Species name: Rhamnus cathartica
Common name: European buckthorn, common buckthorn
Location: Ontario
The European buckthorn is native to (you guessed it) Europe, but also north Africa, and all the way east towards western Asia. It was introduced to North America in the early 1800s, and has since become a menace. The fruit start as green unripe berries, turn a pinkish-red (if you catch them that way; that colour doesn't last long), then a very dark black. These are a favourite fruit of birds, and it turns their poop an awful black-purple colour that is an absolute disaster to get off a white car. Interestingly enough, buckthorn seeds require the digestive and abrasive action of a bird's "waste management system" in order to germinate; unlike many tropical plant species that require the same germination mechanism, it can be any bird species. This shrub (or sometimes tree, depending on how much light and space it is given) is a great example of the idea that just because a bird can eat it doesn't mean a human can; the berries have a very strong laxative effect, and if enough are eaten they can cause severe dehydration (an effect that was exploited in the 18th and 19th centuries medicinally; it is no longer used for this purpose).
A couple of years ago the City of London did a tree survey based on aerial images and citizen reporting, to determine not only the percent cover of trees in London, but also the species of tree. I still don't quite understand how this worked with aerial images, since the resolution can only be so good; apparently they used some sort of combination of infra-red, ultra-violet, and true colour images to get a rough estimation of the species of tree. Sometimes the actual species could be distinguished, other times it was a vague common name like "oak tree" (which could apply to about 15 different species, some native and some not). The results were shocking, and yet they don't seem to have had any impact on species planted by the City. The top "tree", by leaf cover and by number of individuals, was the buckthorn (this is probably both glossy buckthorn and common buckthorn). The next tree was the Norway maple, another non-native and invasive species, and the tree rounding up the top three was either the sugar maple (number of individuals), or the black walnut (leaf area). Despite most of the trees in London being non-native and invasive species, there were two trees planted on my street this summer because of the ash tree purge (nothing like cutting down a native North American, healthy tree to replace it with a non-native invasive species): Norway maple and American elm (which will likely either succumb to the Dutch Elm Disease, or die from salt exposure; they are not salt-tolerant and so don't serve well as boulevard trees in a cold-temperate climate!). I still don't understand why we can't start planting trees that are not only native to our area, but also suitable for the given habitat. I can't possibly see how they're too expensive; you go out into a forest, collect some seeds, and put them into soil in a greenhouse. Free trees!
If the fact that buckthorns are invasive wasn't enough, to add to that they're also the alternate host for some of North America's most devastating agricultural diseases. They harbor the soybean rust and crown rust of cereals during the winter, and when the new crops are mature enough the fungi "jump hosts" and infect a new round of plants. While the soybean rust is not a huge problem in Canada (not yet, at least; it is being tracked by Agriculture and Agri-Foods Canada and is moving further and further north every year), it can be devastating for fields in the southern United States, leading to up to 70% crop loss in a single year.
Moral of the story: if you see a buckthorn in North America, grab a hacksaw and cut it down. Or you could do as was suggested to me by a friend and naturalist: "roundup vaccinations". Drill a bunch of holes in the trunk, and spray roundup in each hole. Act baffled when it dies later that season and has to be removed. Then suggest native species to be planted in that location instead!
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