CHAPTER 11
Habitat
HABITAT IS THE RANGE of environments in which a species occurs.1 To understand current habitats, it helps to consider how they evolved over time. Past habitats of British grouse developed as a result of geological processes, climate, landforms, soils and human impact. In this chapter we discuss how these factors affect today’s habitats and grouse abundance. We emphasise Britain and Ireland, but the principles apply elsewhere. Habitats of grouse abroad are mentioned in Chapters 3-6, and many publications describe and illustrate them.2
British and Irish grouse live in an unusual variety of landscape for such a small part of the world.3 Red grouse have their home on moorland, a semi-natural habitat that is mostly human-induced, while ptarmigan live on high Scottish hills, southern extensions of the subarctic wilderness. Black grouse thrive on low moorland with rushy bogs or scrub, and in open birchwood or pinewood. The capercaillie is the ‘Great Cock of the Wood’, the crown of Scotland’s magnificent old Caledonian pinewood.
GEOLOGICAL PROCESSES
Bedrock
Over 1,000 million years ago, Scotland and the northern half of Ireland lay on the southeast margin of a continental plate that geologists call Laurentia.4 To the southeast lay two separate continental plates, Baltica and Avalonia, the latter including England.5 All three later collided to fuse, involving intense mountain-building that began over 400 million years ago. The roots of these mountains comprised coarse-grained igneous granite, formed by uprising, cooling and hardening of molten rock deep in the earth. The mountains became the Caledonian range, stretching from eastern USA and Canada through Greenland to Scandinavia, Svalbard and Scotland.6 Later, Europe and America separated by continental drift. Millions of years of erosion removed rock many kilometres thick, and most of our granite hills today are the worn remains of these huge mountains, formerly of Himalayan proportions.
The sedimentary rocks found across Britain and Ireland were derived from sediments settling in water, such as sandstone from soil sediments and limestone from animal skeletons. During the formation of the Caledonian mountains, intense heat or pressure metamorphosed some of the sedimentary rocks into harder schist. Where surface rocks fractured, molten rock (lava) spurted from volcanoes, and as it flowed down it cooled quickly to form fine-grained igneous rock.7
Grouse land in Britain and Ireland overlies a great variety of bedrock, such as hard quartzite or granite beside soft limestone, or cliffs with white quartzite above brown sandstone (Table 23). This leads to unusually varied landscape, soil, flora and fauna.
TABLE 23. The main and subsidiary bedrock on British and Irish grouse land.
| REGION | MAIN | SUBSIDIARY |
|---|---|---|
| Highlands | Micaceous sandstones, siliceous and feldspathic sandstones, granite, quartzite | Mica-schist, dark or graphitic schist, limestone, basalt, hornblende-schist, diorite, gabbro |
| Southern Uplands | Shale, greywacke | Granite, andesite, basalt, limestone |
| Northern England | Limestone, millstone grit | Andesite and other volcanic rocks in the Lake District, granite on Dartmoor, siliceous sandstone on Exmoor |
| Wales | Shale | Slate |
| Northern Ireland | Basalt and other volcanic rock, mica-schist | Granite in the Mourne Mountains |
| Republic of Ireland | Limestone | Granite in Wicklow, mica-schist, siliceous sandstone |
Effects on grouse
Geological processes left most ground in Britain and Ireland as lowland plains or valleys, productive for grouse. With the advent of humans in a temperate climate, however, these lowlands became ideal for cultivation, so that suitable habitats for grouse became unsuitable fields and settlements. Since then, grouse have used the remaining land, rejected for cultivation because it was too steep, too rocky, too wet or at high altitude.
Bedrock affects soil chemistry, which in turn affects the chemical composition of food plants. Heather and blaeberry over base-rich bedrock have a higher content of the vital elements nitrogen and phosphorus than over acidic granite, and red grouse, ptarmigan and mountain hares are more abundant there.8
Most acidic rocks are hard and coarse-grained, contain much silica, and weather into poor soils. Base-rich rocks are softer and finer-grained, with a higher content of basic substances such as lime, potash and magnesia. Coarse-grained rocks weather into loose barren soils, whereas fine-grained rocks break down into cohesive fertile soils. As a result, a hill over base-rich bedrock tends to have less bare ground than a hill of the same altitude over acidic bedrock, and often the type of vegetation growing there differs strikingly. Assessing bedrock can therefore be a guide to habitat quality (Table 24).9
TABLE 24. Assessing the richness of bedrock for soil fertility and vegetation.
| RICH | Black or graphitic schists; carbonate-bearing forms of sandstone, shale and igneous rocks; chlorite and calc-schists; limestone; marble |
| INTERMEDIATE | Andesite; basalt; diorite; dolerite; gabbro; green beds; hornblende-schist;# mica-schist; shale; slate |
| POOR | Granite; quartzite; greywacke; micaceous and siliceous sandstones and metasandstones; quartzofeldspathic gneiss; serpentinite and other ultrabasic rocks* |
#This covers ‘epidiorite’, a term now disused by geologists.
*These are so basic as to be toxic, with poor soils and mainly grassy vegetation.
Notes
1. The above is a general classification. At any one locality the rock may be deeply weathered or fault-shattered, with a secondary deposition of carbonate, which increases base-richness. We should also consider topography, climate, altitude, and the rock’s hardness, grain size and solubility. For example, some limestone is so soluble that hilltops are covered with infertile flint-gravel, because the chalk that formerly embedded the rock has been washed away along with the fertile soils, which are now confined to valleys and lower slopes. Old Red Sandstone is poor and weathers slowly. After glaciers grind it down, however, its fine grain aids the development of high-quality soils at low altitude.
2. The Moine rocks of the west and northern Highlands, and the rocks of the Grampian Group in the central Highlands are predominantly composed of poor-quality rocks. This contrasts with the Dalradian rocks of the southern and northeast Highlands, where rich and intermediate rocks are more common.
3. The table can apply also to Alpine land and woodland. Except in southern England and the highest Alpine land, a caveat is that glacial deposits may have been derived from different distant bedrock (loose rocks on the surface are useful clues to this). Also, deposits that have developed on site – such as acidic or fen peat, alluvium from streams, or wind-blown sand – can locally override the influence of underlying bedrock.
FIG 124. Effect of bedrock on vegetation in Glen Derry. The grassier part of the slope on the left has fairly rich diorite below, while the heathery area on the right overlies infertile flaggy metamorphosed sandstone. (Adam Watson)
CLIMATE
Before and during the last ice age
Much of Britain once lay near the Equator, where tropical forest decomposed to form coal. During hot periods, chemical action rotted some hard rock such as granite, leading to better soils than is usual for an acidic rock. Wind, rain and streams wore down the hills to form valleys and plains.
For many millions of years, periodic glaciations were separated by warmer interglacial periods in one of which we now live. During the most recent major glaciation 18,000 years ago (Table 25), ice up to 1,500m thick covered Scotland, and was joined to Scandinavian ice that deposited Norwegian rocks on the Scottish coast. Ice covered Ireland, extending to the Bristol Channel and almost as far as Oxford and London. The sea lay 100m lower than now, and land joined England to the Continent and Britain to Ireland, until rises in sea-level overran these bridges.10
TABLE 25. Dates of some events that help explain habitats today.
Glaciers wear down mountains and cover land under the ice with ground-up rock called till or boulder clay, which contains much silt and clay, and is compacted by the great weight of ice. At the sides and in front, glaciers push up moraines, which are less compacted and hence more freely draining. Huge rivers flowing from the melting ice wash most silt and clay out of any till or moraines in their path, producing fluvio-glacial deposits of very freely drained sand and gravel, with rounded, water-worn stones.
A minor glaciation following the last ice age
After much of the ice melted 14,500 years ago (see Table 25, line 2), woolly mammoths lived in Scotland. Then, 13,000 years ago, the climate warmed rapidly, resembling that of recent millennia. Pollen deposited in peat and in lake beds shows that grass abounded, with juniper and willow colonising, followed by birch.11 The climate subsequently cooled again and birch vanished, to be replaced by grass, heath with much crowberry, and sedges on wet ground. A minor glaciation ensued 12,000 years ago, as an ice-sheet centred on Rannoch Moor covered much of the west Highlands, and corrie glaciers and small ice-caps topped other hills in the Highlands, the Lake District, Snowdonia and Ireland. Lowland Britain and Ireland supported crowberry heath, including Arctic species such as least willow. Bones of collared lemmings from this period have been found at Edinburgh, extinct giant elk in Scotland,12 wolverines in England south to Devon, and an Arctic marine fauna in the Firth of Clyde.13
After the last minor glaciation
The minor glaciation ended as warmth arrived abruptly 11,500 years ago (see Table 25, line 7). Juniper scrub spread widely, to be replaced by birch and hazel almost everywhere but the high hills. Pine colonised most of England, although it was generally replaced later by oak, elm and lime on fertile lowland soils. On English
FIG 126. Rounded hills on Baffin Island, of a similar altitude to high British hills. A glacier with moraines is on the right, and ice-caps behind. This is what our hills would have looked like during the last glaciation about 12,000 years ago. (Adam Watson)
and Welsh uplands there grew birch, hazel and oak, with Scots pine on poorer ground. Pine covered much of Ireland to the Atlantic coast, and expanded in Galloway and most of the mainland Highlands during the period of warmest summers.
At the forest maximum 8,000 years ago, moorland dominated by crowberry occurred in a narrow belt above the uppermost woodland, and on exposed coasts, bogs, and forest clearings after lightning fires. The uppermost limit of woodland (the timberline) had moved uphill,14 so Alpine land occupied a far smaller area than now.
At this time much more broadleaved woodland grew than pinewood. An oak-elm-lime mixture dominated southern England, oak-elm-hazel in northern England and much of Ireland and central Scotland, and birch-hazel-oak on east Scottish lowlands and west Scotland. Birch-hazel scrub grew on the north coast of Sutherland and Caithness, and in the Western and Northern isles. Alder and willow abounded on riversides and other wet ground, and aspen with holly and, locally, yew grew on better soils. Shallow lakes were bigger and more numerous than now, and bogs covered much lowland.
Richard Tipping mapped Scotland’s ancient woodlands using radiocarbon dating of pollen. This method cannot distinguish woods from scattered trees, though ‘there would have been very few places from which no trees could be seen’.15 Such maps may give a ‘false impression of uniformity’, but ‘Small-scale climatic,
FIG 127. Ancient pine roots, embedded in thick peat, exposed by erosion that has cut a deep hag in the heather moorland. The piles of dead purple moor grass, left by a spate of rainwater, indicate continued erosion. (Stuart Rae)
geological, and topographic contrasts over short distances would have introduced an astonishing beauty, richness and diversity of woods within individual valleys.’
With the advent of a wet climate, peat spread over large tracts of Britain and Ireland, especially in the rainy west. Pine retracted from Galloway and Rannoch Moor, heather expanded and the timberlines dropped lower. By 4,000 years ago, pine had gone from many western regions but remained in the drier east.
CLIMATE, HUMANS, DEFORESTATION, INFERTILITY AND PEATLAND
Frank Fraser Darling stated that man caused the deforestation and infertility of the Scottish Highlands from AD800.16 In fact, most deforestation occurred BC, but the question remains how much deforestation, infertility and peatland were caused by prehistoric man and how much by changes in climate. It is generally agreed that man caused most of the deforestation in the northeast Highlands. Some scientists disagree that humans deforested the west and thus caused its infertility and peatland,17 though others hold that humans played a part by enabling or accelerating peat formation.18 Interactions between human and climate are sometimes ignored, yet human and climatic influences are not necessarily exclusive.
For forest to be converted to peatland, dead plant litter must remain partly decomposed. This occurs in waterlogged soils that lack sufficient oxygen for decomposition to take place, which in turn are more likely to occur in a wet climate. Because processes that encourage evaporation of water – such as grazing or cultivation- discourage the creation of peat, the rate of peat formation can depend on the balance between climate-induced and human-induced changes in water balance. Trees lose much water to the atmosphere by evaporation and transpiration, replacing it from their roots and thus drying the soil. Because deforested heath has less foliage than trees, it draws up less water, soils become wetter, and water tables may rise, which should favour peat.
As discussed above, it has been hard to separate climatic from human influences affecting the development of peatland, but here we note two studies that separated them. In west Glen Affric, an area once forested but now dominated by treeless wet heath, thick peat expanded long before forest collapse.19 And near Hudson Bay in Canada, a region unaffected by human deforestation or agriculture, peat thickened and timberlines retreated south,20 during periods when peat also grew and forest declined in Britain and Ireland. The evidence points to a widespread oceanic climate in the northern hemisphere, expanding peat and extirpating woodland.
To conclude, human-induced deforestation is not necessary to turn forest to peatland, but in some parts of Britain and Ireland it may be sufficient and may have exacerbated climatic impacts. When there is a fine climatic balance between waterlogged and freely drained soil, tree removal may tip it towards the former and hence the creation of peat.
HOW GROUSE WOULD HAVE FARED DURING AND AFTER THE ICE AGE
The only large ice-free refuge in Britain and Ireland during the last main ice age 18,000 years ago lay south of the Bristol Channel and the Thames, then an Arctic tundra with willow and rock ptarmigan. Bones of willow ptarmigan have been found in Hungary, Monaco and Italy, far south of the species’ present range, and those of rock ptarmigan in southeast England, the Channel Islands, central Europe and Virginia, with more willow than rock ptarmigan in southern or lowland areas.21 After the ice melted, valleys and floodplains would have offered fertile soils and bogs for grouse. Southern England warmed, leading to the growth of scrub and then woodlands of birch and pine, suiting willow ptarmigan and then blackgame and capercaillie.
Hazel grouse probably lived in Britain after the last main ice age. They would have thrived in the woodlands, which contained birch, hazel, willow, alder and aspen, along with some pine, yew and holly. Also suitable would have been the last phase of broadleaved woodland that colonises after fire or wind-throw in pinewood, such as that found in Finland today, where there is a succession of willow ptarmigan, blackgame, hazel grouse and capercaillie, in that order.22
Blackgame would have favoured open woodland, scrub and moorland near trees. Their remains have been found on the north Caithness coast and on North Uist in the Outer Hebrides, dating from the Pictish period before the Norse colonisation, in places devoid of natural woodland or scrubland today.23 Capercaillie would have abounded in pinewood, and during summer in blaeberry-rich birchwood and oakwood. The large expansions and retractions of pine and other tree species (see Table 25, lines 9-14, 16, 19-23) would have ensured dynamic rises and falls in the numbers and range of the five grouse species.
LANDFORMS AND SOILS
Landforms result from the interaction of geological processes with climate, and include large-scale features such as valleys, down to small ones such as hillocks and hollows. They affect grouse by providing physical cover, and by influencing soils and vegetation.
Soil formation
Soils are formed by the interactions of parent material, comprising weathered rock that has not moved, and deposits moved to the locality by ice, water, gravity and wind; climate; soil organisms, plants and animals, including humans; land relief, such as mountain, floodplain, altitude, slope and aspect; and time.24 Scientists classify soils by the sequence, thickness and colour of their ‘horizons’, or layers, starting at the surface with litter from dead plants and working down the ‘profile’ (vertical section in a soil pit) to the parent material (Fig. 128).25 Freely drained, uncultivated moorland and woodland soils in Britain and Ireland generally show several distinct horizons (see Fig. 129), whereas a single thick layer of peat (see Fig. 130) accumulates on very poorly drained ground. Drainage is influenced by gradient, topography (for example, a basin drains differently from a hilltop of the same gradient) and the nature of the soil material. It ranges from excessive (as in gravel), through free, imperfect, and poor, to very poor (as in peat).
P living plants
L litter (dead plants) from previous growth
F fermenting litter being rotted by decomposer organisms, with some of the original plant structures obvious to the naked eye
H ‘humus’ layer of well-decomposed organic matter with few or no recognisable remains. (In wetter soils there is a dark, peaty O horizon below H, not shown here – see Fig. 130. It comprises organic matter that accumulates owing to little decomposition in wet anaerobic conditions)
A generally ‘eluvial’ (washed out; so called because material has been moved from A to B by water percolating downwards), with an upper part that is typically darker than the lower, stained by dark matter washed down from H
B generally ‘illuvial’ (washed in), including clay and silt washed down from A
C unconsolidated mineral material, little altered by soil-forming processes, and consisting of weathered rock particles above bedrock or deposits, the ‘parent material’ of the soil
FIG 128. Different horizons, or layers, in a soil pit. In this diagrammatic podzol, horizons L-H are organic (derived from plant and animal remains), A-B are mainly mineral (derived from rock), C is all mineral, and A, B and C denote increasing depth. (Drawn by Dave Pullan)
1. Organic soil, or topsoil (F-A), lies above paler mineral soil, or ‘subsoil’ (B-C). Cultivation disrupts L-A and sometimes B.
2. Uncultivated podzols have very acidic surface layers that lack a crumb structure. The humus found here is often called ‘mor’ or ‘raw humus’, and is home to few or no earthworms. Horizon A has an upper dark layer with some humus, above a thicker ash-grey layer with little humus.
Removal of clay, silt and nutrients from horizon A results in a coarser, less cohesive, infertile topsoil. In the lower part of horizon A, sand grains become bleached, hence its ash-grey appearance, and iron compounds are precipitated as yellow-brown oxides within the upper layer of horizon B.
In iron humus podzols, humus from horizons H-A darkens the upper layer of horizon B, forming a Bh horizon. Iron humus and peaty podzols can have a dark reddish-brown iron pan, 1-2mm thick, below the Bh horizon. A bright yellowish-brown layer lies below the Bh horizon of iron humus podzols, and under the A horizon of other podzols.
3. Peat has thick O horizons. Organisms are scarce in acidic, wet, anaerobic, cold conditions, so hill peat is very infertile. Peat has been defined as soil with > 30cm thickness of peaty layers, but can be many metres thick. ‘Fen peat’ is derived from plants such as grass above base-rich parent material, as opposed to heath above acidic parent material. It can be fertile when drained, as in the Cambridgeshire fens.
4. In brown earths the A horizon contains mineral material intimately mixed with ‘mull’ humus (from a Danish word for mould). This A horizon has a well-developed crumb structure and is home to many earthworms and other decomposer organisms, and organic surface layers are thin or absent because litter decomposes quickly. The A horizon is dark brown, the upper B horizon brown and the lower B horizon a paler brown, the layers grading into one another.
FIG 129. The soil profile of an iron humus podzol on heather moorland. Under the heather (P) is a thin layer of litter (L) of the same brown colour as the heather stems, and obvious only at the highest point of the pit. Below this lies a thicker, darker brown, fermentation layer (F). Then comes a band of very dark grey (almost black) humus (H), containing tiny mineral grains, which are a sign of completely humified material and hence not peat. The bottom of the humus grades into a thin uppermost blackish layer of mineral soil coloured by leached humus (upper A), grading in turn into a very thick and much paler ashy grey (almost pale grey) layer of leached mineral soil with less humus (lower A). Below this is a thin, very dark brown band, slightly darkened by humic deposits in the uppermost subsoil (upper B), a thick yellowish-brown layer of leached subsoil (B), and even thicker layers of reddish-brown subsoil (lower B). The C horizon (below B) is not shown. (Gordon Miller)
The fertility of most soils depends largely on the minerals they contain, which are particles usually derived from local bedrock. A common exception occurs after ice grinds bedrock in one area and deposits it above a different rock elsewhere, whereupon the acidity and nutrients in the mineral soil reflect the deposits. Also, water flowing across underlying rock and issuing at the surface has a chemical content that reflects the bedrock.
FIG 131. Molehill on a grassy flush that has been enriched by nutrients from groundwater, set on a heathery slope. (Stuart Rae)
These influences profoundly affect vegetation. On infertile moorland, including that above thick peat, flushes are conspicuous from their abundant rushes, grasses and flowering herbs, indicating soils locally enriched by waterborne nutrients. Bog myrtle, which fixes atmospheric nitrogen, adds further fertility in many moorland flushes. At Alpine flushes, bright green moss and herbs signify fertile conditions, due to continuous irrigation by nutrient-rich water.
Natural soil development and human modification
By ‘soil development’, scientists mean the changes that result in a mature soil in a steady state, with a full set of horizons. The natural processes involved in soil development are weathering, leaching, podzolisation, gleying, and peat formation (Table 26; see also Fig. 128).
TABLE 26. Processes of natural soil development in temperate regions.
# Podzolisation occurs on freely drained soil derived from acidic parent material, in climates where precipitation exceeds the plants’ water loss via evaporation and transpiration. Very freely drained sand or gravel leads to iron podzols, abundant in the boreal forest of Scandinavia and northern Russia, but in Britain infrequent because of the oceanic climate. There are British examples, however, such as in northeast Scotland. A bigger excess of precipitation, or poorer drainage, induces in turn iron humus podzols, peaty podzols, gleyed soils and thick peat.
* Waterlogging also occurs in peaty podzols after wet weather, where an impermeable layer (an iron pan) bars water from percolating further down. This creates a perched water table, with freely drained soil underneath the pan down to the general groundwater table at greater depth. Where the parent material (C horizon) comprises glacial till or moraine, there is often a thick cement-like induration that also bars downward percolation. In receiving sites such as hollows where water gathers, gley soils are then common.
** Litter decomposition can be fast in dry conditions but slow in wet ones, and fast with base-rich litter such as from ash trees and grass, but slow with acidic litter such as from heather or pine. Wet conditions arise from a large excess of precipitation over evapotranspiration, due to heavy precipitation or to poor drainage under low precipitation. Because wet conditions often impede downward leaching, wet mineral soils can be more fertile than podzols. On the other hand, organic matter, especially litter that is acidic, decomposes slowly in wet conditions, often favouring peat formation. Plants on thick hill peat depend for most of their nutrients on deposits in precipitation. Locally, however, groundwater flowing over bedrock or through subsoil comes to the surface, and spreads nutrients downhill in flushes that enrich both peat and plants.
Much of northern Scotland falls within the world zone of boreal coniferous forest, while most of southern England and much lowland elsewhere in Britain and Ireland lies in the next zone to the south – deciduous forest. Scientists generalise that climate largely determines these zones, though soils and vegetation may be locally influenced more by relief (as in bogs) or by parent material (as with limestone).
Some soils have poorly developed horizons because of their youth, or because some factor has prevented the stability needed for horizon development, such as soil or sediments deposited by streams (alluvium) or resulting from movement on slopes (colluvium).26 Scientists generally agree that natural soil development is very slow – at least 1,000 years is the suggested time for a temperate soil to reach a steady state.27 A scientist would need Methuselah’s 969 years to solve this problem decisively by studying the same sites for centuries, for when undisturbed natural soils are checked over several decades they usually show no material change.
The skeletal soils occurring after glaciation tend to be more fertile than many of the soils that they develop into subsequently. Soils in the first few millennia after the last ice age would have been generally fertile, not yet impoverished by leaching. Since 7,300 years ago, however, there have been several periods of a cool, wet climate, which would have facilitated leaching. John Birks concluded that acidification since the ice age has been natural, with widespread development of infertile soils ‘as a result of the acidic nature of much of the local bedrock and till, coupled with intensive leaching under a cool, moist climate’.28 There are local exceptions, such as fertile soils on base-rich bedrock, and on alluvium, colluvium, flushes, and ground altered by humans.
Humans change many soils, often strongly modifying the effects of natural factors.29 Cultivation for farming disrupts upper horizons, so that formerly infertile podzols come to resemble relatively fertile brown soils and have been classified thus in soil surveys.30 Drains and modern ploughs shatter indurated layers,31 thus lowering groundwater tables and increasing nutrient uptake, but also causing soil erosion. Artificial fertilisers and pollutants provide extra nutrients.
Soil type and vegetation
Although many differences in soils arise because sites differ physically, soil scientists agree that certain vegetation can affect soil type. For example, pine and heather produce acidic litter that usually decomposes slowly, whereas ash trees produce base-rich litter that decomposes rapidly. The former tend to lead to podzols, and the latter to brown earths. However, effects of vegetation are sometimes overstated, for instance the suggestion that birch encroaching on heather turns podzols into brown earth within several decades.32 To test this, birches were planted in Yorkshire heather, but after three decades the soil ‘remained a fully differentiated podzol’.33
Plants often favour certain soils and some species are good indicators of soil fertility. For example, podzols are more acidic than brown earth, and heather or pine predominate there, while herb-rich grass or ash trees prevail on brown earth. This happens usually because dominant plants outcompete others, not because the latter cannot grow there. Heather, blaeberry and pine can all grow on fertile natural soils and on fertile cultivated soil.
To conclude, mature podzols with well-developed horizons are less fertile than young soils from the same parent material, but brown earth and cultivated podzols are more fertile. Other things being equal, fertile soils support vegetation that tends to have more plant species, and to be more nutritious, than infertile soils nearby.
PREHISTORIC HUMAN IMPACTS ON SOIL
Concentrations of hazelnut shells show that hunter-gatherers lived in the mild climate of the island of Rum 9,000 years ago. In South Uist, pollen of birch and hazel declined 8,000 years ago as pollen of grass and heather increased, signifying human clearance of woodland and expansion of pasture. On lake floors in lowland Scotland, increases of grass and sedge pollen coincided with the occurrence of charcoal 2,000-5,000 years ago, signifying human-induced deforestation along with fire and grazing.34 All heather moorland lies below the current potential natural timberline. Humans have destroyed all natural timberlines in Britain and Ireland, save for fragmentary relics in the Cairngorms (see Fig. 132). Timberlines – which are the highest altitude at which natural woodland can grow – are evident in many countries with less overgrazing and burning, such as Canada.
Cultivation of cereals spread to Britain from the Continent, the first signs in Scotland appearing 5,800 years ago in the mild climate of lowland Arran and Kintyre. Large-scale impacts became evident 5,000-4,000 years ago on great tracts of lowland across Britain and Ireland, as human deforestation produced moorland pasture for farm animals or fields for cultivation. In an example 5,000 years ago, birch-hazel scrub declined on Orkney’s exposed coast, as humans and farm animals turned an understorey of tall herbs and ferns into pasture within 200 years.35 Human deforestation of upland began later but proceeded on a massive scale, along with the use of fire and introduction of farm animals, leading to the replacement of woodland by moorland.36
Farmers cleared most forest in Britain and Ireland, but this was not the case in northern Europe. Our gentler relief and longer growing seasons favoured
FIG 132. Relic timberline of Scots pine at Creag Fhiaclach, Glen Feshie, reaching almost half way up the slope. Juniper scrub extends above this limit. Willow and birch scrub has been extirpated by browsing. (David Duncan)
cultivation,37 so farmland now covers 77 and 68 per cent of Britain and Ireland, respectively, but only 3 per cent of Norway. Agriculture on deforested land maintained more people (estimated at over 300 times more) than hunting in forest.38 Furthermore, the amount of pasture available after deforestation greatly exceeds that on the shaded, drier soils under mature woodland, so it supports more farm stock. Because of our mild winters, stock can stay outdoors and need less supplementary feed from summer crops.39 Hence, stock density in Britain and Ireland exceeds that in countries with cold winters, and effects on grouse habitats are more severe.
Cultivation on what is now moorland or woodland
Cultivation began to affect much British and Irish upland from 5,000 years ago, continuing there until early in the first millennium BC. In Shetland and Mayo, for example, recent removal of peat for fuel revealed prehistoric field systems on mineral soil under the peat. These dated from 4,800 and 4,500-3,200 years ago, respectively, the Mayo ones starting after forest clearance and ceasing as peat developed.40 On Jura, pollen of cultivation weeds dating back 4,000-3,000 years has been found beside settlement remains, on land that had been broadleaved woodland and is now wet heath above thick peat.41
On the many eastern moors and woods that remained peat-free, archaeologists readily found stone-clearance heaps (Fig. 133) and enclosures from prehistoric farming. If you visit a site marked on a map as ‘field systems’ or ‘settlements’, you will see moorland or woodland, often treasured for its supposed wildness. However, 3,000 years ago the same scene would have been a hive of industry: many people summered here, living in circular huts (see Fig. 134) roofed by poles and thatch, growing crops, and tending cattle, sheep, goats and pigs.42
Later, from around 3,000 years ago (see Table 25, line 25), a wetter, windy climate with cool summers lasted for centuries, ending most cultivation of moorland before the birth of Christ. In a later warm period, a smaller expansion of cultivation peaked in the 1100s. Evidence of this can be seen in the plough ridges, enclosures and settlements on much moorland such as the Lammermuirs, well above the upper limit of fields since 1850.43 These areas were subsequently abandoned in the cold, wet centuries of the Little Ice Age up to 1750, as moorland took over again.
After cultivated fields revert to woodland, the resultant vegetation differs from that in primary forest. These differences relate to changes in soil properties brought about by agriculture, which persist for at least three centuries and are perhaps almost irreversible.44 In northern France, deforestation to make way for fields during the Roman occupation in AD50-250 was followed by abandonment
FIG 133. Stone clearance: boulders cleared from land in Glen Esk that was used for cultivation by prehistoric farmers more than 2,000 years ago. (Adam Watson)
FIG 134. Prehistoric hut circle, measuring 9m across, on heather moorland, Deeside. The stone circle, now overgrown by heather, was once the foundation of a hut made of wooden stakes (wattles), comprising a circular wall 1.8-2m high and a roof apex reaching 6m high.42 The abandoned grass fields at lower altitude in the background were farmed in the 1800s. (Adam Watson)
and reversion to forest, yet the altered soil fertility and vegetation persist today.45 Similar cases in other continents are becoming more widely known.46 Cultivation also favours soil organisms that decompose plant litter, thus reducing the likelihood that an infertile acidic peat will form.47
We infer from this that ancient cultivation has boosted soil fertility and the nutritive value of food plants on much modern moorland and woodland, and thus increased the abundance of red grouse, blackgame and capercaillie today and in the recent past.
HABITATS OF RED GROUSE
Before deforestation
The former natural vegetation on most land that is now moorland would have been forest, with some open woodland and scrub, and bogs too wet for woodland to develop. It would have reverted to forest after lightning fires or wind-throw.
To judge from the more natural conditions found in other parts of the species’ range, the main habitats of Lagopus lagopus before deforestation in Britain and Ireland would have occurred on a belt of heath48 between the timberline and the Alpine zone. In addition, they would have been found on exposed coasts, forest bogs, and in forest after fire or wind-throw kept the land open for several years until regenerating new tree canopies closed overhead. Today, densities of red grouse increase in plantations established on moorland49 and in naturally regenerating woodland, until canopy closure. After deforestation, their range and abundance would have soared at the expense of woodland grouse, especially capercaillie.
Types of moorland
Heather dominates most moorland, hence the term heather moorland. Heather moorland with much blaeberry occurs almost entirely in the zone of former forest. That forest would have had a blaeberry-rich understorey, holding a smaller proportion of heather than on most of the heather moorland that replaced it.50 Heather with blaeberry is the main vegetation in the north Pennines and high ground in North Yorkshire.51 In Wales, it shares dominance with a blaeberry-grass mixture that occurs more locally in Scotland and signifies less heather as a result of overgrazing.52
Peatland is moorland with thick peat, usually defined as at least 30cm of organic matter above the mineral soil (see Fig. 130). It forms the chief land type in the far north and northwest of Scotland and in northwest Ireland, as well as in substantial areas elsewhere in Britain and Ireland. One variety of peatland, called blanket bog because it blankets the landscape across slopes and flat ground, has very thick wet peat and vegetation with sparse heather. Another, called raised bog, develops as a dome on lowland basins, its heather-dominated vegetation signifying a drier climate. Peatland differs from heather moorland because it receives more water than can drain away.53
The main vegetation on peatland is wet heath, dominated by sedge, rush and moss. On gentle slopes where the peat is over 1m thick, deergrass and cotton-grass predominate, whereas on the wettest flat bogs the cross-leaved heath and Sphagnum moss take over.54 Heather grows sparsely on peatland, with a greater abundance on slopes than on flat bog. One seldom sees blaeberry, but crowberry thrives on peaty hummocks and slopes.
Heavier rainfall accounts for the key regional difference between wet heath in the west and heather moorland in the east. Relief, however, imposes local variation, so that even in the far west we find heather moorland on steep slopes, hillocks and ridges. And in the east, despite its low rainfall, wet heath occurs on very poorly drained basins, and on gentle slopes below groundwater springs. Wet heath also covers much flat, thick peat in east Sutherland and Caithness, where it is locally called ‘flow’.55
How natural is moorland?
The moorland that replaced natural forest has what botanists call ‘semi-natural’ vegetation. Although this comprises the same species that grow under old pinewood, their relative abundance differs greatly,56 with far more heather. Hence deforestation increases heather, and muirburn tends to favour heather even more, because plants of this group withstand fire better than some other species. Because muirburn prevents natural succession to the climax woodland,57 heather moorland is a man-induced ‘fire climax’. Where burning ceases or declines greatly, trees and scrub soon colonise. One reason for muirburn is that young heather regenerating after fire provides a more nutritious bite for farm stock. Another unnatural feature of moorland since the late 1800s has been muirburn carried out in small patches to increase numbers of red grouse. Standards of muirburn on grouse moors have declined more recently, involving bigger burnt patches or fire rotations that are too short in some places and too long in others.58
Almost all moorland is burned more frequently than the occasional sporadic fire caused by lightning in ‘wilderness’ (land affected primarily by natural forces). Also, except in rare cases where they are fenced out, farm animals or red deer occur at higher densities than large wild mammals in wilderness. Foxes and crows thrive on human-induced extra carcasses of sheep and deer, and on road-kills, feed for farm animals, and waste in rubbish dumps. Now unnaturally common, they are unnaturally serious predators.
Declines in heather-dominated land, associated with overgrazing, began before 1750 in north Wales and Galloway.59 In 1850-1900 the declines spread in Wales, Scotland and Ireland, and they have continued ever since. Overgrazing favours grasses, sedges and rushes, because the growing point of these plants is at the very
bottom of the stem and stays undamaged if an animal eats the shoot tip. In contrast, the growing point on heath, trees and scrub is at the tip, the first piece to be eaten.
A striking feature on otherwise treeless wet heath is that trees and scrub abound on islets, large boulders, cliff ledges, stream gullies and ravines – in other words, places inaccessible to sheep and deer.60 This raises the idea that grazing may have caused treeless wet heath. However, other factors occur besides grazing. First, sites where trees grow beside wet heath suffer no muirburn. Also, tree establishment would be favoured in the inaccessible sites because soils there are freely draining, owing to rapid run-off over rock lying close to the surface or to steep slopes. Moreover, steep slopes in areas of wet heath have fertile colluvial soils, which are good for tree establishment. The nearby wet heath differs radically, because its poorly drained wet peat is hostile to tree establishment.61 Much wet heath would probably remain as such, therefore, even if its present unnaturally high levels of burning and grazing were to cease.
Sustainability of current use of moorland
Some believe that moorland degrades soil fertility, and that such deterioration began first in the west. Declines in bags of red grouse during the twentieth century usually came earlier in western regions of Scotland and Ireland, and declines in the land’s carrying capacity for sheep and in its ‘lambing percentage’ (number born per 100 ewes) also occurred there.62 It has been claimed that burning and overgrazing exhaust soil, that nutrients are lost as sheep and cattle carcasses are exported to lowland areas,63 and that nutrients in rain may be insufficient to replace these losses.64
This was studied at Moorhouse, a largely peaty grouse-moor in Cumbria, by finding whether nutrient inputs from precipitation matched outputs in streams.65 Inputs exceeded outputs, except possibly for phosphorus, a nutrient that is usually limiting for plant and animal nutrition on peatland, so any net loss of it could be critical.66 The estimated inputs of nutrients were probably too low, however, for they excluded the large amounts in fog, rime, and dry aerial deposition,67 in flushes from groundwater flowing across rock and through subsoil, and in the accretion of fresh soil from rock weathering. Also, sheep-ranching on Moorhouse did not reduce soil fertility in experiments lasting 11-31 years.68 On the other hand, muirburn on grouse-moors involves small, relatively low-temperature fires, whereas large, hot fires entail bigger losses of nutrients.69 Hence the frequently hot, large, short-rotation fires that typify western sheep-walks would entail greater nutrient losses. The question remains open.
Declines in lambing percentage and carrying capacity for sheep might be explained by climate change, as has been suggested for grouse bags (see Chapters 3 and 14), especially an increasingly wet climate in the west.70 Another explanation is vegetation change caused by overgrazing, resulting in less heather and more bracken, and on sheep-walks a spread of unpalatable grass, sedge and rush. Yet another might be a lack of gamekeepers or shepherds.
Another possibility for these declines is that soil erosion exceeds the formation of new soil. On eastern grouse-moors, where most fires are narrow and where sheep, cattle or deer occur usually at low density, we regard soil erosion as immaterial, with the important exception of peat hags. In western and northern parts of Scotland and Ireland, however, where shepherds burn wide fires every few years, sheep at high density have bared much ground and eroded the peat,71 and during severe downpours we see run-off that is heavily charged with peat and mineral soil. An eventual decline in grouse food on such areas seems likely.
To sum up, the area of heather moor in Britain and Ireland first increased as a result of deforestation, long before the Roman invasion, and in the last few centuries has decreased owing to overgrazing and other human factors such as tree-planting and reclamation to agricultural grassland (see Table 25, lines 32-33). Whether the remaining area now supports lower densities of grouse because of human-induced declines in fertility is moot. This is less likely on grouse-moors than on heavily burnt overgrazed sheep-walks above very poor soils, where much soil can be lost by erosion.
HABITATS OF ROCK PTARMIGAN
Internationally, the montane zone is the belt of coniferous woodland on mountains, below the subalpine zone’s stunted trees and the treeless Alpine zone higher up.72 The Alpine zone is home to Scottish ptarmigan. Here, glaciers during the last ice age gouged out corries and left ice-smoothed rock that was later shattered by intense frost to become boulder fields, and melt-water channels eroded the bedrock and boulders. These landforms affect ptarmigan abundance by providing cover, as well as habitats for blaeberry, a favoured food plant.
Alpine soils vary more than those on moorland or woodland, because of the greater variety of topography, climate and snow-lie. On fairly steep slopes, colluviation keeps soils fertile. Springs that shed groundwater have moss-dominated vegetation, which supports many insects eaten by chicks (Fig. 136), while the large expanses of freely drained soils favour the heath that comprises the adults’ main food plants. Also, churning by frost prevents the development of induration in the upper soil layers, thus maintaining free drainage. Heath dominates the dry, eastern Scottish hills, but it and ptarmigan also abound in the
FIG 136. Herb-rich subalpine flush, amongst short, wind-clipped heather, Cairnwell hills. (Stuart Rae)
wet west on ground where bedrock lies near the surface, providing rapid run-off and freely drained soil.
Largely prostrate vegetation creeps along the windswept ground, and exposed places have patches of bare, scoured soil. Plants grow slowly here, and much of the annual increment dies back from the effects of wind and weather, so the vegetation stays at virtually the same height over the years. Many moorland species grow in the Alpine zone, including abundant heather on the lower half of it, but the few muirburn fires that extend up from the moorland peter out quickly because there is too little fuel and much bare ground here. Heather-dominated vegetation extends to higher altitudes on exposed ridges, and luxuriant crowberry grows among stable large boulders. Blaeberry dominates colluvial slopes with small boulders, and also hollows that hold snow until late spring. Though scarce on boulder fields and among heather, ptarmigan abound on luxuriant crowberry and, especially, on blaeberry.
Snow can fall in any month in the Alpine zone, and in cold winters there may be no thaw for months, although thaws reach the highest summits occasionally in most winter months. Because wind almost always accompanies heavy snowfalls, exposed ridges remain largely snow-free, while blown snow gathers in hollows. In snow-free periods, gales blast surface grit and pebbles through the air, ripping out vegetation. Fog often prevails, and during frost the tiny fog droplets freeze into thick, hard crystals of rime on windward surfaces. Hoar frost from frozen water vapour sometimes covers all surfaces more loosely.
In shallow hollows, deep snow lasting until June prevents frost-churning of soil, the shelter reduces abrasion of plants by wind, and irrigation by melt-water induces more plant growth in dry weather. As a result, a thick topsoil develops here, with little bare ground. Mat grass dominates the vegetation, providing easy walking for people but unfavourable for ptarmigan. It contains hardly any heath plants for food, and a ptarmigan that stands on it appears conspicuous to predators because of the uniform background.
In deeper hollows, snow lies so long that it drastically shortens the growing season, so that plants are dotted sparsely on bare gravel. Soil below long-lying snow is more fertile than usual for that parent material, owing to extra inputs of nutrients from invertebrates stranded on the snow and from wind-blown plant debris and aerial dust. Droughts do not limit plant growth here, provided that some snow remains as a source for irrigation.
Snow delays the onset of plant growth. Near the edge of melting snow patches, plants start growth long after those on snow-free ground have passed this stage and become fibrous. Like a prolonged spring season, this changing band lasts for weeks, allowing plants that are newly freed from winter’s grip to start fresh, nutritious growth. Ptarmigan favour these places in mid- and late summer.
Although far more natural than moorland, the Alpine zone has been damaged by sheep and deer, resulting in less heath and more grass on many hills, especially the base-rich ones.73 Only high Scottish hills over infertile acidic rock, which are hence less attractive to sheep and deer, still have near-natural Alpine vegetation.
HABITATS OF BLACK GROUSE
The more natural conditions found in Scandinavia and Russia offer clues as to the original habitats of British blackgame. These would have been forest-steppe (probably the best habitat according to the literature), scrub regenerating after fire or wind-throw, open bogs that are too wet for trees, and scrub above the timberline and on exposed coasts. Blackgame favour the broadleaved scrub that regenerates first after fires in boreal forest. Willow ptarmigan in Sweden colonise ground soon after fires or clear-cutting, whereas black grouse come later, at the stage when scrub or young copses have grown.
Blackgame do well in ancient native pinewood with glades of open ground that are maintained by grazing sheep, deer, cattle or horses. Birds are found in such areas at all seasons, though in autumn they often leave to eat berries on nearby moorland, and in spring to crop the shoots of cotton-grass. Their numbers increase in coniferous woods planted on moorland,74 gradually declining as the tree canopy closes, although foresters in recent decades have reduced or eliminated the essential heath understorey and so discouraged blackgame.
In a few valleys, blackgame live mainly on moorland, although they visit nearby pines, birches and other trees to feed, especially in deep snow. They favour edges between moorland and farmland, especially where copses or scrub provide buds, catkins, leaves and berries as food. Much of the freely drained lower moorland beside modern fields carries the remains of field systems from prehistoric cultivation, which would have improved soils and the nutritive value of today’s vegetation. Nearby peat-bogs hold cotton-grass, whose shoots afford good food
in spring. Grazed fields provide good sites for leks, as well as early growing weeds for a nutritious bite in spring.
Greyhens select dense, mature heather or rushes for nest sites, far taller than vegetation used by nesting red grouse and very well hidden from above. Overgrazing by sheep, cattle or deer, or too much burning or scrub removal, eliminates this valuable cover.75 Hens with small chicks strongly favour groundwater flushes with tall rushes or grass, along with some short herbs. Here, the continuous flow of nutrient-rich water prevents development of the soil to a podzol, instead inducing a fertile young soil with abundant invertebrates for chicks.
HABITATS OF CAPERCAILLIE
Capercaillie would have occupied a large area when pinewood reached its maximum extent (see Table 25, line 15). Scots pine with some birch formerly covered much of Ireland and Scotland, especially in the west and centre, and in Scotland eastwards to lower Deeside. It also dominated parts of England and Wales that have infertile, freely drained soils. By the 1700s, almost all pinewood had gone in England, Wales and Ireland, and loggers clear-felled most of Scotland’s remnants. Prolific natural regeneration of ‘young firs’ (Scots pines) usually followed in Scotland, because deer were absent or scarce, and landowners planted many pines. These came too late for capercaillie, however, which need trees several decades old.
Pinewood with much blaeberry is the best habitat for this species (see Chapter 6). Several types of pinewood vegetation have been recognised in Norway and Britain, but the British classification is somewhat oversimplified.76 A fresh attempt, with grouse habitats and soils in mind, would be useful.
Most soils under ancient pinewood in Scotland are unsuitable for farm crops because of very freely drained glacial deposits, steep slopes, flushes, thick peat, boulders or surface bedrock. Two of the biggest such woods, Abernethy and Rothiemurchus, lie on outwash sands and gravels from the great Spey glacier.77 Although it has been asserted that the survival of Rothiemurchus wood testifies to its care by past generations,78 in reality it is an accident of the last ice age, which left infertile soils unsuitable for cultivation.
SUMMARY
The ways in which habitats have developed through geological processes, climate and human impact help to explain habitats today. Bedrock can have profound effects on natural soils, vegetation and grouse abundance. After the last ice age, warming led to the growth of Arctic vegetation and then to forest with scrub, before humans arrived. We discuss how grouse fared then, when rich habitats covered entire countries. Later, a wet climate caused deforestation and peat expansion in west Britain and Ireland, favouring red grouse at the expense of woodland grouse. Prehistoric farmers deforested most of the lowland, confining grouse to poor soils with moors or woods. Human deforestation on eastern upland created moorland pasture on poorly drained soil, and allowed cultivation of much freely drained soil. Most of the cultivated moorland was abandoned as the climate deteriorated, and became pasture. We infer that increased soil fertility from ancient cultivation still boosts the nutritive value of plants on much moorland and woodland today, and thus the local abundance of red grouse, blackgame and capercaillie. Freely drained moorland reverts to forest if humans stop burning and reduce overgrazing. Only acidic infertile Alpine land is nearly natural. Sheep have reduced heath on many base-rich hills, probably extirpating ptarmigan from south Scotland, England, Wales and Ireland.