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The Flandrian: the case for an interglacial cycle

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1. Zone IV the Pre-Boreal.

Following the temporary climatic setback of pollen Zone Ill, the Loch Lomond Stadial, of the Late-Devensian, the Flandrian or post-glacial amelioration begins when in Europe the ice at the Raa and Central Swedish moraines begins to decay, and in Britain the remaining corrie and valley glaciers also start to finally waste away. This sudden and rapid rise in temperature is dated as between 10,200 BP and 10,300 BP and marks the opening of the Pre-Boreal period, or pollen Zone IV. This amelioration of climate and cessation of soil disturbance allowed both the rapid sucession on stable maturing soils of the surviving vegetation of the Late-Devensian (which meant in Britain the rapid spread of closed birch woodland both northwards, and to higher altitudes), and also the migration northwards of more thermophilous elements from southern refugia.

The Pre-Boreal in Britain is mainly a time of birch woodland with both Betula pubescens and in England also Betula verrucosa (pendula) which had also arrived in Zone II, while other trees of the Allerøod/Winderrnere Interstadial such as Sorbus aucuparia, and Populus tremula also accompanied the birch in its postglacial expansion. During the Loch Lomond Stadial (11,000.-10,200 BP) the birch declined widely, and possibly became extinct in the British Isles, but before 10,000 BP it was again well established in in the midlands and northern England, southern Scotland, Wales, and even Eastern Ireland (Birks 1989). In the south and east pine is already present by 9,000 BP in Zone IV (Bennett 1984), although it is not known whether it survived through Zone III from the Allerød, although, according to Jessen it was probably absent from Ireland at this early stage in the post-glacial it appears to have been present in SW Ireland before 8800 BP. Here it possibly arrived independently from the English populations as it was absent at this time from Wales. There are, however, two characteristics of the Late-Devensian/Flandrian (late-glacial/postglacial) boundary that deserve special comment.

1. The first is the rapid rise in Juniper pollen frequencies, a feature also experienced at the start of the Allerød, and in both cases this is believed to be associated with a free flowering of prostrate forms of Juniper, such as Juniperus nana the dwarf Juniper, as the temperatures rose rapidly, as well as an invasion by the shrub Juniper, Juniperus communis. Indeed there is some evidence for the apparent hybridisation of these two species in an ameliorating environment, as yet ecologically unsaturated and without undue shading and competition from a closed tree canopy. This juniper pollen maximum has been dated as for example at Scalesby moss as between 9,607-10,203 BP.

2. Secondly, the Zone Ill/IV boundary is invariably associated with a recession in the NAP/AP ratio. As Davis and others have shown by absolute pollen counts this is not necessarily an absolute decline in the NAP. pollen but rather a vast relative increase in arboreal pollen as the woodland spread and the canopy closed. So, although a recession of herbaceous and dwarf shrub pollen is to be expected, the lack of macrofossil evidence does not necessarily mean the absence of the species in question. Indeed there is evidence that many such plants persisted into the post-glacial, and some even through to the present. For example, Betula nana persisted in lowland sites into Zone IV and there is even some evidence of hybridisation with the tree birch, Betula pubescens. Jessen also reports the persistence of Salix herbacea in western Ireland where too Empetrum was still present in fair quantities, but the last remains of the Empetrum rich heaths that had characterised the Late-Devensian disappeared from Ireland in the Pre-Boreal. Other late-glacial plants like the sea buckthorn Hippophaë rhamnoides, now a plant of coastal sand dunes in south and south east England, persisted at inland sites in the Pre-Boreal, while other late-glacial pollen types like Helianthemum, Artemisia, Polemonium, Rumex, and Galium, although still found in Zone IV diminish progressively in frequency from the base up. On the other hand the expected influx of thermophilous elements from southern refugia migrating northwards under the influence of a rapidly ameliorating climate is only represented at this very early stage in the post-glacial by aquatics.

2. Zones V and Vla, b, and c: the Boreal

The succeeding Boreal period represents a continuation of the expansion of forest, and with the migration under conditions of increasing warmth of deciduous trees from the south, it marks a stage of shifting forest composition. Godwin sums up the main characteristics of this time as;

1. the strong expansion of the hazel (Corylus avellana), especially in the west and north,
2. the powerful expansion of pine into a predominantly birch dominated Pre--Boreal countryside,
3. accompanying or following the pine expansion, the onset of the replacement of pine by expanding mixed oak forest, especially the trees oak and elm.

The Boreal embraces the whole of Zone V and Zone VI, the latter being divided in sub-zones a, b, and c. In Zone V birch was still dominant in Ireland and in the northwest of Britain, but Betula verrucosa is present for the first time in Ireland, as too is indisputable macrorossil evidence of pine. Birch expands late in those areas of the Highlands which were ice covered during the Loch Lomond readvance (9500-9000 BP), and in the southwest Peninsula possibly as a result of soil instability and immaturity following glaciation and/or periglaciation (Caseldine & Maguim 1986). As previously noted pine was already present in southern Britain in the Pre-Boreal, expanding between 9,000 and 8,500 BP to central England the southern Lake District and the northern Pennines, so that in Zone V it became abundant throughout England. In northwest Scotland there appears to have been an apparently independent early expansion of pine either derived from trees which survived in refugia at or to the west of the current position of the western Scottish coast, or resulting from a very rapid expansion (before 8,500 BP) through western Ireland and southwest Scotland where it was confined to marginal sites only. However, not until Zone VI did it really expand in the north and west and effectively replace birch woods in the west. In Scotland it expands eastwards into the Grampians at about 7,500 BP, and by 6,000 BP it was expanding altitudinally in the Caimgorms (Dubois & Ferguson 1985) and reached Skye and Sutherland a thousand years later. However, it appears to have been absent or rare throughout the Flandrian in the southwest of England, the Cumberland lowlands over much of lowland southern Scotland (south of Rannoch Moor) and lowland south Wales and northeast England. All through Zone VI pine remained a very important component throughout British woodlands, though in the southeast and perhaps the east of England it was giving way to the trees of mixed oak forest at the end of this time.

Hazel, Corylus avellana, which present as an early poslglacial immigrant in Zone IV only begins to expand at the opening of Zone V. Indeed it is this expansion of the hazel that characterises the Boreal Period through the whole of Western Europe. In Ireland hazel quickly achieves high values (100% A.P., or more) [this is Jessen's 'Hazel/Birch Period']. In south and southeast England the expansion was slower, though in the north and west hazel pollen frequencies were similar to or higher than those encountered in Ireland. It is Zone VI, however, that sees the really remarkable expansion of the hazel [Godwin's 'Pine/Hazel Period' in England, and 'Hazel/Pine Period' in Ireland]. As was pointed out above, hazel pollen is more frequent in the west in Zone V. but in Zone VI it is much more common in pollen diagrams from across the country. This phenomenon is probably related to the rapid rise in sea level that was taking place at this time, bringing a more oceanic influence to the south and east of the country - a topic to which we shall return shortly. In the British Isles the hazel pollen count is much higher than on the continent, although here too its frequencies are always pronounced. It reaches its highest frequencies, however, in Ireland: Dunshaughlin x17, Carrowreagh x12, Castlelacken x12 total arboreal pollen. Most English sites have values between 100% and 400% TAP.

Hazel appears to have been absent during the Loch Lomond stadial and first became established in the British Isles around the Irish Sea basin and along the western seaboard of Scotland spreading inland and eastwards except in the east central Highlands where perhaps the existence of a mountain barrier restricted its spread from the coast as a frost sensitive lowland shrub. The hazel probably arrived in the Irish sea basin by water transport (can survive 30 days floating in fresh or salt water), expanded rapidly on fertile soils in coastal areas and then spread inland and became locally abundant (except in eastern Scotland) forming a hazel dominated landscape which persisted for a thousand years in the absence of competition from taller, longlived, dense shade producing trees (Birks 1989).

As we have seen no such hazel dominated period occurs in any of the earlier intergladals and these high values in the Boreal Period of thc Flandrian must imply widespread hazel scrub, especially in the oceanic west. It has further been suggested that these high frequencies represent hazel, normally associated as an understorey shrub in mixed oak forest, out stripping the trees of this forest under highly favourable conditions of soil and climate as it migrated back into what was to become the British Isles. Deacon (1974) plots this rate of return and spread and suggests that the hazel, perhaps as a result of changing genotype, was able to survive in refugia much closer than was the case at the onset of earlier interglacials. On the basis of hazel's first appearance she places some of these in northwest Europe. From these more northerly refugia migration was more rapid than from southerly refugia, and although their location is uncertain there is a strong indication that some of these refugia lay to the west of the British Isle (ie. on the continental shelf, much of which must have been dry land at that time) as well as in south west Scandinavia.

The hazel favours base rich soils with a mull humus and tends therefore to show a preference for fresh calcareous soils. In the Boreal this is borne out in Ireland by lower hazel pollen frequencies in thc southwest where the soils are naturally base deficient and acidic. In England, especially in the east and south under what were perhaps more continental conditions, we must envisage a passing period of pine forest on fresh, relatively unweathered soils with a hazel undergrowth. It is interesting to note that this rather uncommon assemblage of species still occurs today on the island of Bornholm in the Baltic. Hazel pollen frequencies, however, show considerable variation both regionally, and between the sub-zones of Zonc VI; variation, the cause of which is still not fully understood. Nevertheless, the hazel maximum was generally passed by Zone Vlc.

Zones V & VI are also separated by a rapid rise in the frequencies of oak and elm which occurs in the latter; elm in particular reaching surprisingly high frequencies in lowland sites, especially in Ireland. In some sites in the south and east of England there are low frequencies of oak and elm pollen in Zone V, with both trees arriving by 9,500 BP. The maximum extension of both trees was complete by 6,000 BP, but although Ulmus glabra was pressent through out this range Ulmus carpinifolia was additionally present in East Anglia in the, early Flandrian. By Zone VIc lime and alder Tilia and Alnus) are present, and it is the expansion of the latter, the alder, that marks the end of the Boreal and the transition to thc Atlantic period. Thc evidence suggests that both Tilia cordata predominantly, and some Tilia platyphyllos was present by 7,500 BP. Prior to 7,500 BP alder Alnus glutinosa is present locally and mainly in coastal situations suggesting well developed coastal alderwoods, but its inland expansion occurs after that date. Before we move on to consider Zone VIIa, the Atlantic, we must give some attention to what had been happening to the sea-level during this early part of the post-glacial.

2.1. Early Flandrian Sea Level Changes.

At the start of the Flandrian, sea levels were Iow, but as the Pre-Boreal and Boreal Periods progressed the broad avenues for migration afforded by the floors of the southern North Sea, English Channel, and the Irish Sea, all then dry land were incrementally reduced by a progressive if intermittent eustatic rise in sea level. Ireland was the first to bc cut off, and as a result its flora and fauna are impoverished relative to the rest of Britain (eg only 2/3 of the Angiosperm flora of England, Wales & Scotland is native to Ireland). The lime, a late Boreal immigrant, and a thermophilous tree species never reached Ireland, and , therefore, is not a native tree there. The rest of Britain was severed from the continent at about 7,000 years BP marking the close of the Boreal and the opening of the Atlantic. After this time then migration to Britain was only possible for a plants and animals with effective long range dispersal mechanisms, and these are relalively few. So with the exception of later introduced species the native British flora and fauna consists of those species that had arrived by the end of the Boreal.

Apart from its significance for migration the Flandrian rise in sea level is important in a way to which I have already alluded. It means that at the end of the Boreal Britain was entirely surrounded by sea, and this resulted in a marked increase in the oeeanicity of the climate especially in southeast England which hitherto had been more continental than the rest of Britain. To this increased oceanicity (and hence climatic ‘wetness') is attributed the rise in alder pollen which signify thc openning of Zone VII the Atlantic period. The alder, which favours damp base rich soils takes its place in the expansion of mixed oak forest at this timc, forming an important element in that forest, probably at the expense of pine and birch. At highcr altitudes the alder is associated vith birch and some oak, and probably formed extensive birch/alder or oak/alder/birch woods in the uplands, particularly in Scotland.

Before we turn our attention to the Atlantic Period we must recognise that the second half of the Flandrian is a period during which the effects of human cultures become progressly more discernible in the overall process of vegetation change, affecting in particular the absolute and relative composition of Flandrian forest ecosystems and through his effect on the surface water balance affecting the development and extent of wetland communities. A good deal of controversy did, and still does, surround the valuation of thc roles of Man and climate in the vegetation change observed during this period. First, we shall treat the remainder of the Flandrian, briefly, concentrating on the evidence for climatically controlled vegetation change. We shall consider this period again at more length when discussing the human factor and its effect.

3. Atlantic Period, Zone VIIa - Thc Climatic Optimum.

Pollen Zone VII occupies a long time span and its opening is marked by a rise in the frequency of alder pollen, mainly at the expense of birch and pine. In the north and west this expansion is slower. Zone VII is divided into VlIa, the Atlantic, and VIIb the Sub-Boreal. The Atlantic is known as the climatic optimum of the Flandrian and is marked not only by the rise in aIder, but also by the expansion of oak, elm (Ulmus glabra) and both species of lime (Tilia cordata, and T. platyphyllos). Lime in particular, beginning its expansion at the end of the Boreal, Zone VIc, reaches relatively high frequencies in Zone VIIa. We can envisage much of the country, therefore, as covered by mature, relatively stable climax mixed deciduous forest, with markedly thermophilous elements present. This optimum period taking in late VIc and VIIa is gcnerally believed to indicate a climate warmer than at present and also possibly a drier one; that is the evaporation rate has fallen since as a resuIt of a combination of cooling and increased precipitation. The Atlantic, then is marked by the maximum extension northwards of thermophilous elements, for example the lime amongst the trees, and such extension must havc involved fertile seed production and successful establishment suggesting that between 6,000 and 5,500 BP July and August mean temperatures must have been 2-3° C higher , than today. The ivy (Hedera helix) a good climatic indicator occurred much further to the north in Zone VIIa than it does now, both in Britain and in northern Europe and Scandinavia. The altitudinal limit of forest and the treeline were, according to accumulating evidence, significantly higher than the present potential natural limit; over 600 m. (2000 ft.) in the Caimgorms, and perhaps over 900 m. (3000 ft.) in sheltered sites. Nevertheless, and in spite of this impression of an extensive closed forest covering most of the country, the vegetation would not have been uniform and regional variations would have occurrcd even at thc climatic optimum. Even the forest composition would vary regionally, with lime perhaps assuming dominance over the oak in the south and east of EngIand and the oak in turn giving way to pine & birch in the highlands and eventually to birch in the far north, and to scrub/woodland in the islands. The Atlantic, however, also sees the initiation of peat formation, both blanket peat in the uplands and raised bog peat succeeding hydroseral development in both the lowands and uplands. It was at this time, towards the end of Zone VIIa that the blanket bogs of the southern Pennines, Scotland and Ireland all began. This trend is traditionally interpreted as a t response to increased oceanicity both at high altitudes and in the west. The development of high level blanket bog began of course to depress the tree line and there are many examples of roots and stumps underlying blanket peat of Atlantic age that can be encountered today revealed from beneath the entombing peat by peat erosion.

4. Sub-Boreal Period, Zone VIIb

The passing of the optimum begins with the opening of Zone VIIb the Sub-Boreal period. The climate of this period was thought to be warmer and drier, for there is evidence that the bog moss Sphagnum becomes less prominent and ericaceous heath plants, and then trees invade the bog surfaces. In spite of this evidence the decline of certain of the constituents of the mixed oak forest tree flora was taken to show that the optimum was passing. At the beginning of Zone VIIb there is a marked decline in elm, a distinctive characteristic of the pollen diagrams which was taken by Godwin to define the VIIa/VIIb boundary. He also points out that at the same time the lime curve also starts to decline, especially in the north and considers that these declines in the thermophilous trees reflects a deterioration of climate and questions the original interpretation of the sub-boreal as warm and dry. The decline of elm is often associated with a rise in ash, Fraxinus excelsior. The big decline in lime, however, occurs at the end of VIIb, often with an increase in the birch, and this event was regarded as marking the VIIb/Vii boundary. It is at this time that the large leaved lime Tilia platyphyllos disappears from Britain as a native tree.

5. Sub-Atlantic, Zone VIII.

Godwin then considered that as our most warmth demanding tree the lime decline in England and Wales was the clearest expression of climatic revertence and he used it to define the VIIb/VIII boundary. The lower values for pine pollen through the sub boreal especially in the north being due to a decrease in summer warmth.

About 2,500 yrs BP (dated both by radio carbon and by archaeology) an abrupt change occurs with the climate becoming both cooler and wetter. Mixed oak forest remains dominant over much of the country, but with considerable changes in its composition. Elm and lime have declined considerably in importance and are probably absent from parts of the country. Birch and ash have expanded to take their place, while in the south and east of England hornbeam Carpinus betulus expands (nb it is a late-temperate tree in interglacials) as too does the beech, Fagus sylvatica.

The best evidence for the climatic deterioration of the Sub-Atlantic comes from bog stratigraphy. Here there is an abrupt change from the humified peats of the Atlantic and Sub-boreal with some Sphagnum and much Calluna and sometimes a layer of pine and other tree stumps, to the unhumified Sub-Atlantic Zone VIII peat mainly consisting of Sphagnum peat with species like Scheuchzeria now confined to one locality in a bog pool on Rannoch Moor in Scotland and indicating a very wet bog surface. This change recognised first on the continent by Weber is described as a Grenzhorizont

In lowland raised bogs there is evidence of flooding at this time. For example in the Somerset levels the marginal fens which previously had been only moderately wet were flooded with base rich water enabling the saw sedge, Cladium mariscus to spread up on to the bog plane over the acid Sphagnum peat. The top of the bog was above the flood level but was wet enough to support Scheuchzeria palustris. The development of the raised bog after the recurrence surface (Grenzhorizont) appears as the standard growth of the bog surface producing very unhumified peat.

Finally further evidence is found for climatic deterioration in renewed mass movement and solifluction at high altitudes particularily in Scotland. For example White and Mottershead (1972) dated a buried peat/soil beneath a solifluction lobe/terrace at 5145 BP ie the Atlantic/Sub-Boreal boundary. Similar dates obtained from the Cairngorms (King and Sugden) of 4855, and 2860 BP, the latter Sub-Boreal/Sub-Atlantic.

6. The Anthopogenic Factor in Holocene Ecological Change

By the climatic optimum of late Zone VIc and Zone VIIa the Flandrian (Holocene) forests had achieved their fullest development in terms of species diversity and probably also in terms of their structural complexity. We must visualise, therefore, mile after mile of primaeval mixed oak forest over almost the whole of the British Isles only giving way in the north - in the Scottish Highlands - to vast forests still dominated by pine - the primordial Woods of Caledon. Although usually referred to as mixed oak forest the canopy trees and the under storey vegetation varied regionally, with the lime and the elm assuming great importance in central, south, and eastern England. It has been assumed that man had no more than the slightest affect on these forests up to the optimum. Population densities were low and the prevailing cultures were those of Mesolithic hunter-gatherers. The interpretation of the Pre Boreal and Boreal Periods, therefore, as a time of autogenic succession towards the establishment of these deciduous forest ecosystems would appear to remain valid. In contrast, the latter part of the Flandrian (Holocene), following the climatic optimum, is characterised by retrogressive climatic change: change which we have already treated as being the result of climatic deterioration, impoverishment of soils, and the spread of bog. Since roughly the early sixties this view has been open to reinterpretation. It has become clear increasingly that many of the vegetation and ecological changes, and particularily those used to define the zonal boundaries of the Flandrian (Holocene), owe as much to the influence of Man as they do to the direct impact of climatic change.

Copyright: Iain White, University of Portsmouth 1995, 2002. All rights reserved
This page is maintained by Iain White - Page last updated July 2002
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