Alien plants have increased in the city


Many more plants, but much more alien. The Alma Mater of Bologna has made a comparison between a botanical catalog of the late nineteenth century and the today's mapping of the green in the city and the results say that the species present within the circle of the Bolognese walls have almost tripled, but at the expense of those originating in the territory .
It is a story that begins in 1894, when a Bolognese botanist, Lucio Gabelli, decides to survey all the plants that grow in the city. Beating palm to palm the historic center - at the time surrounded by medieval walls - it records every species it encounters, in the gardens, along the road, even among the cracks in the walls. He even discovers a flower originating from Peru (a purple heliotrope), which somehow took root in Bologna. The catalog that delivers to the history of botany is the starting point - today, one hundred and twenty years later - of the work carried out by a group of researchers from the University of Bologna.
Scholars have literally retraced the footsteps of Gabelli's journey, cataloging all the species that grow between parks and gardens, avenues, flower beds and sidewalks, walls, columns and buildings.
Objective: to understand how green has changed and how it changes in the city.
And at a certain point, the surprise: along the way the researchers have found the purple heliotrope: it continues to bloom, every year, for over a century, at the same point where Gabelli had noticed it.
But apart from this extraordinary case, the comparison between the two catalogs shows that in the last one hundred and twenty years the urban flora of Bologna has changed radically. "The warming of the climate, the changes in the city's architecture and the progressive intervention of man on the urban environment have profoundly changed Bologna's floristic biodiversity", confirms Annalisa Tassoni, professor of the University of Bologna who coordinated the study. "A change that has seen the multiplication of alien species, introduced above all as ornamental plants, to the detriment of those native to the area, which have been reduced significantly".
From the results of the research - published in Scientific Reports, a journal of the Nature group - it emerges that the species that inhabit the historic center of Bologna have almost tripled, going from 176 at the end of the 19th century to 477 today; at the same time, however, the number of alien ones has more than doubled, from 12% to 30% of the total. "These vast changes are probably linked to the profound transformation of the historic center of Bologna in the last century: the disappearance of the cultivated areas and the medieval walls, the overbuilding, the extensive reconstructions of the second post-war period," says Professor Tassoni. "All these events led to the disappearance of species linked to the agrarian economy of the past, such as cereals and fruit trees. On the other hand the large-scale introduction of ornamental plants, in gardens and on balconies, has allowed many species not originating from the territory to spread and take root ".
More diversity on the one hand, therefore, but on the other less "typical" for the Bolognese flora. However, the increase in alien species is not necessarily negative. "In historic city centers the natural environment is often almost completely absent," explains Mirko Salinitro, a researcher at the University of Bologna and first author of the study. "In these contexts, alien species are sometimes the only ones capable of colonizing spaces that would otherwise remain empty, thus creating habitats that can favor precious pollinating insects, for example." In short, plants - local or alien - are able to survive in environments that, also due to the increase in temperatures, become increasingly hostile.
The editorial staff
The study was published in Scientific Reports, a journal of the Nature group, with the title “Impact of climate change and urban development on the flora of a southern European city: analysis of biodiversity change
For the University of Bologna the authors are Annalisa Tassoni and Mirko Salinitro of the Department of Biological, Geological and Environmental Sciences and Alessandro Zappi of the "Giacomo Ciamician" Department of Chemistry, to which is added Alessandro Alessandrini of the Institute for Artistic and Cultural Heritage and natural resources of the Emilia-Romagna Region.
Cover photo by Alessandro Alessandrini


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