1. History and
Distribution of Mixed Oak Forest.
Following the end of the
last glacial phase of the Quaternary Ice Age, about 10,000 years ago, the ice
that had covered much of Britain withdrew northwards and birch, pine, and then
mixed deciduous forest migrated northwards from refuges to the south in Europe.
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One
of the most powerful techniques for reconstructing Holocene and Pleistocene
environmental change is Pollen Analysis or Palynology. This
technique relies on the assumption that plants produce pollen and spores in
huge quantities, but quantities which are broadly in proportion to the
frequency of each plant in the overall vegetation cover. These small light
grains are then distributed in the atmosphere, more or less at random, from
which they eventually fallout, or are rained out. This pollen rain
represents in the relative proportion of different pollen types, a reflection
of the make up of the regional vegetation from which the pollen has been
derived. When the pollen fallout on to a growing surface such as that of a peat
bog, or into a lake with accumulating sediments they form a stratigraphic
sequence of micro fossils which records any changes through time in the
pollen rain and hence in the vegetation (and by implication the climate) from
which it was derived.
Such biostratigraphical sequences can be represented as profile
pollen diagrams and calibrated against either absolute or relative time scales.
When a significant number of reliable pollen diagrams are available, and these
diagrams cover a reasonably wide geographical area, the pollen data can
be mapped in various ways. By recording the times of first arrival of a
pollen type at a wide range of sites it is possible to show using isochrones
(lines of equal age) the rate and pattern of spread of species into
and throughout Britain in the Flandrian, or Holocene. |
By between 7,000 and 5,000 years BP at the so
called climatic optimum of the Holocene or post glacial
(Flandrian) period essentially the present pattern of forest types had
been established in the British Isles. There has been some subsequent
contraction in these forests occasioned by climatic deterioration promoting the
spread of ombrogenous mires in both lowland depressions and at high altitudes
where it is associated with a retreat of the altitudinal treeline. Nonetheless,
this was essentially the primeval virgin forest which Oliver Rackham has called
the wildwood (Rackham 1976).
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If you wish to explore the establishment of this wildwood in
the early Flandrian, and its subsequent modification after the climatic optimum
of c. 5000 yrs BP you can transfer to the Quaternary Palaeoecology Web Site at
this point by clicking on the icon. In particular look at the materials under
Q8b, and Q9. |
The majority of the country; England, Wales, much
of Ireland, and southern Scotland was covered by closed mixed deciduous forest
that represented the extreme northwestern portion of the vast temperate
(summer) deciduous forest formation of west and central Europe which
stretched to the east as far as the Urals. Deciduous forests also covered much
of Ireland but with some differences in composition, and it is also probable
that in Ireland as in Scotland, the pine remained a native (Birks et al 1975). In Scotland these
deciduous forests penetrated the Highlands, on better soils, along the valleys
and straths, but gave way northwards and at higher altitudes to native forests
of Scots pine which formed a natural treeline at altitudes up to 600m. (2,000')
as at Creag Fiaclach in the Cairngorms today.
Above this treeline the pine was followed by
sub-alpine juniper or birch scrub. These pine forests represented the oceanic
extension of the vast boreal coniferous forests of Europe and they gave
way northwards, in the northwest Highlands to boreal deciduous woodlands
of birch; woodlands which continue through northern Norway and Lapland along
the Arctic seaboard of Eurasia.
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contorted,
semi-prostrate pine
at the altitudinal treeline |
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sub-alpine juniper
scrub
in gulley above treeline |
Throughout its large distribution in southern
Britain mixed deciduous forest showed considerable variation (in spite of the
fact that Britain's tree flora is poor, with only 60 native species of which
only 12 are really common, although later introductions now virtually
naturalised increase this figure if one considers not only native species) in
its floristic and faunistic composition, and in the details of its community
structure.
For example, certain warmth demanding or
thermophilous tree species such as the small leaved lime, Tilia cordata, were more prevalent in the south and
east where they formed a significant component of the forest, perhaps assuming
dominance over the oak in East Anglia, a fact still reflected in present
distribution patterns, and preserved in place names such as Lyndhurst in the
New Forest - a name derived from the alternative colloquial term for the lime -
'the linden'. Other trees with southern distributions are late postglacial
immigrants. The beech, Fagus sylvatica, for
example, the other leading dominant of European deciduous forests, although it
reached Britain before migration routes from the continent were severed by
rising sea levels c. 7,000 years BP, it did not expand its representation as a
forest tree until man began to clear the oak and open up the climax forest.
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leaves and fruits of the ash,
Fraxinus excelsior |
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Some species such as the elm, Ulmus glabra, and the ash, Fraxinus excelsior, showed a preference for soils of
high base status, particularly over limestone and may have formed distinct
forest variants assuming dominance over the oak as in the ashwoods of the
Derbyshire and Yorkshire Dales today. It must be noted, however, that as with
the beech, the post glacial expansion of the ash is also related to forest
clearance and probably took place at the expense of the elm. At the end of the
Atlantic period c. 5,000 BP throughout most of Europe there is a sudden and
synchronous drop in the quantity of elm pollen, the elm decline, which has been
variously attributed to climatic change, fungal disease, and the impact of
Neolithic agriculture, or to some combination of these factors.
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heathy oakwood with
calcifuge herbs |
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oak on calcareous soils with calcicolous
shrubs such as |
This soil or edaphic variation would have been
even better demonstrated by the composition of the subordinate tree and shrub
strata, or underwood, and of the field and ground layer vegetation: - with
heathy varieties on poor acid soils, - with a wealth of calcicolous shrubs and
herbs on soils over chalk and limestone, - and damp wet variants with alder,
Alnus glutinosa, and willows, Salix spp., and sedges on waterlogged mineral soils
or over fen and mire peats. |
