Case Studies

Cladonia furcata

A Cladonia thallus develops initially as a squamulose growth form (referred to as the primary thallus) and the primary thalli later produce upright stems, the podetia, as fruticose secondary thalli. The podetia may be simple stalks with apical apothecia or they may be much branched, depending on species and, again depending on species, the original squamules may persist or disappear to leave just the podetia. Many ground-inhabiting lichens are susceptible to damage by trampling and the fruticose lichens are especially susceptible since the thalli are upright and, whether simple stalks or complexly branched, are therefore easily broken when trodden on. By contrast the crustose lichens suffer least, especially if they are growing on trampling resistant surfaces such as concrete or bitumen. Crustose lichens on soil would tend to suffer more, simply because the soil may be broken up by heavy foot traffic.

The paper by Lange and others that is listed below documented the growth of a lichen species growing in the 1 to 2 centimetre wide gaps of a cobblestone footpath in the Würzburg Botanical Garden (northern Bavaria, Germany). The path in question was the main thoroughfare in the botanical gardens and was used by over 30,000 people each year as well as by the gardens' maintenance vehicles. The lichen had a squamulose growth form, the individual squamules no more than 1 to 2 millimetres in diameter and growing densely to look like a small greenish carpet. The heavy foot traffic ensured that many squamules were broken though in the more protected areas, such as at the path edge, branched podetia could develop. However, none of these were more than a few millimetres long, most were damaged and neither mature podetia nor apothecia were ever seen along the path. It was impossible to determine the species from the squamulose thalli growing along the path so some thalli were transplanted to a protected sand-bed nearby. The first podetia started to appear after three months and after a year of undisturbed growth the podetia had an average length of about 13 millimetres, with the longest a little over 18 millimetres. The average number of branchings per podetium was three, with a maximum of six. After another 8 months there were irregularly branched podetia from 20 to 30 millimetres high and it was possible to identify the species as Cladonia furcata.

Normally Cladonia furcata has a non-persistent primary thallus and the podetia develop rather quickly, within a few months of the start of squamule development. Often rudimentary podetia can be seen on young squamules and the podetia may persist for several years before they finally develop apothecia. A paper published in 1917 reported the results of in situ studies into growth rates and recolonization. As part of his project the researcher cleared small quadrats of all vegetation and then observed them over several years to see what happened. Here is his account of one of the quadrats:

...a soil quadrat 1 m. square on high ground in an open wood, denuded 5 cm. deep, removing all plant parts, except roots of seed plants which extended deeper into the soil and could not be pulled out. The quadrat, before being denuded, was covered by lichens and mosses, with about 85 herbs and 2 seedling trees scattered within its limits. Cladonia furcata was the only abundant lichen. Cladonia pyxidata was present in several small patches, and a small cluster of Cladonia mitrula was seen. After 1 year 36 clusters of Cladoniae were noted within the quadrat, all so small as to be visible only on careful observation and too rudimentary to be determined. Herbs, mosses, and seedling trees were also appearing. After 5 years the quadrat had reverted to the condition found in the surrounding area so far as the lichens were concerned. Cladonia furcata was again dominant and of the same size and appearance within as without the quadrat.

Thus Cladonia furcata can colonize bare ground and produce well developed podetia within a half dozen years. After spore production the lichen dies and it has been estimated that the maximum life span is about 8-10 years but there is some evidence to suggest that sterile thalli may live longer. A similar phenomenon is known in a variety of flowering plants which may live in a non-flowering state for many years or decades but which die very soon after producing seeds.

The path in the Würzburg Botanical Garden had been colonized by Cladonia furcata sometime between 1977 and 1981 and by 1997 there were several hundred thalli along the path. The persistent, compacted squamules were well adapted for surviving conditions of constant trampling. However, the transplant experiments showed that the ability to produce podetia and spores was still present and was suppressed only while the thalli were being trampled. Lange and his co-authors suggested that the ability of Cladonia furcata to survive for many years in squamulose form, yet still produce podetia and apothecia readily may be due to the fact that in this species (in contrast to other Cladonias) the sex organs are differentiated during podetial development or at the top of a mature podetium. Therefore trampling, while it may destroy young podetia, does not disrupt any sexual processes.

References

Fink, B. (1917). The rate of growth and ecesis in lichens. Mycologia, 9, 138-158.

Lange, OL; Green, TGA & Türk, R. (1998). An unusual growth form of Cladonia furcata: the trampling-resistant primary thallus colonizing a paved pathway. The Lichenologist, 30, 583-595.