Our rich and diverse urban flora: an exciting study from Paris

Results of an exciting new study, focusing on the flora of the Paris area have been published. The aim of the study was to better understand the “filters” that allow some species to establish successfully in our cities (while other plant species may fail to establish). These filters can be environmental (cities pose challenges such as high temperatures, dry and impervious soils…) or biological (species that are already there offer stark competition).

The Ile-de-France region, surrounding the city of Paris comprises around 50% of agricultural land, 24% of forests, 16% of built land (including housing, industrial areas, quarries, worksites and transport).

In this paper, researchers used records collected by citizen scientists, part of the Vigie Flore program, with 620 plots (10 m²) surveyed between 2009 and 2017.

They chose to study a unique measure called “species originality“. The idea is that a species is “original” if it has unique and distinctive traits among the other species living in the same area. A species could be “original” if it is much taller than all the other plants in the area, or if it relies on a very unusual pollinator. To evaluate originality, they selected eleven traits for all the species recorded by amateur botanists. These included for example: start of flowering period, duration of flowering, life span, type of pollination (wind, insects, self-pollination…), type of seed dispersal (by animals, wind or human activity), plant height, leaf size, seed weight…

The researchers discovered that species richness (the number of different species) increases as we get closer to cities. With fertilised soils and higher average temperatures, cultivated plants are able to escape from gardens, and add to the diversity observed during surveys.

Among the most original urban species found during the study are fleabanes (Erigeron sp.) and narrow-leaved Pepperwort (Lepidium ruderale). These urbanophile species (found more frequently in cities than in the surrounding countryside) have specific traits that help them survive in urban environments: early and long flowering, helped by higher urban temperatures; annual or biannual life span more adapted to rapidly changing urban environments (for example spraying of weedkiller), wind-pollination due to the lack of insects; high leaf area to increase competitiveness in rich soils.

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Although what the authors call “urbanoneutral” species (that is, species found both in urban and countryside environments) such as Dandelions (Taraxacum spp.) or annual meadow grass (Poa annua) are the most frequent, the urbanophiles, which are less frequent, could play unique roles in urban environments.

Indeed, the originality (distinctiveness) increased with urbanisation. Urban environments offer, often on a small area, a wide diversity of habitats: for example, a park could harbour acid grassland flora, and aquatic species in its pond; an industrial estate or a roof garden, brownfield flora; a ornamental garden, exotic winter-flowering species… This means that urban species can display a wide variety of traits, often adapted to their little, specialised ecological niche.

Within a specific habitat, the authors believe that competition also limits the establishment of species that are too similar. Having “original” species, with different flowering times or seed dispersal strategies, allows resources to be available for everyone.

Although this is the first study of this type, it would be interesting to see if the plants in other cities follows the pattern observed in Paris. This could have implications on how we manage our urban flora. Could the loss of some of these plants, through changes in land use (for example the conversion of brownfield land to housing) or human selection (through weeding) disturb urban ecological processes, and lead to other issues (decrease in insect diversity, increase in space available for invasive species…)?