| Alnus glutinosa is
the only species of alder native to the British Isles
and it is very probable that all Holocene fossil records
of Alnus in this area are referable to it (Godwin 1975).
Pre-Holocene records may include other species such as
Alnus incana but there is no direct evidence for
that as yet and all Pleistocene records may be regarded
as Alnus glutinosa at present. Macroscopic and
pollen records occur in almost all of the previous interglacial
periods of the Pleistocene (West 1980). In the earlier
Pleistocene temperate stages Alnus pollen frequencies
are steady throughout the interglacial, but in the later
interglacials Alnus is most frequent in their middle sub-stages,
as in the Holocene. During the Cromerian Interglacial
(Oxygen Isotope Stage 11) Alnus was abundant and
is present at several sites in the Cromer Forest Bed (Godwin
1975). Alnus pollen is recorded throughout the
Hoxnian Interglacial (Oxygen Isotope Stage 7 or 9) and
is particularly abundant in the mid-interglacial warm
phases at several sites in East Anglia and the Midlands
of England, when a strong macrofossil record also occurs
(Godwin 1975). The Irish record is similar. Although recorded
in every phase of the Ipswichian Interglacial (Oxygen
Isotope Stage 5) Alnus pollen frequencies are much lower
except where associated with alder macrofossils and Alnus
may not have been generally abundant (Godwin 1975, West
1980). Alnus pollen does occur in glacial stages
of the Pleistocene, including the mid-Devensian (Chambers
and Elliott 1989). Late Devensian records are quite numerous,
and occasional macrofossils also occur, as at Willow Garth
on the Yorkshire Wolds (Bush and Hall 1987). These records
may be derived from earlier deposits or result from long
distance transport (Tallantire 1992), but they may represent
local presence. Alnus macrofossils have been reported
in an early Holocene context in the south of England (Clarke
and Barber 1987, Waller 1993) but Alnus pollen
frequencies, although widely present (Chambers and Elliott
1989), remain very low in the early stages of the present
interglacial (Bennett and Birks 1990). As Alnus is a high
pollen producer and its pollen is well transported by
wind and water, these low frequencies suggest no major
local populations of alder around these sites at this
time, although the existence of small isolated populations
remains possible. Such very early centres of Alnus
growth, probably controlled by local edaphic conditions,
would not be easily visible palynologically unless located
very close to pollen sampling sites. Huntley and Birks
(1983) suggested a gradual spread of Alnus into
Britain from glacial refugia in eastern Europe, but the
presence of a long period in the early Holocene with widespread
but low Alnus pollen values suggests a continuity of population
at a low level well before the main increase of Alnus
pollen in the mid-Holocene (Bennett 1983, 1986). Chambers
and Price (1985) reported an early radiocarbon age of
almost 8,500BP for the rise of Alnus in coastal
mid-Wales, and suggested a westerly refugium from which
Alnus spread quickly when conditions became favourable.
The existence of local isolated centres of alder growth
suggested by the early Holocene Alnus records may
have provided locations from which Alnus could
have expanded quickly without immigration from external
sources. Local, site-dependant events could have governed
the regional spread of Alnus (Birks 1989). The
mid-Holocene expansion of Alnus frequencies is
a major pollen stratigraphic feature on British pollen
diagrams (McVean 1956b, Bennett and Birks 1990) often
rising very sharply to abundance after a lengthy period
of sporadic or low values although at some sites the rise
is much more gradual (Birks 1986). Local conditions must
be the controlling factor. Usually Alnus replaces
Pinus but at sites where Pinus was never
common taxa like Salix and Betula are reduced.
Birks (1989) maps the expansion of Alnus in south
and east England at c.8,000BP, spreading into central
England and Wales by 7,500BP. By 7,000BP Alnus
had spread into eastern Ireland and southern Scotland
and by 6,000BP it was common in almost all parts of the
British Isles except the western and northern fringes
(Birks 1989). Against this trend, however, are local centres
of early establishment and strong variation within regions
(Bennett and Birks 1990), often being much later in upland
areas than in adjacent lowlands. The rise of Alnus
pollen is therefore diachronous (Smith and Pilcher 1973)
and dependent upon site factors and alder's specific habitat
requirements as well as environmental trends such as a
change to an oceanic climate. Wetter climate and the spread
of lowland wetlands after c.7,5000BP certainly increased
the suitable habitats for Alnus and encouraged
its spread. Abundant Alnus macrofossils in river
valley floodplain and coastal carr peats reflects this
great expansion of alder in the mid- and later Holocene
(Brown 1988). In these sediments Alnus pollen frequencies
can be superabundant (Janssen 1959) reaching between 80%
and 100% of tree pollen. In the mid-Holocene a great reduction
in the rate of sea-level rise and marine transgression
caused a relatively slow rise in the ground water level
over large coastal areas, creating conditions suitable
for the development and maintainance of great areas of
alder fen-woods. The coincidence of charcoal and other
indicators of disturbance at the time of the rise of Alnus
pollen (Smith 1984, Chambers and Price 1985) suggests
that at some sites Alnus, which may be regarded
as a pioneer tree, was favoured by vegetation disturbance
by humans or even beaver. Alnus is light-demanding and
may not easily colonise closed-canopy deciduous forest
without disturbance. Human drainage of most of the lowland
wetlands of Britain has greatly reduced the abundance
of Alnus.
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