142
THE ESSEX NATURALIST
Although Basildon started to implement smoke abatement in 1957 (and
to-day the major part of the area is controlled) it is undoubtedly domestic
heating and industry which are the main sources of the high levels of
sulphur dioxide recorded for the town. Together with low level emissions
from surrounding urbanised and industrialised areas and probably also
'fall-out' from high level emissions (i.e. from tall chimneys, especially from
power stations, which have been responsible for the large increase in
recent years of high level emissions), the rural areas are also affected. This
can be seen from the sulphur dioxide levels at the rural sites of Loft Hall,
Orsett and Begrum's Farm, Mountnessing (see table).
The lichen flora of these rural areas correlates fairly well with the
pollution scales of Hawksworth & Rose (1970), where epiphytic lichens are
used as indicators (due to their extreme but slightly differing suscepti-
bility) of different levels of air pollution. However, the lichens present both
on hypertrophicated (e.g. near farms) and on non-hypertrophicated mature
trees in open situations, are indicative (i.e. following Hawksworth & Rose)
of slightly higher sulphur dioxide pollution levels than actually occur. For
example, on hypertrophicated bark in the least polluted areas (about
70μg/m3 SO2, or zone 4 of the scale of Hawksworth & Rose) the species of
lichens present are indicative of zone 3 (=about 125 μg/m3 SO2). The
probable reason for this is that there are pollutants other than suplhur
dioxide, of which the most likely are agricultural chemicals or other
gaseous material. Hawksworth & Rose (1970, p. 146) stress that, among
other things, such pollutants will cause discrepancies.
Over virtually the whole area, the only lichen present on trees is Lecanora
conizaeoides. Most other epiphytes occur only very rarely and mainly in
the rural areas in the very northern part of the area on such hedgerow trees
or stumps as Salix, Ulmus and Fraxinus and in fairly open woodland e.g.
Thrift Wood, Bicknacre. The proximity of water, especially if accompanied
by small variations in topography, even depressions in fields, provides
another valuable microclimate enabling species to survive. The most
diverse lichen flora with some of the rarest species occurs especially on the
inclined trunks of Salix in these conditions. More typically, however,
epiphytes have retreated to the boles of trees, and the survival of a number
of species on old tree stumps overhanging ditches is characteristic of this
area and other parts of Essex I have studied. Several rare species have been
found in Thrift Wood, Bicknacre, which is in dire need of further study.
It is in this region as a whole (north of the area studied) that the epiphytic
lichen flora becomes richer, due to the lower sulphur dioxide levels in the
air (Witham is included in the table to indicate this) and also because of
the richness of woodland and old trees around such areas as Danbury
Common, Woodham Walter Common and Braxted.
Since there are no naturally occurring rock outcrops in this area (which
consists mainly of London Clay with outliers of later Eocene sands form-
ing the low hills together with glacial and post-glacial sands and gravels)
the only saxicolous substrates are those created by man. By far the most
important of these lichenologically, are the gravestones in the older church-
yards, and usually the churches themselves. Many of these are made from
Jurassic limestones and retain a 'relict' flora (cf. Laundon, 1967). 62% of