Academic ecologists these days spend a lot of time thinking about invasive species: How fast are they likely to spread? What traits allow a certain species to succeed where others fail? What is the impact on native plant and animal communities? With our warming climate, alien species, previously innocuous, might escape from ‘captivity’. Garden plants are of course one of the most common sources. The charity Plantlife recently compiled a list of the plant species most likely to become invasive in the UK in the near future.
Particularly high risk were two tree species, the Tree-of-heaven (Ailanthus altissima) and the False-acacia (Robinia pseudoacacia). The Tree-of-heaven has already spread itself all over London, and seems a good bet to do the same thing in other UK cities in the near future. Both these trees can be found occasionally in Sheffield’s gardens, parks and streets; unfortunately, both produce fertile seed in Britain, and also sucker. Indeed, one of the main problems with these trees in other countries has been their ability to sucker into native woods, potentially transforming them into monocultures. False-acacia suckers particularly can travel considerable distances: the next time you walk past the University Arms pub, up Brook Hill, have a look at the trees at the corner with Hounsfield Rd and you might be able to spot False-acacia shoots popping up at some distance from the main tree. The Tree-of-heaven suckers profusely when the main bole is cut down; the resulting thickets can be seen in some gardens around Sheffield, at the corner of Carr Rd and South Rd in Walkley, for example.
For naturalists keen to monitor potentially harmful changes to our region's flora, the seedlings of these species will be particularly important to keep one's eyes open for.