| Tilia is mostly insect-pollinated
and so is considerably under-represented in the pollen
rain. Percentage frequencies on pollen diagrams therefore
generally underestimate true Tilia abundance. The
Tilia pollen record for previous Quaternary interglacial
stages has been considered by West (1980) and although
pollen data are only available mainly from East Anglia,
this location broadly corresponds with Tilia's
most abundant Holocene distribution and is probably representative.
Tilia records are mainly confined to the warmest,
middle substages of past interglacials. For Oxygen Isotope
Stage 5 (c.f. Ipswichian interglacial) Tilia percentages
are low but for the preceding interglacials are much more
significant, reaching up to 5% of tree pollen. Tilia must
have formed an important constituent of the thermophilous,
deciduous, mid-interglacial woodland in these past temperate
phases, as during the present interglacial. The pollen
record supports its status as the most thermophilous British
forest tree. The great majority of pollen records for
the Holocene and before are referable to Tilia cordata
(Godwin 1975). There are many pollen records from which
the Holocene palaeoecology of Tilia cordata may
be reconstructed. Huntley & Birks (1983) have mapped its
spread in a northwesterly direction across Europe. Birks
(1986, 1989) has mapped its empirical limit (consistent
presence) in south-east England to around 7,500 radiocarbon
years ago, although as an under-represented pollen type
it was probably present in low numbers substantially earlier
than that. The recovery of Tilia cordata macrofossils
by Kullman (1998) in the Scandes Mountains of central
Sweden and AMS dated c.7,000BP, well to the north of its
previously believed Holocene limit, suggests that T. cordata
may have been present in Britain in low populations from
much earlier in the Holocene than the pollen record suggests.
Its more general spread through the forest to the north
and west in Britain was slow because of its low rate of
population expansion (Bennett 1986) and its need to immigrate
into already established Quercus and Ulmus forest.
Pollen data suggest (Pigott & Huntley 1980), however,
that by about 5,500BP Tilia had reached the line
between the southern Lake District and south Durham which
climatic and edaphic factors determined as its northerly
expansion limit (Pigott & Huntley 1978). The application
of correction factors derived from modern pollen studies
to its low postglacial pollen frequencies (Andersen 1970,
Bradshaw 1981) suggests that Tilia cordata may
well have been a very important, and perhaps even dominant,
component of the undisturbed mid-Holocene mixed deciduous
forest in lowland northern, central and southern England
(Birks et al. 1975, Moore 1977, Greig 1982, Bennett 1989).
Although generally pollen frequencies do not exceed c.15%
of tree pollen in their most abundant phases, at some
sites much higher values are recorded. These usually occur
in very small peat sites or just after peat inception
and probably reflect very local vegetation (Greig 1982).
Waller (1994) has suggested that wetland basin size is
critical in determining the pollen percentages of the
poorly transported Tilia grains. Tilia populations
were probably much larger than relatively low average
Tilia pollen frequencies suggest. Tilia pollen
is robust and can preserve well when other types are degraded,
particularly in soils, so high frequencies may reflect
differential preservation. Keatinge (1982) has shown that
concentration of Tilia grains by stemflow may account
for higher frequencies in these circumstances, indicating
local woodland presence. Tilia pollen frequencies
decline from about 5,000BP onwards, consistent with cooling
climate after the mid-postglacial thermal optimum. The
decline is often very sudden and forms one of the few
pollen stratigraphic marker horizons in the later Holocene.
Human influence was probably a more important factor than
climate and Turner (1962) has shown that the Tilia
decline was not a synchronous event but occurred at
different times at different sites, usually in association
with pollen of arable weeds and bracken spores, indicators
of increased forest clearance and farming activity. In
many cases this event correlates with Bronze Age forest
clearance in the millennium before 3,000BP, but individual
examples occur in Neolithic, Iron Age and Medieval times.
Whenever it occurred, however, Tilia pollen values never
recover their former levels and Tilia seems to
have been unable to regenerate after clearance. The selection
of the better loessic soils for arable farming would have
particularly affected Tilia and the subsequent
removal of those soils by erosion would have prevented
its return. Prehistoric and later human communities would
have found Tilia to be a valuable tree, providing
leaf fodder, quality timber and stringy bark, or bast,
useful for rope fibre (Godwin 1975). This would have resulted
in a destructive impact on the tree's populations until
its extensive coppicing in later periods. |
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