London's Changing Flora
Not only does London's human population change over time but so does that of its wildlife. The advent of the urban fox is a well- known story, but what of the plants? Mark Spencer explains..
Much of Greater London is now inhabited and dominated by non-native plants. Many of these are the plants we treasure in our gardens, but increasingly the plants of London's streets, roadsides, railways and waterways are from other parts of the world. Many, such a London rocket , Sisymbrium irio (pictured right) have been here for hundreds of years, others, like membranous nettle
(Urtica membranacea) were first noticed in 2007.
Why are these plants interesting or important? Non-native plants are often accused of being invasive and indeed some, such as buddleja (Buddleja davidii) and Japanese knotweed (Fallopia japonica) are. Others, like Oxford ragwort (Senecio squalidus) and broad-leaved everlasting-pea (Lathyrus latifolius) inhabit London's landscape causing no serious harm to our native wildlife. The 'heat island effect' in London means that the capital is often several degrees warmer that the neighbouring contryside. As the world's climate changes London will almost certainly get even warmer. These effects are already being seen, recently there have been reports of lemon and grapefruit (Citrus spp.) growing and fruiting in London, as well as avocado (Persea americana).
When factors such change in the environment and potentially invasive species combine the results may prove very serious. Tree-of-heaven, Ailanthus altissima (left) has been grown as a popular garden and street tree in London for many years, it is also an invasive species in many parts of southern Europe. Recently, the number of seedlings and young plants of Tree-of-heaven on London's streets and railways has increased massively, we may be seeing the start of a major problem; Tree-of-heaven forms dense thickets that out-compete native plants, its roots damage built infrastructure, and fallen trees block waterways causing localised flooding.
As our non-native plant communities spread, many of our native plants are becoming increasingly threatened. Marsh Sow-thistle, Sonchus palustris (pictured right) once grew alongside the river Thames, it is now almost extinct and the last remaining population in south-east London teeters on the edge of oblivion. The reasons for the loss of our native plants are complicated but poor management of our remaining greenspace is now a major contributory factor. Currently, a few native plants such as pyramidal orchid (Anacamptis pyramidalis) seem to be benefitting from the recent changes in the climate.
Since the publication of Rodney Burton's Flora of the London Area (1983) there have been many changes, broadly speaking native plants are getting less numerous and the non-natives are expanding. The time has now come for a new work to document these changes and the LNHS botany committee are now investigating how this should be done. If you would like more information, please contact Mark Spencer: lnhs_plant_recorder@hotmail.co.uk
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Photos: Mark Spencer