely wrong to call the Highlands windswept; it cannot even be said truly that the coastal areas are windswept, for here on the shores of Loch Sunart where I am writing, the trees round the house are of beautiful symmetry and of great height, yet only 150 yards away on a shingle beach the wind blows much harder. The summits of the hills are windswept and the outer coasts are still more windswept, and it should be understood that the outer coasts of the North-West Highlands are the windiest part of Great Britain; much windier, for example, than the Shetland Isles or the outer Norwegian coast, or Valencia Island off south-west Ireland. The gales above 4,000 feet are worse than on the coast; the meteorological data from the Ben Nevis Observatory period gave an average of 261 gales a year of more than 50 m.p.h.

On the outer coasts of the Highlands gusts of 100 m.p.h. occur from time to time, and in certain places, where the configuration of the hills governs the play of wind, there are freak gusts and up and down draughts of excessive strength. In December 1938, I was going over the hill of North Rona during a three-day southerly gale to fetch water from the well on the southern cliff face. The wind was not so bad as it had been in the night, but I had to go on hands and knees over the ridge at 300 feet, from which there was an unbroken downward sweep to the sea on the south. When I reached the edge of the 70-foot cliffs which were at an angle of 30° from the vertical, I saw the turf at the edge of the cliff being lifted like the edge of a blanket, and the outer fringes of it were being torn off and flung inland just as a blanket would wear in a wind. All this is common enough in the islands; I had seen it before. But a few yards inland I saw two bare patches in the turf where two boulders had rested for years; the black surface of the bare patch was a good inch below the turf, and the boulders themselves—a foot to eighteen inches across and about eight inches high—were rolled uphill a distance of about three feet. The seals might well have shifted them had they been there, but no seals wandered in that part of the island. Only the force of the wind could have moved those stones, and as I still cannot believe that any wind we know could turn up the dead weight of a boulder well set with a flat bottom in the turf, presumably the cliff being set at that angle had the effect of multiplying the force of the wind at the upper edge; and a fairly large area of turf must have been lifted and stretched in some of the gusts, with the result that the boulders would be thrown out of their sockets uphill. The effect of wind is much less farther inland from the sea. The tree line on the coasts may be no more than 200 feet—assuming that trees will grow at all—whereas it is 1,800 feet on the western side of the Cairngorms. The prevailing wind in the Highlands is from the south-west. Such winds come off the relatively warm waters of the North Atlantic Drift and are laden with moisture. The weather is rarely cold during the time they blow. If the observer is far enough out from the high hills to see what is really happening in the sky, he will note that the south-westerly gales are predictable from the movements of the clouds before the wind is felt at sea level, or he may see a great bank of cloud out to the west in the Atlantic the night before. The south-westerly gales are gusty even out on the coasts, but they are so moisture-laden that one’s sense of smell is heightened: earth and sea have a beauty of their own at such a time through the scents they convey. The observer is watching the sky for signs of the end of the gale and sees a break of blue sky for a moment; then he notices that the clouds are no longer moving from the south-west but from the west. Soon he feels the wind to be coming from the west at sea level and the clouds are moving from north-west. Finally the wind veers farther to north-west and falls light in the north. That is the end of the gale.

The trough of low barometric pressure is left behind and the recording needle marks a steady rise and then levels off as the wind reaches the north. These gales have a closely similar pattern: in winter they may last several days; or only twenty hours in summer, sometimes completing the pattern day after day, beginning in the early morning with short gusts which are the forerunners and falling light in the late evening. It is the West Highland coast which shows up these gales as if under a magnifying glass. They will be mere breezes a few miles inland or at the head of a sea loch. Only their raininess will be felt there. The barograph shows less pronounced movement also, back in from the coast.

It is obvious that the effect of wind on the outer coasts is very great, for there is not only the period of great gales in the winter, when there are no leaves on the shrubs and no leafy vegetation at ground level, but there is the continual wearing of the summer period. I have seen leaves die from shaking through three days of blowing. Such trees as exist take on a distorted appearance, not one branch or twig managing to survive on the windward side of the trunk. It is a remark commonly heard that a tree has been bent by the constant action of the wind, but this is not true. Distortion is brought about by the continual lack of survival of all growth on one side. The distal ends of twigs are killed and the tree develops more and more a fuzziness of short annual shoots from the main stem. The influence of wind on the coastal region is further complicated by the spray which it may carry, for most broad-leaved plants object to a deposition of salt on their foliage. This is a subject we shall touch on later in the book.

North winds are relatively uncommon in the Highlands, but are recognized as bringers of snow in winter, snow which sets up its own train of events in natural history. A north wind in June or July means the best of sunny weather, but in August the north wind brings rain. East winds blow most regularly in spring, but gales from the south-east occur as well in the West Highlands. They are very cold for the district, as the south wind can be as well, for the air has come over a mountainous region where it must become chilled. The southeasters are dry winds and have a desiccating effect on the autumn herbage, sufficient to curtail the grazing season in some years. The south winds of summer mean a leaden sky and rain.

To conclude, the climate of the Scottish Highlands and Islands is rapidly changeable and far from uniform. The coastal climate is maritime or oceanic, but in the Central and Eastern Highlands it is more continental—more extreme temperatures, less wind, less rain and drier air. The meteorological tendency to drier air in the central region and the Dee Valley is further added to by the capacity of the ground to drain rapidly.