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- Cladonia sp, foto fitis, sytske dijksen
Lichens
Lichens are strange organisms. As a famous lichenologist (lichen scientist) once said, “Lichens are a case of fungi that have discovered agriculture”. By agriculture, he was referring to the plant part of the lichen, the green algae. The fungi provide a home and retain water, while the algae provide food. Fungi make thread-like structures encapturing the algae. Being a plant, the algae can produce energy from the sun via photosynthesis. Sometimes cyanobacteria take the place of the green algae. The organism formed is neither fungus or algae, but a lichen. The most common group of lichens found in the dunes are Cladonias, such as caribou moss. Thirty of the forty Claudonia species found in the Netherlands grow in the dunes. There are even species that grow on sea dikes and tolerate salt spray. More than 600 species of lichens are found throughout the country.
On Texel
Texel is rich with lichens: in the dunes, on dike stones, on walls and trees. Lichens prosper so well on the island because there is no polluting industry and therefore relatively clean air. They are particularly obvious in the winter when most other growth has died.
- Lichens as environmental indicators
- Red pixie cup, foto fitis, sytske dijksen
Lichens can grow in extreme conditions such as wind, drought and storm, but are very sensitive to air pollution. They have no roots and live almost literally from the rain and air. You aren't bound to find many lichen species in areas where factories emit sulfuric pollutants. At the end of the 1980s, many such factories in the Netherlands started placing filters in their chimneys to remove the sulfur. Since then, some species have returned. Climate change since the 1990s has caused a decline in a number of northern species, while several southern species are now found in the Netherlands.
However, another problem is nitrogen. Nitrogen deposition helps moss and grass species to grow more abundantly, displacing lichens. Lichens need bare ground to grow and when moss and grass spread, bare spaces disappear. The spreading of heath star moss in particular is a major threat. Studies of lichens growing on wind-blown sandy areas in the Netherlands have shown that the influence of nitrogen deposition is detrimental to lichens, particularly the rarer species. Because the air is cleaner in areas along the coast and on the Wadden Islands, lichen species in sandy areas are less affected here. Momentarily, it's areas more inland where smaller and rarer species are disappearing.
- Growth forms and reproduction
- lichens on stone, foto fitis, sytske dijksen
Instead of leaves, roots and stems, lichens have thalli. There are a number of characteristic growth forms: leafy, crusty, scaly and branched are the most common forms. Lichens can reproduce sexually as well as asexually. The fungus produces spores just like mushrooms and must land where there is also a suitable alga. The more common methods are vegetative, such as small broken off pieces of thallus that are transported by wind or water to a new place.
Lichens grow very slowly. They need sunlight but are easily overshadowed by higher plants which grow more rapidly. That is why lichens grow where other plants have more difficulties, such as on rocky mountainsides, bare dunes, dikes, sidewalks and even on branches or trunks of trees. Some people think that lichens kill the tree but they only use it as a base. After all, they don't have any roots that extract nutrients. The reason sickly or dying trees seem to be covered in more lichens than healthy trees is related to the increase in available sunlight.
Species that grow on rocks grow very slowly, only several millimeters per year, but they can live for centuries. Other species grow faster, up to 3 centimeters per year, but don't get as old. The growing season for lichens is generally late in autumn.
- Gray dunes
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