I’m now starting to think that lichenometry might be one good link between the geology and ecology of the Himalayas. Because lichens can obtain everything they need to grow from the air around them, and the water it contains, they are the ultimate pioneer species. The hyphae of the fungal partner in the relationship provide a framework to shelter the more fragile algal cells. These algal cells photosynthesise and, in turn, supply carbohydrates to allow more fungal growth. Other necessary minerals can be acquired in trace amounts from rainwater and from the imperceptibly-slow dissolution of the substrate on which the lichen is growing. This allows them to grow on bare stone, as well as on more hospitable surfaces such as soil and tree bark.
Crustose lichens growing on old volcanic rocks above Ullswater, English Lake District
Foliose lichen (Usnea sp) on tree bark
Lichens’ growth strategy means they can survive in the most hostile of terrestrial environments but they trade this advantage off against very limited potential for rapid growth in more favourable conditions. Victims of their own success, as soon as a little anchorage is produced and a rudimentary soil forms, other more raypidly growing species, such as grass and, in this case bilberry (Vaccinium myrtillus), take hold and rapidly out-compete them.
However, because lichens grow at a rate of a just a millimetre or so a year, measurements of their growth can be used by geomorphologists, as well as archaelogists and paleontologists, to indicate how long something has been exposed at Earth’s surface. Lichenometry is particularly useful for dating surfaces less than 500 years old, where radiocarbon dating is less accurate, and one application is in measuring the rate of glacier retreat, particularly pertinent in a warming world. The possibilities of applying this in a Himalayan context would definitely be worth investigating on our trip.
Map lichen, Rhizocarpon geographicum, often used for lichenometry.
[…] This beautiful orange lichen, a relative of the bright yellow Xanthoria parietina which increasingly covers rocks and trees along roadsides in the UK, is found at high latitudes and altitudes, on both acid and basic rocks, all around the world. Whereas X. parietina is flourishing because of its tolerance to sulphur and nitrogen oxides in our atmosphere, X elegans survives a different set of stresses. It is orange because the cells of the algal partner are rich in carotenoids (principally mutatoxanthin), which both act as accessory pigments to make photosynthesis more efficient and protect the cells against damage from too much sunlight. The lichen is darkest in colour when it grows in particularly dry or exposed spots and can survive in extreme desert conditions, where there may be as little as 60 mm of rain a year. Because colonies grow at a slow, constant rate once established, it makes a good candidate for lichenometry, along with our old friend Rhizocarpon (see “The wonderful world of lichens“). […]
[…] Rather like lichens, where the fungus harbours algal cells which supply it with carbohydrates (see The wonderful world of lichens), legumes host a bacterium (often Rhizobium leguminosum) in a symbiotic relationship. These […]
[…] thing that does give trees colour at this time of year is the abundant yellow lichen cladding many branches. This is probably Xanthoria parietina, which has increased dramatically over […]