Bulletin of the British Museum (Natural History)

LICHEN GENUS MICAREA IN EUROPE 93
Habitats in lowland pastoral areas
The lowlands of eastern England, the English south midlands and parts of eastern Scotland
contain few 'natural' habitats suitable for Micarea species. In addition, these same areas suffer,
at least to some extent, from the effects of incoming pollution (especially sulphur dioxide and its
derivatives) and indigenous pollution in the form of agricultural biocides and fertilisers.
Nevertheless, the diligent collector will usually discover suitable niches where Micarea species
are to be found. The most commonly encountered member of the genus in lowland agricultural
and suburban districts is M. denigrata which can grow on worked wood in a wide range of
situations, and on old tree stumps in hedgerows. M. prasina can be found on trees and shrubs in
small woodlands planted as fox coverts or for the rearing of pheasants. It may also be found on
sheltered stems of shrubs in waste ground, marginal land, or gardens, where M. nitschkeana may
also occur.
A major refuge for wildlife (including lichens) in lowland Britain is provided by churchyards
(Anon, 1973, 1978). Although records of Micarea species are rather few, M. denigrata can often
be found on timber and, more rarely, on siliceous memorial stones; and M. lignaria and M.
peliocarpa have been recorded on the siliceous stone-work of old walls and memorials.
Distribution
Britain
Maps 1-28 present the distributions of all the Micarea species occurring in the British Isles, with
exception of M. lithinella which is known from just one 10 km grid square (44/86). It is seen from
these maps that records for most species are concentrated in the western and/or northern
districts. This is partly due to the fact that suitable habitats are most numerous in these areas,
and it is probable that several species (e.g. M. bauschiana, M. lignaria, M. melaena, M.
peliocarpa, and M. sylvicola) would be more evenly distributed if there were more available
habitats in the lowland areas of the south and east. It is impossible to discuss in isolation the
effects of climate or substrate availability on plant distributions. Substrate availability is largely
dependent on topography and geology, but the physical and chemical nature of an existing
substratum can be much influenced by the prevailing climate. For example, the bark of trunks of
mature Quercus in high rainfall districts tends to be more leached, soft, and friable (i.e. more
suitable for Micarea spp.) than in low rainfall districts. As a second example the species of the
Micareetum sylvicolae (see p. 92) are ombrophobous (avoid frequent direct wetting), but are
also aerohygrophilous (require ± constant high humidity). In western Britain the high rainfall,
high incidence of cloud cover (i.e. low duration of possible sunshine), and incoming moist
air-stream from the Atlantic on the prevailing westerly winds result in high levels of relative
humidity (and low levels of saturation deficit) more or less throughout the year (see Climato-
logical Atlas of the British Isles, London: HMSO (1952)). Under such conditions a well
developed Micareetum sylvicolae can be found on underhangs in a wide range of lowland
habitats. Eastern districts generally have a lower rainfall and lower incidence of cloud cover
resulting in lower levels of relative humidity, such that the Micareetum sylvicolae tends to be
more confined to narrow river valleys with a long history of continuous tree cover. A western
bias to the present day British distribution of many lichens is often due to the fact that the eastern
side of the country is generally that most affected by industrial and urban development,
intensive agricultural practices, and the resultant forms of air pollution (Hawksworth, Coppins
& Rose, 1974; Coppins, 1976).
A climatically determined western distribution, attributable to the General Western Group of
Coppins (1976), is shown by Micarea adnata, M. alabastrites, M. cinerea, M. lignaria var.
species have a eu- Atlantic or sub-Atlantic
endoleuca, M. stipitata, and M. synotheoides . These
European distribution, and are confined to areas with an annual rainfall of over 800 m (mostly
over 1000 mm) distributed over at least 160 wet days ['wet day' = period of 24 h in which 1 mm
(or more) of rain is recorded]. The genus Micarea is poorly represented in southern (Mediterranean)
Europe, and it is not surprising that few British species exhibit a marked southern
tendency. The best example of such a species is M. pycnidiophora, which is mainly confined to