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.
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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.
