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A History of Trees in Britain
from the end of the Ice-Age
to the Roman Occupation

Adrian D Middleton
University of Sheffield
Certificate in Plant Studies
'Of Trees, Woods and Hedges'
Spring 1996

No history stands in isolation, and the history of trees since the last 'Ice Age' - the 10,000 years known as the Holocene - must be placed in the context of the previous million years - the period known as the Pleistocene.

A breakdown of the Pleistocene is shown in Figure 1. The naming, sequence and dating of the sub-divisions is contentious and is based on fragmentary evidence from unrelated sites and conflicting dating techniques.

The period was one of climatic fluctuation with 'Ice Ages' in which parts of Britain were covered by glaciers; 'interglacial' periods when the climate was warmer and trees and other plants re-established themselves; and 'interstadials' in which the climate improved to a lesser extent.

Figure 2 shows the maximum extent of glaciation in Britain. Even at its height, the worst Pleistocene glaciation did not cover the south of England, and the last major glaciation, the Devesnian did not cover the Midlands or East Anglia. A later minor glaciation around Loch Lomond, only covered parts of the Highlands.

The period since the last glaciation can be regarded as another interglacial which is still in progress, and as Figure 1 shows, it represents only a very short time when compared with previous interglacials.

Earlier interglacials shows a sequence of plants establishing themselves in an area, and each such sequence follows a similar pattern or cycle - see Figure 3.

Figure 3 - Typical Zones within an Interglacial (Based on Bowen 1978)

End of glacial period and start of interglacial.

• Pre-temperate Zone : development of forest vegetation following the late-glacial phase. Betula and Pinus are characteristic trees, with light demanding herbs and shrubs.

• Early Temperate Zone : development of mixed oak forest with Quercus, Ulmus, Fraxinus and Corylus on rich soils.

• Late Temperate Zone : expansion of late immigrants such as Carpinus, Abies and Picea with accompanying decline of mixed oak forest partly due to deteriorating soil conditions.

• Post Temperate Zone : return of boreal trees, Betula and Pinus, forest thinning and development of open communities. Ericaceous heaths often continue into the following early glacials.

Return to glacial conditions

These sequences are seen from the hard outer layers of pollen grains preserved in peat and other deposits. The shape and ornamentation of the grains allows identification to genus and sometimes to species level, and the proportions of the different grains can be interpreted to indicate the make up of the surrounding flora.

Great care has to be taken in this interpretation since the proportions can be influenced by many external factors - see Figure 4.

Figure 4 - Factors affecting the interpretation of Pollen Analyses (based on several sources)

• Different species produce widely varying levels of pollen. Higher production is typical of wind pollinated plants, especially some trees. Other trees, such as the Rosaceae, being insect pollinated, are hardly seen in the pollen record.

• Frequency of flowering varies. If a species is near its geographical limit it may flower rarely, and as the influence of man increased, techniques such as coppicing also reduce the frequency of flowering.

• Some pollen disperses over a wider area than others. Pine pollen, being 'winged'? is typically seen in modern marine sediments, and can be over-represented in analyses.

• The representation of the pollen analysis may affect its interpretations. Typically, pollen analyses are shown as percentages per species or genus through a section of sediment - they cannot be shown in absolute terms because the rate of sedimentation is not known to be constant. The proportion is therefore affected by changes in the overall amount of pollen being produced.

Within each warm cycle, some sort of forest established itself in Britain. Prior to the Anglian glaciation, this mixed forest included Tsuga (Hemlock) and Pterocarya (Wingnut), which did not re-establish after early cold cycles (not true glaciations), but it was dominated by species seen in each later cycle - e.g. Pine, Alder, Birch, Hornbeam, Oak, Spruce and Elm.

In each cycle, the floral zones moved south as the climate cooled and north as it warmed. This movement was impeded by the east-west 'grain' of the geography of much of Europe, and in each cycle some species failed to return as the climate improved, e.g. Tsuga and Pterocarya. After the Gipping glaciation, Abies (Fir) failed to return, and the last record of native Picea (Spruce) is found in the Chelford Interstadial prior to the latest glaciation.

Figure 5 breaks down the most recent interglacial - i.e. the period since the last Ice Age - into 'Pollen Zones' commonly labelled I through VIII.

The end of the glaciation was not sudden. Over 3,000-4,000 years the climate varied between Tundra (Zones I and III) and Cool-Temperate. Zones I and III supported a grass-sedge tundra flora now more typical of arctic-alpine areas, and are known in Europe as the Older and Younger Dryas after Dryas octapetala (the Mountain Avens). 'Trees' were restricted to low growing junipers, arctic willows and arctic birch (Betula nana).

It was once thought that the glaciation was so severe that all plants were wiped out throughout Britain. This 'tabula rasa' theory has now been abandoned, and many arctic-alpine plants survived through the worst conditions. Some trees also remained in isolated pockets especially in the south, though possibly as stunted forms whose flowering would be restricted by the climate.

As the conditions improved trees spread from these remnants, and from 'mainland'? Europe - Britain and Ireland at this time were connected to one another and to Europe by a land-bridge, the sea-level being up to 400ft lower than now. These changes are first seen in the Alleröd interstadial (Zone II) as the pollen of Birch (Betula pubescens and B. pendula) and Aspen (Populus tremula), which formed copses at this stage rather than continuous woodland.

The end of Zone III saw changes which have been taken as the base of the 'Holocene'?. The relatively sudden change in temperature is marked by the rapid increase in juniper pollen as previously stunted forms took advantage of the conditions, grew rapidly, and began to flower profusely. This early flora also includes Sea Buckthorn (Hippophae rhaminoides), seen early in other interglacials, which required the open light conditions allowed by the absence of tree cover.

The rapid invasion of pioneer species such as birch and pine increased the tree cover, isolating many shade-intolerant species in niches such as sea cliffs and mountain tops, and changing the herbaceous flora from that of open ground to that of the forest floor.

Through Zones IV, V and VI a series of invasions spread from Europe and across all or part of Britain.

• Pioneer species such as Birch and Pine spread across the whole of Britain in Zone IV. Birch spread from the east across what is now the North Sea. Pine (Pinus sylvestris) spread from two centres, one in the south and a separate centre in Scotland where it may have survived the glaciation in isolated areas, or it may have spread from Ireland.

• Hazel (possibly from the land-bridge south of Ireland) and Elm and Oak (from the south) spread through Zone V to replace Pine (and to some extent Birch) throughout southern and eastern England. By the end of Zone VI, Pine was restricted to the north and west.

• Alder (Alnus) arrived in Zone V, but its main expansion took place later.

• Lime (Tilia) was a late arrival across the land-bridge (late in Zone VI) since it requires warmer conditions. It spread later but only ever reached as far north as the Lake District.

• Ash (Fraxinus excelsior) crossed into Britain but until some time later it is only found at low levels and restricted to the south-east.

• Some Mediterranean species, including the Strawberry tree (Arbutus unedo) reached western Ireland (across the land bridge forming the Atlantic coast), and were isolated there.

These invasions ended as the land-bridges became submerged (at the end of Zone VI), and therefore defined what we now take to be the 'Native' flora of Britain. Since many species failed to re-establish before this time, the flora of Britain, and more so of Ireland, is less rich than in former times, and than that of the Continent.

Zone VIIa - the 'Atlantic' Period - was a warm and wet period in which the climate came under the influence of the Atlantic Ocean, partly due to the loss of the land-bridge. On flatter mountain tops, the forest was replaced by blanket bogs, and on lower ground Alder become widespread especially in wetter areas. Lime reached its maximum extent, and much of Britain was covered by a mosaic of Oak, Elm and Lime forming the 'Mixed Oak Forest' of the 'Wildwood'. Figure 6 shows how this 'Wildwood' varied across Britain from the dominance of Lime in the south-east to remnants of Pine and Birch forest on the north and west.

During this wet period, the soils became leached and depleted, and possibly influenced by man clearing areas of forest, Birch became re-established in some areas.

Man's influence is also inferred at the end of the Atlantic period - about 5,000 years ago - in the decrease in Elm pollen across north-western Europe affecting both Ulmus glabra (in Britain) and U. minor (in Denmark). This coincides with a reduction in other frost-sensitive species and a drop in temperature, but may be related to man clearing or managing the trees to provide fodder and bedding for agriculture, so reducing the level of flowering. A more recent theory is that it may relate to diseases, like Dutch Elm Disease, again possibly related to man 'damaging' the trees.

Other local temporary 'Landnam' clearances of the forest are also interpreted as signs of Neolithic agriculture. They relate to increases in cereal and open-space 'weed'? pollen and the presence of charcoal from the burning of the forest. The cleared areas were abandoned as the soil deteriorated, allowing in Birch followed by Oak and Lime.

At the same time as the 'Elm Decline', Ash (Fraxinus excelsior) increases in importance and spreads across most of Britain as a pioneer species in cleared areas. Lime pollen also begins to decline, possibly due to the climate becoming more continental, but also due to the use of its leaves as fodder.

Man's impact increased with the introduction of metal tools in the Bronze Age (from ~3700 years ago), and through into the Iron Age the climate cooled to a low between 2,900 and 2,300 years ago. Locally, where the forest was cleared it was replaced by heath vegetation, and in some areas Bronze Age trackways - made of coppiced wood - were overwhelmed by a sudden growth of fresh peat.

At this time Beech and Hornbeam begin to appear consistently in the south of England. Though these trees arrived before the loss of the land-bridge, their spread at a time when the climate seems not to be in their favour may be related to man, though Hornbeam is also seen after the 'climax'? in other interglacials.

From this time until the Roman occupation (in 54 BC), and beyond, the tree cover of Britain is influenced by two factors - man and the cold-wet climate.

• Already before the Romans, some new species had been introduced from Europe including the Grey and White Poplars and the Smooth leaved Elm.

• In well drained parts of lowland Britain, large areas were cleared for agriculture including the growing of corn, and as ploughing and manuring techniques improved the fields become permanent. When the Romans arrived corn was even being exported.

• In highland Britain, some areas of woodland were gradually being replaced by bog and heath.

When the Romans arrived some of the wildwood, possibly up to 40%, had been cleared, but the remainder, which in some areas was already were being actively managed, provided a basis for many of the industries which they introduced. These industries used the woodland as a renewable resource which continued until the Industrial Revolution and the increase in the use of coal.

The Romans also introduced several species from Europe and the Mediterranean, notable the Sweet Chestnut. Some of these species, and other later introductions, were well suited to the British climate, and some had been present in earlier times. These thrived and though not strictly 'native'? they have now become part of the natural history of Britain.

Bibliography

Bowen, D.Q. (1978). Quaternary Geology. Pergamon. Oxford

Milner, J.E. (1992). The Tree Book. Collins and Brown. London.

Pennington, W. (1969). The History of British Vegetation. EUP. London

Rackham, O. (1986). The History of the Countryside. Wiedenfeld and Nicholson. ##

Wilmstra, T.A. (1978). Palaeobotany and Climatic Change. In Gribben, J. (ed.) Climatic Change. CUP. Cambridge.

## Not quoted in original essay!!