| Corylus avellana was
the first of the temperate deciduous forest trees to immigrate,
establish itself and then become abundant in the postglacial
period. Its sudden rise to very high pollen frequencies
marks the start of the mesocratic phase of the present,
Flandrian, interglacial (Birks 1986) and is the first
stage in the development of dense, mixed deciduous forest.
This Corylus pollen rise is an almost ubiquitous
feature of Holocene pollen diagrams and reflects an extremely
rapid increase in population (Bennett 1983), probably
with no real competition from other taxa and no effective
environmental constraints (Birks 1986). Radiocarbon dates
for the Corylus rise usually fall between c.9,400
and c.9,000BP, although it occurs later in more environmentally
marginal regions such as the uplands and on isolated islands
such as Mull and Arran (Innes 1999, Boyd and Dickson 1986).
Here the rational pollen limit (rise to abundance) of
Corylus occurs more typically from c.8,800 to 8,500BP.
Birks (1989) has presented an isochrone map for the Corylus
pollen rise suggesting the earliest spread of hazel was
through the Irish Sea area, including south and eastern
Ireland. Similar conspicuous increases in Corylus
pollen values occurred in previous interglacials but were
less pronounced and occurred later in the interglacial
cycle than in the Flandrian (West 1977, 1980). Huntley
(1993) has considered the possible reasons for the rapid
early Holocene expansion of Corylus. He concludes that
the greater climatic tolerance of Corylus gave
it an advantage over other thermophilous trees like Quercus
and Ulmus in the more seasonally extreme climates
of the early Holocene. Other factors such as the location
of its glacial refugia (Deacon 1974), faster migration
rates and the possible effects of Mesolithic human activity
(Smith 1970) are judged less likely to be responsible.
The early Holocene spread of Corylus avellana has
been discussed in detail by Tallantire (2002). Corylus
abundance was reduced after several centuries by the immigration
and spread of Quercus and Ulmus. Hazel produces
pollen in great quantities and also flowers at an early
stage in the year giving it increased pollination advantage,
resulting in over representation in pollen counts. However,
under a heavy oak-elm tree canopy it flowers very little
and then the bulk of hazel pollen may result from bushes
growing at the woodland margin and in clearings. Moderate
Corylus values in mid-Holocene pollen diagrams
suggest it mainly grew as a shade-tolerant woodland understory
shrub. Exceptionally, Corylus abundance continued in areas
of favourable geology such as the limestones of parts
of northern England (Bartley et al. 1976) or western Ireland
(ref). Corylus is heliophytic and is favoured by
forest opening. Very high Corylus pollen percentages
are often recorded following the creation of woodland
clearings. In the early and mid-Holocene these are often
associated with charcoal, as hazel is more fire resistant
than most other trees, and have been attributed to the
effects of Mesolithic activity (Simmons 1996). Corylus
pollen, wood, and fruits have been commonly identified
in Late Quaternary deposits and the distinctive nuts of
hazel are often recovered from British peat and alluvial
sediments. They preserve well in waterlogged conditions,
where they have been incorporated in such great quantities
that hazel's former abundance seems certain. Sub-fossil
nuts show teeth marks of rodents where they have been
hoarded and some preserved nut distributions suggest former
shoreline detritus of streams and ponds (Godwin, 1975).
Corylus nuts remain viable in water for lengthy
periods and Birks (1989) suggests that water currents
may have been a main long-distance transport agent of
Corylus in the early Holocene. At many prehistoric
human settlements hazelnuts occur in large quantities,
suggesting that they were collected for food. The distinctive
microscopic anatomy of hazel wood allows definite identification
of well-preserved specimens of wood and charcoal. The
wood has been found in the Somerset peat levels, where
Neolithic track-ways occur constructed of parallel, straight
rods, 3-4m long and shown by annual rings to be 8-17 years
old (Coles ref). This suggests early coppicing and hazel
rods were used in the construction of many types of artifact,
such as fence lines and fish traps (Coles ref). Hazel
is palatable to sheep but not cattle and unlike other
tree and shrub species hazel does not appear to have been
adversely affected by the onset of Neolithic clearance
and farming, after which its pollen frequencies generally
increased.
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