This high number of non-native species within cities may actually improve ecosystem services (e.g. It is argued that with the increase of global travel, which has led to a greater circulation of plant species and the spread from gardens of introduced alien plants 7, 8, 9, 10, there has been an enhancement of species richness counterbalancing or even exceeding the potential local extinction of native species 11, 12. In addition, in some semi-natural environments within cities, endangered native plant species show consistent populations 5, 6 making urban habitats preferential sites for their conservation. General data indicate that urbanization reduces native species diversity at a regional scale, while, on the contrary, total plant species richness often increases in the city centres in comparison to wildlands, due to the highly heterogeneous patchwork of urban habitats coupled with the human introduction of exotic species 4. ![]() Although floristic diversity 3 in urban areas is generally poorly documented, inside cities where comprehensive surveys have been undertaken plant species biodiversity is often remarkably high 2. The development of urban environments is one of the principal causes of land use change worldwide 2 and believed to be the major cause of biodiversity alteration. Whereas in 1900 only 10% of people were residing in cities, this number now exceeds 54% and is expected to increase to 66% by 2050 1. The world’s population is experiencing a dramatic shift from rural to urban living. In particular, the temperature index showed a significantly higher amount of macrothermal species in accordance with a warmer climate and the urban heat island effect. Climate change and the presence of artificially irrigated areas within the city has led to an increase in both hygrophilous and drought-resistant species. Analysis of variation in Ellenberg’s indicator values over the last 120 years evidenced a shift toward shade-tolerant species, mainly connected to the increased presence of parks and trees within the city. The correlation between urbanistic features and plant richness indicated that the main factor affecting plant richness is the presence of green spaces (parks, tree lines, flowerbeds, etc.). The comparison of species life forms pointed out a noticeable recent expansion of phanerophytes and geophytes at the expense of therophytes and hemicryptophytes. Since 1894, a large increase occurred in the number of taxa (families from 41 to 101, species from 176 to 477) and alien species (from 22 to 144), with a 65% total species turnover. The city of Bologna was selected as a case study since two specific and complete studies have been carried out in a 120-years timespan, one in 1894 and one in 2018. Ecological studies in cities are not only aimed at investigating floristic diversity, but also represent informative test cases for understanding ecological system dynamics and responses to urban and climate changes since cities represent microcosms of environmental changes happening globally.
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