The Essex Naturalist
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- , 1955. Das Phytoplankton des Siisswassers. Vol. 16, part 4. Euglenophyceen. Schweizerbart,
Stuttgart.
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Schweizerbart, Stuttgart.
Komarek, J & Fott, B, 1983. Dos Phytoplankton des Siisswassers. Vol. 16, part 7. Chlorococcales.
Schweizerbart, Stuttgart.
Scourfield, D J, 1944. The nannoplankton of bombcrater pools in Epping Forest. Essex
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Chlorococcales. Essex Naturalist 31: 193-198.
Lichens
J F Skinner
Southend Central Museum, Victoria Avenue, Southend-on-Sea, Essex SS2 6EW
The last five years have been most interesting from a lichenological point of
view, with the rapid spread of several conspicuous species as well as a steady
increase in the number of smaller species found. At the same time, certain
developments give rise to concern for our county's lichens.
In the late 1980s, a large grey foliose lichen, Parmelia perlata, was first noted
in the county as isolated thalli on beech trees in Epping Forest (Hawksworth &
McManus 1992), on ash in Hatfield Forest and on willow at Canewdon. This
lichen is common in well-lit situations, mainly on trees, throughout much of
lowland Britain where mean sulphur dioxide levels are below about 35μgm-3.
Levels of this gas have been falling slowly but steadily for the last twenty years
and it seems that a concentration has now been reached at which P. perlata can
thrive. This phenomenon is not confined to Essex but has been observed over
much of central and eastern England. Other pollution-sensitive species have
also done well in these five years. P. caperata, a common lichen in unpolluted
areas and with similar habitat requirements to P. perlata, was, until 1989, only
known from 13 sites in Essex, mainly as isolated thalli on trees in parkland
situations and the very sheltered Roman River Valley (Skinner 1983). In 1989,
Peter Earland-Bennett found it on willow in Wickford town centre and in the
Billericay area. It now turns up regularly, especially on willows, and Tim Pyner
has found it at a number of places during bryophyte surveys.
A smaller and less conspicuous Parmelia, P. revoluta, has also increased in
abundance. With Peter Earland-Bennett, I found it completely coveting the
east-facing side of an ash trunk in Cockaynes Wood, Alresford (TM054218), in
1994.
It should be mentioned that, although the improvement in air quality in
south-east England is most welcome, there has been, over much of north-west