Abstract
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. 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. 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 comparison of species life forms pointed out a noticeable recent expansion of phanerophytes and geophytes at the expense of therophytes and hemicryptophytes. 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.). 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. Climate change and the presence of artificially irrigated areas within the city has led to an increase in both hygrophilous and drought-resistant species. 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.
Highlights
The world’s population is experiencing a dramatic shift from rural to urban living
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]
Original site conditions in urban habitats have changed tremendously and present living conditions for plants are completely different from those occurring during the last century[16]
Summary
The world’s population is experiencing a dramatic shift from rural to urban living. Whereas in 1900 only 10% of people were residing in cities, this number exceeds 54% and is expected to increase to 66% by 20501. 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] This high number of non-native species within cities may improve ecosystem services (e.g. air quality, pollinator insect diversity)[13,14] and it is expected that the establishment of garden ornamental plants will keep increasing in the future[15]. Climate records and urban structure of the study sites make it possible to evaluate the influence exerted by humans on local floras[17] In this context, the city of Bologna can be considered a perfect case study for the assessment of long-term floristic changes since two floristic surveys were carried out with a gap of more than 120 years. Throughout the paper, we will refer to the historical flora of 1894 as “Gabelli, 1894” and to the modern flora of 2018 as “Salinitro et al, 2018”
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