Environmental Engineering Reference
In-Depth Information
discovered Hypogymnia physodes with the 'pollution lichen' at Chelsea Physic
Garden in 1986, at that time the innermost record of a foliose (leaf-like) lichen
in central London (Gilbert
1986
). The specimen is now in the Natural History
Museum (BM) lichen herbarium. However, the order of species return did not
follow the order of loss ('zone skipping'), reflecting availability of colonising
sources and biological factors (Hawksworth & McManus
1989
). Clearly, pre-
viously defined indicator species, frequency data and zone scales no longer
reflected air quality changes. There were 38 species recorded in Regent's
Park in central London in 2001, including the new colonist, Flavoparmelia
soredians ( James et al.
2002
) on young oak trees, a species with an essentially
Mediterranean distribution (Rose
1995
). Ornamental cherry (Prunus spp.) trees
were colonised by healthy Lecanora conizaeoides.ASO
2
sensitive cyanobacterial
species, Peltigera neckeri, was found by an 11-year-old (and who identified it as
a Peltigera species) during a British Lichen Society field visit on 7 January 2001
to Brompton Cemetery, 5.3 km from Charing Cross (the accepted centre of
London). It was probably imported with chippings (Hawksworth et al.
2001
).
Subsequently, two quantitative studies (using different recording methods)
were carried out by Linda Davies (on ash Fraxinus) using fine ADMS dispersion
modelled pollution data and Ren´ Larsen (on oak Quercus) using monthly
average pollution concentrations and trees georeferenced with GPS. Trees were
selected in both studies away from roads where NH
3
and NO
x
concentrations
would be lower than those at roadsides (Cape et al.
2004
). Davies recorded
74 lichen species (and 14 moss, 7 fungal and 3 algal species) on ash (Davies
et al.
2007
). Larsen recorded 64 lichen species (and 4 bryophyte species) on oak
(Larsen et al.
2007
). In areas of highest NO
x
, species recorded belong almost
exclusively to the families Candelariaceae, Physciaceae and Teloschistaceae (Davies
et al.
2007
). Both studies established correlations between frequency, NO
x
and bark pH, which suggested that transport-related pollution and bark pH
influence lichen and bryophyte diversity today. Hypogymnia physodes, the first
macrolichen to be recorded in central London (Gilbert
1986
), was virtually
restricted to outer London, apart from some of the larger parks, as at
Wimbledon Common and Hyde Park. It is no longer present in Chelsea Physic
Garden. Spatial distribution patterns in both Larsen's and Davies' studies
suggest phytotoxic effects and critical level exceedances for NO
x
as shown
previously for SO
2
. Although there has been a dramatic decrease in SO
2
concen-
trations, emissions of oxides of nitrogen (NO
x
) in London have changed very
little over the period since Laundon's pioneering study (Laundon
1967
,
1970
).
However, the sources and emission heights are now quite different (GLA
2002
).
Historically, NO
x
was emitted at chimney height and was a product of domestic
and industrial coal burning. Nowadays, NO
x
in London is mainly emitted from
vehicles at ground level where dispersion is often impeded (Davies et al.
2007
).
Climatic factors, although not yet quantified in London in relation to lichen
