| Hedera helix is pollinated
by flies and wasps and even though the flowering stems
of the plant are in the woodland canopy or open scrub
habitats and the plant is a high pollen producer its pollen
is therefore quite poorly dispersed (Huntley and Birks
1983) and is seldom recorded in high frequencies. Its
pollen grains are easily recognisable and Hedera helix
pollen has been recorded from temperate stage sediments
of earliest Pleistocene age in the British Isles. Pollen
records become increasingly common from the Hoxnian interglacial
(oxygen isotope stage 7 or 9) onwards (Godwin 1975). In
the Hoxnian and Ipswichian (oxygen isotope stage 5) stages
Hedera is very common in mid-interglacial, supporting
its status as an indicator of warmer, mesocratic conditions
(Birks 1986) which has been deduced from the distribution
of Hedera pollen records in the Holocene. Hedera
persisted longer in the more oceanic Hoxnian, reflecting
an intolerance of cold, continental climates which it
still exhibits today (Godwin 1975). Hedera pollen
frequencies remained higher for longer in the mid-Holocene
in more oceanic Ireland than in Britain or north-west
Europe. Iversen (1944) and Troels-Smith (1960) have discussed
the value of Hedera helix as a climatic indicator
in the Holocene, with winter temperature a probable limiting
factor. Hedera helix is probably only slightly
more tolerant of winter cold than Ilex aquifolium,
with its thermal range restricted to areas with average
winter temperature for the coldest month not falling below
-1.5OC. Lower temperatures reduce fertility and can also
directly cause mortality. There are a few records of Hedera
helix pollen from Pleistocene cold stages in the British
Isles (Godwin 1975) but the susceptibility of ivy to low
winter temperatures means that these are probably attributable
to reworked pollen. Very high Hedera pollen values have
been recorded from probable mid-Ipswichian sediments in
Hampshire (Barber and Brown 1987) where it may have been
growing in valley bottom dry fenwood, a habitat in which
Godwin (1975) suggests it may have been particularly common.
Factors other than climate may have influenced the past
abundance of Hedera helix and very high percentages have
been recorded associated with herbivore grazing. Scrapings
from a rhinoceros tooth from Hoxnian sediments at Clacton,
Essex contained 37% of Hedera pollen (Godwin 1975).
Troels-Smith (1960) noted the Holocene importance of Hedera
as a fodder plant and Simmons and Dimbleby (1974) have
discussed several sites where high Hedera percentages
are associated with prehistoric human activity from Mesolithic
to Bronze Age times. Gathering of Hedera while
flowering as a winter fodder may account for such exceptional
high frequencies. Forest clearance creating more open
conditions may also have caused increased Hedera frequencies
and Garbett (1981) investigated the possible effects of
elm disease at the time of the Ulmus decline in creating
more standing dead trees and so perhaps favouring Hedera,
but without finding significantly increased Hedera
percentages.
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