Paulson[1141], from his observations of lichens in Hertfordshire, has concluded that the presence or absence of lichens on trees is influenced to a considerable degree by the nature of the soil. They were more abundant in woods on light well-drained soils than on similar communities of trees on heavier soils, though the shade in the former was slightly more dense and therefore less favourable to their development; the cause of this connection is not known.
c. Lignicolous. Lichens frequenting the branches of trees do not long continue when these have fallen to the ground. This may be due to the lack of light and air, but Bouly de Lesdain[1142] has suggested that the chemical reactions produced by the decomposition of the bast fibres are fatal to them, Lecidea parasema alone continuing to grow and even existing for some time on the detached shreds of bark.
On worked wood, such as old doors or old palings, light and air are well provided and there is often an abundant growth of lichens, many of which seem to prefer that substratum: the fibres of the wood loosened by weathering retain moisture and yield some nutriment to the lichen hyphae which burrow among them. Though a number of lichens grow willingly on dead wood, there are probably none that are wholly restricted to such a habitat. A few, such as the species of Coniocybe, are generally to be found on dead roots of trees or creeping loosely over dead twigs. They are shade lichens and fond of moisture.
The species on palings—or “dead wood communities”—most familiar to us in our country are:
- Usnea hirta.
- Cetraria diffusa.
- Evernia furfuracea.
- Parmelia scortia, P. physodes.
- Xanthoria parietina.
- Placodium cerinum.
- Rinodina exigua.
- Lecanora Hageni, L. varia and its allies.
- Lecidea ostreata, L. parasema.
- Buellia myriocarpa.
- Cladoniaceae and Caliciaceae (several species).
These may be found in very varying association. It has indeed been remarked that the dominant plant may be simply the one that has first gained a footing, though the larger and more vigorous lichens tend to crowd out the others. Bruce Fink[1143] has recorded associations in Minnesota:
On wood:
- Teloschistes chrysophthalmus.
- Placodium cerinum.
- Lecanora Hageni, L. varia.
- Rinodina sophodes, R. exigua.
- Buellia parasema (disciformis), B. turgescens.
- Calicium parietinum.
- Thelocarpon prasinellum.
On rotten stumps and prostrate logs: Peltigera canina, Cladonia fimbriata var. tubaeformis, Cl. gracilis, Cl. verticillata, Cl. symphicarpia, Cl. macilenta, Cl. cristatella.
Except for one or two species such as Buellia turgescens, Cladonia symphicarpia, etc., the associations could be easy paralleled in our own country, though with us Peltigera canina, Cladonia gracilis and Cl. verticillata are ground forms.