He notes that fitness and how the lichen partners work together are dependent on environmental conditions. Usually, once a lichen association has been established the mycobiont does not switch partners. In this case the fungus associates with a cyanobacterium in shady, humid conditions to form small, shrub-like thalli.
However in drier or more exposed conditions, the fungus associates instead with green algae to form large, flat lobes. Mosses are also not lichens , according to the Forest Service. Though at first glance some may superficially resemble a lichen, mosses are actually primitive versions of plants and are capable of independent photosynthesis.
Lichens are key players in a variety of environmental processes. For example, cyanobacterial photobionts participate in nitrogen fixation. Lichens also contribute to a phenomenon known as biological weathering. The lichen mycobionts can break down rocks and release minerals by producing certain chemicals. Lichens can also disrupt rock surfaces simply by physically attaching to them, and by the expansion and contraction of their thalli, according to a article published in the journal Catena.
Weathering can lead to the eventual disintegration of rocks, according to the article. While this is a disadvantage, especially when lichens grow on building stones, it is also an essential step for the formation of primitive soils.
When lichens decompose, the organic matter that is left behind, along with particles of rock and dust trapped by thalli provide material for the development of primitive soils. The lichen species Cladonia rangiferina , commonly called reindeer lichen, are an important source of winter forage for most North American caribou populations and key components of a winter diet except in areas with shallow snow cover or that have mild winters according to the Forest Service.
Lichens can be useful for creating dyes, indicating air and substrate quality, and providing a food source for reindeer and caribou on the Seward Peninsula. Check out some of the most common lichen genera you'll find around Bering Land Bridge! Cetraria are small to large foliose lichens with narrow or channeled lobes. Often yellow, brownish, or blackish, these are extremely common on the tundra and an important element in reindeer and caribou diets.
More commonly known as reindeer lichens, Cladina species vary widely and cover a wide range of land and habitats. These are typically shrubby and vary in color ranging from white, green and yellow. As the common name suggests, reindeer lichens are an important food source for local caribou and reindeer populations.
They are found in abundance throughout the tundra. Not to be confused with Cladina, Cladonia lichens are stalk-like, and usually best identified by their cap of red fruit or their cup-like structure. Be aware though, these can be tricky -- all lichens with these characteristics fall under the Cladonia genus, but not all Cladonia have red fruit caps or that cup structure!
They are not attached by their entire lower surfaces to their substrates. Indeed, some foliose lichens are just centrally attached to their substrates with the rest loose, so making it possible to see both the lower and upper surfaces very easily.
Xanthoparmelia substrigosa below is an example. Those three growth forms will account for the majority of genera that most people are likely to see. It is also worth mentioning the concept of a squamulose lichen since the genus Cladonia is very widespread and often shows a squamulose growth form.
But the species in the genus also produce upright fruticose structures called podetia - sometimes with the appearance of fairly simple stalks, sometimes flared at the apex and so presenting a somewhat trumpet-like form. In this photo you can see a Cladonia colony growing on soil. There are numerous squamules on the soil but you can also see a number of the upright podetia with broader apices.
Moreover, at the margins of some of the broader apices you can see additional podetia developing. Here is a closer view, showing an enlargement of part of the previous photo.
You can also see that the podetia themselves also have flake-like squamules. Cladonia is not the only squamulose lichen genus, just a very commonly seen one.
A colony of a squamulose lichen looks like a scattering of small flakes or scales on the substrate. A byssoid lichen has a somewhat wispy appearance, like cotton-wool teased out to some degree. Leprose lichens have a powdery or granular appearance. You can find combinations of growth forms in some lichens. For example, some species are crustose centrally but somewhat foliose at the margins. Just in case you're interested, such a lichen is called placodioid or placoid and Placopsis perrugosa is an example.
In most cases a species will always have the same gross morphology but a number of species are known to show some plasticity. This box gives one example. Usually Siphula coriacea is characterised by erect, bluish grey lobes, shown here. The species is known from mainland Australia and New Zealand, in heathlands and grasslands. In moister, sheltered areas the lobes may be over a centimetre in length but in the drier rangelands of inland Australia the lobes are markedly shorter, often only a few millimetres in length.
An unusual form has been found in Idalia National Park, western Queensland. The thalli consist of more or less circular disks, rather than of erect lobes.
The discoid thalli, shown here , are from two to eight millimetres in diameter and, though different in gross morphology, match the usual forms of this species in both chemistry and micro-morphology. Typical forms of Siphula coriacea were found elsewhere in the same general area. All of these expressions may be called usefully imprecise descriptive terms. They are useful in the same way that expressions such as shrub and tree are useful when talking about plants.
They are imprecise in that sometimes it may be difficult to place a particular specimen in a particular growth form 'pigeon-hole'. Similarly, occasionally you may wonder which of shrub or tree is the better term to describe a particular plant.
What of the byssoid lichens? Logically you could argue that a byssoid growth form is three dimensional as is a clump of cotton wool and so a byssoid lichen is really just a very delicate fruticose lichen. Some lichenologists consider squamulose and placodioid forms as simply variants of crustose.
As you can see there is a variety of terms more than listed above and some debate over the boundaries between them. It is useful to be aware of these issues, since different books or websites may use some terms in slightly different senses, but there is no point in getting bogged down in terminology. For most purposes it is enough to be comfortable with the terms crustose, foliose, fruticose and squamulose as defined above.
In general a particular species will show the same growth form, no matter where it grows. Occasionally, for some reason perhaps genetic, perhaps environmental , a species that is usually, say, crustose might grow in a fruticose form. Such occasional, but dramatic, differences in growth form in the one species are well-known to many gardeners.
A plant species that usually grows as a tree may be found growing in, say, a prostrate form. Often such plant variants are highly valued horticulturally and propagated vegetatively to preserve the variant form and sold as cultivars of the species in question. You may come across the terms macro-lichen and micro-lichen.
These are two more examples of usefully imprecise terms. Roughly speaking a macro-lichen is one that is foliose or fruticose and the rest are micro-lichens. Note that this has nothing to do with size, despite the impression given by the prefixes macro and micro. Last, but not least, the interior of lichens is often a place richly infused with complex secondary fungal chemicals found nowhere else in nature, and these compounds are likely to play a role in protection from UV radiation, desiccation, and grazing by herbivores as well.
However, there are also good arguments in favour of the controlled parasitism camp. Up to half of the carbon fixed by algae is immediately converted to fungal sugars which are inaccessible to the alga itself. In fact, it is thought that many early stages of developing lichen spores may survive using such a parasitic or saprophytic strategy.
Lastly, there are many lineages of lichen fungi that are parasitic on other lichens — the so-called lichenicolous lichens! In some cases, non-lichen fungi have evolved from lichenised forms. These can be specialised opportunistic parasites or saprophytes or even symbionts, competing for nutrients with other fungi in the lichen thallus. The symbiosis may be more complex than this. Recent work by Spribille et al has found yeasts embedded in the cortex of ascomycete macrolichens, and their abundance correlates with previously unexplained variations in phenotype.
There is also convincing evidence for a consistent presence of non-photosynthetic bacteria within the thalli of all lichens, although the role of these bacteria is as yet unknown.
Interestingly, a role for non-photosynthetic bacteria was suspected for many years, as the relichenization of separately cultured fungi and algae in the lab was facilitated by the presence of bacteria.
In fact, a legacy of exclusion from accepted mycological research persisted until the s, despite their obvious affinities with non-lichen fungi. With the advent of molecular biology, the shared history of lichens and non-lichens has been elucidated and accepted , and we now know that the fungi that form lichens have evolved from many only distantly related lineages across the fungal tree of life, uniting them and their non-lichen relatives in the Kingdom Fungi.
Lichen fungi are a heterogeneous group; they are similar only ecologically, in that they share the nutritional strategy of gaining carbon from an internal symbiotic photosynthetic partner, the photobiont. In the study of lichens, the name and classification belongs to the fungal partner, which in most cases is the dominant member of the association, at least in terms of biomass.
Lichen fungi have evolved independently several times within the mushroom-forming fungi and relatives the basidiomycetes , but much more commonly, from within the cup fungi the ascomycetes. Probably more than ten distinct major lineages of fungi within the ascomycetes are lichenised. Current estimates suggest that one fifth of all known fungi and half of all ascomycetes are lichenised, with about 28, species worldwide.
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