A Lichen Invader

There is a great deal of concern regarding invasive species.  However, when most people think of invasives they usually think of plants or animals. Did you realize that there is a recently introduced lichen that is rapidly expanding its territory in southwestern British Columbia? 

This invading species is the Maritime Sunburst Lichen (Xanthoria parietina). It was first recorded from the province about 15 years ago, from the Fraser Valley. The 1994 British Columbia Ministry of Forests publication The Lichens of British Columbia does not list it as occurring here because the first record of Xanthoria parietina in BC did not occur until about 5 years after this book was published. Now the situation has changed considerably. This is no longer a rare species, but a common one that is doing what invasive species usually do. It is rapidly expanding its range. It now grows as far east as Hope, and as far west as Qualicum Beach and Parksville on Vancouver Island, as well as on Galiano Island. It is well established in Steveston, and at Ambleside Park in West Vancouver. One area where this lichen does not appear to be introduced is within the city limits of Vancouver itself. It is reported to be sensitive to air pollution, and that may explain this seeming anomaly.

Some significant questions are, where did it invade from, and how did it get here? As with so many questions regarding origins, these are unknowns, but there are some quite logical explanations. This is a common species in eastern North America and Europe, and it has been known for some time from the Pacific Coast states to the south of us, where it is probably not native. It grows on deciduous tree trunks and branches, and occasionally on concrete. The vast majority of them grow on planted trees in urban areas, with only a few observations on native alder trees. Most likely the Maritime Sunburst was growing on horticultural trees which were brought from one of these three areas. When a living tree or plant is transported from one area to another, it is not just the plant that is transported. There is a whole ecosystem of small organisms, most of them microscopic, that come with it. 

Xanthoria parietina is a very showy distinctive lichen, so it is fairly easy to spot at a distance and to note how fast it is spreading. In open sunny areas, and most street trees are in open sunny areas, it is bright orange. These are large leafy, wrinkled lichens, and in a good growing site, may be up to 15 cm. across. They are usually much smaller than this, because they eventually make contact with others, so that large areas of tree trunk may be entirely orange in color. In shade they take on a yellowish gray tint. The difference in color is a good example of an organism protecting itself. The orange is a pigment that protects the lichen from ultraviolet light. In the shade less of it is produced, and so a more yellowish tone is produced. On closer inspection you will see that the lichen surface has several tiny saucer-like discs on it. These are reproductive structures. They produce spores which can be carried by air currents to other trees. Whether these spores successfully establish new lichens on other trees in our area, is another unknown.

Xanthoria parietina 

 Photo by Rosemary Taylor 

Is there a problem? Nobody knows, but there is certainly a problem for other small lichens that grow on tree trunks. The trunk of a tree is an ecosystem of small organisms - lichens, mosses, liverworts, algae, as well as the insects and mites associated with them. The Maritime Sunburst Lichen rapidly overgrows the small lichens around it. Very little research has been done concerning micro-ecological interactions such as this. 

If you live in a community close to Vancouver investigate the urban trees in your neighbourhood. The Maritime Sunburst is probably there, or will be there in the near future.