New Zealand's array of lichen species represents about 10% of the world’s known lichens.
A lichen is an amalgamation of a fungus and one or more photosynthetic organisms (those that make food from sunlight, water and carbon dioxide). No fungi can make their own food, but they have evolved ways to barter or steal it from other living things. A fifth of the world’s fungi do that by partnering with photosynthetic algae and/or cyanobacteria – their partnership is called a lichen.
Lichens function similarly to green plants. Historically they have been treated as plants and studied by botanists.
Lichens are made up of at least two organisms, a fungus and either a green alga or a cyanobacterium (sometimes both). The lichen’s body (thallus) is built by the two organisms working together. How they do that is still poorly understood. Most of the thallus is made of the tangled, thread-like strands of the fungus. The algal partner is usually restricted to a thin layer of single cells below the upper surface of the thallus.
Lichenised fungi receive sugar from their photosynthetic partner. The alga or cyanobacterium within the thallus is protected from the outside environment, and receives water and minerals from the fungus. This mutual aid improves the survival of both partners in habitats where neither could easily survive alone. Most lichens can be found growing on soil, rock, tree bark, and leaves, but some have managed to colonise man-made surfaces such as asphalt, glass, plastic and metals.
Lichens can survive in temperatures that would kill most other creatures. Desert-dwelling lichens can endure heat up to 70°C. In laboratories, they have survived being dry-frozen at close to absolute zero (-273°C), and in Antarctica they can photosynthesise at -23°C and overwinter in total darkness at -70°C.
Lichens thrive in places that unpredictably alternate between being wet and dry. They have a competitive advantage over most land plants, as they shut down their metabolism during dry spells. When the rain comes, they absorb water readily and restart their metabolism.
Many lichens produce unique chemical compounds called lichen substances. Some are brightly coloured sunscreens that protect the lichen from ultraviolet light, while others are poisons that fend off grazers such as slugs and insects, or invaders like bacteria and rot fungi. Some protect against sulfur and other air pollutants.
Lichen bodies have three main forms:
Most lichenised fungi belong to the class of cup fungi (Ascomycetes), and reproduce by shedding fungal spores from splash cups. Splash cups are often brightly coloured and range from thumbnail-size to so small you need a hand lens to see them. When a raindrop hits the cup, the spores bounce out, are lofted into the air, and land faraway. The germinated fungal spore must find an algal partner quickly to form a new lichen.
Most lichens also reproduce without spores. Many shed tiny bits of thallus containing both fungus and alga, so a new lichen can grow without the need for the fungus and alga to find each other first.
In New Zealand, lichens are found on all types of land, from the coast to high-alpine rocks. There are thought to be over 2,000 lichen species, slightly more than the number of native seed plants (about 1,900). But it will be years before the exact number of lichens in New Zealand can be known, because many are too small to be seen easily, and too few lichenologists are searching for them.
Relatively few New Zealand lichens are endemic (meaning that they grow nowhere else) – less than 40% are unique to New Zealand, compared to 84% of the country's seed plants.
The aggressive lichen Xanthoparmelia scabrosa colonises the edges of asphalt roads and carparks in the wetter parts of New Zealand, forming blue-green rosettes. It also takes hold on roof tiles.
Most lichens are killed by sulfur spewed from car exhausts, but Xanthoparmelia scabrosa makes lichen substances that react with the sulfur and render it harmless. This solution is so successful that in places the lichen blankets the entire road.
The trunks and branches of apple and plum trees are sometimes host to clumps of the harmless yellow-orange Teloschistes chrysophthalmus.
Byssoloma adspersum usually grows on rock, but also thrives on timber preserved with CCA (copper, chromium and arsenic), where it leads a charmed life free of competition from other lichens.
The bright yellow-orange Xanthoria ligulata is a common crust-forming lichen of coastal rocks, but it is equally at home on gravestones and exposed slabs of concrete. In shade, its colour changes to a dull blue-grey. In full sunlight it produces an orange pigment that is thought to act as a protective sunscreen for its algal partner.
Placopsis parellina var. microphylla lives on coarse soil. It secretes a glue that binds together sand, gravel, and loose soil particles, and it stabilises riverbeds in Canterbury and Otago, where most plants are washed away during flash floods.
A number of fruticose lichens can be found on ground that has lost its natural cover, such as the banks of roads and tracks that have been cleared or where slips have occurred. Pixie cup lichens (some species of Cladonia) and Stereocaulon can often be found growing in thumb-high tufts at such sites.
Large foliose (leaf-like) lichens are commonly found on tree trunks. The largest are species of Pseudocyphellaria and Sticta, which can grow to the size of an A4 sheet of paper. Another large foliose lichen, Menegazzia pertransita, grows on the trunks of beech trees in wet Fiordland.
On the forest floor, Trapeliopsis congregans flourishes on decaying logs and stumps. Psoroma asperella settles on mosses, and the crustose Thelotrema novaezelandiae overruns liverworts.
New Zealand forests, like tropical rainforests, have a number of lichens that live on the uppersides of leaves.
New Zealand spinners who dye their wool prize the lichen Pseudocyphellaria coronata. Compared with brightly coloured synthetic dyes, lichen dyes, some of which are pleasantly fragrant, yield a wide range of subtle colours depending on what mordant is used.
Light penetrates shrublands, allowing crustose lichens such as Haematomma alpina and the fruticose Usnea species to thrive on the branches and stems of subalpine shrubs.
Subalpine bogs and exposed, damp peatlands have a characteristic lichen cover which is dominated by coral lichen (Cladia retipora) and similar looking fruticose species belonging to the genera Cladina and Cladonia.
Crustose and fruticose species abound in the mountains. If you tramp above the treeline anywhere in New Zealand, you will come across the rock lichen Rhizocarpon geographicum. Climatologists can track global warming by measuring how quickly this lichen invades rocks freshly exposed by melting glaciers.
Rust-coloured Placopsis lateritioides is also at home on alpine rocks, where it forms tough crusts.
Malcolm, Bill, and Nancy Malcolm. The forest carpet. Nelson: Craig Potton, 1989.
Malcolm, Bill, and Nancy Malcolm. New Zealand lichens. Nelson: Micro-Optics, 2000.
Malcolm, W. M., and D. J. Galloway. New Zealand lichens: checklist, key and glossary. Wellington: Museum of New Zealand Te Papa Tongarewa, 1997.