Environmental Engineering Reference
In-Depth Information
James & Davies
2003
). Edward Forster (1765 1849) was the first lichenologist
to examine the area, and he made collections and notes mainly between 1784
and 1796 (Laundon
1967
) which are preserved in the Natural History Museum
(BM) herbarium and library (Hawksworth
1973a
). Collections made over a
300-year period show that numbers of lichen species recorded have varied
tremendously over this period. Lichen species declined when SO
2
concentrations
peaked, with the almost total extinction of lichens sensitive to air pollution and
woodland management (Rose
1976
; Rose & Coppins
2000
). However, the species
returning today are not all the same ones which are present in the BM herb-
arium. More species were found growing closer to London than in more remote
regions ( James & Davies
2003
; Davies
2005
).
In the winters of the late nineteenth and twentieth centuries when station-
ary high pressure systems settled over Western Europe, wind speeds fell and
temperature inversions formed. Pollutant concentrations increased and fog
became widespread in Britain, with London severely affected by these condi-
tions (Brimblecombe
1987
/88; NSCA
2002
). The famous smog which began on
5 December 1952 led to the abandonment of La Traviata at Sadler's Wells
theatre and death of cattle at Smithfield market, and within 12 hours large
numbers of people showed respiratory illnesses. This led to the Clean Air Act of
1956 and Smoke Control Areas. Partly through legislation, social, economic
and technological changes, concentrations of sulphur dioxide have fallen from
an annual high mean of 350
m
gm
3
in the 1970s to an annual average across
London in 2001 of 3
m
gm
3
with no exceedances of the EU objectives for health
or vegetation expected (Bell et al.
2004
). During peak concentrations, the city
of London was so polluted that it was termed a 'lichen desert' ('lavoknen').
This term was originally coined by Sernander (Sernander
1926
) where the
trunks in city centres around gas-manufacturing works, railway stations (steam
trains were in service) were devoid of macrolichens (Laundon
1973
). Laundon
recorded only nine lichens growing on trees within 16 km from Charing Cross
in central London and a single species in Central London (Laundon
1967
).
During the high SO
2
pollution climate prevailing in the 1960s and 1970s,
two-thirds of the lichen flora occurred in churchyards and cemeteries, which
is attributed to the pH buffering capacity of limestones and other calcareous
stones. At Mitcham churchyard in London, Caloplaca flavovirescens occurred
on over 80% of limestone memorials erected in the eighteenth century. The
absence of thalli on modern stones suggested pollution limited recolonisation,
i.e., representing a 'relict flora' (Laundon
1967
). Oliver Gilbert also suggested
that colonies of Parmelia saxatilis near Blyth, Northumberland, were of a relict
nature and perhaps represented a separate ecotype (Gilbert
1971
).
A significant improvement was recorded in the 1980s including species that
had not been seen in London for over 200 years (Rose & Hawksworth
1981
;
Gilbert
1986
; Purvis
1987
; Hawksworth & McManus
1989
). Oliver Gilbert
