Assessing the extinction risk of the spontaneous flora in urban tree bases.

As the spatial arrangement of trees planted along streets in cities makes their bases potential ecological corridors for the flora, urban tree bases may be a key contributor to the overall connectivity of the urban ecosystem. However, these tree bases are also a highly fragmented environment in which extinctions are frequent. The goal of this study was to assess the plant species' ability to survive and spread through urban tree bases. To do so, we developed a Bayesian framework to assess the extinction risk of a plant metapopulation using presence/absence data, assuming that the occupancy dynamics was described by a Hidden Markov Model. The novelty of our approach is to take into account the combined effect of low-distance dispersal and the potential presence of a seed bank on the extinction risk. We introduced a metric of the extinction risk and examined its performance over a wide range of metapopulation parameters. We applied our framework to yearly floristic inventories carried out in 1324 tree bases in Paris, France. While local extinction risks were generally high, extinction risks at the street scale varied greatly from one species to another. We identified 10 plant species that could survive and spread through urban tree bases, and three plant traits correlated with the extinction risk at the metapopulation scale: the maximal height, and the beginning and end of the flowering period. Our results suggest that some plant species can use urban tree bases as ecological corridors despite high local extinction risks by forming a seed bank. We also identified other plant traits correlated with the ability to survive in tree bases, related to the action of gardeners. Moreover, our findings demonstrate that our Bayesian estimation framework based on percolation theory has the potential to be extended to more general metapopulations. Author summary: Understanding how biodiversity is maintained in a urban environment is an important question in ecology. In this article, we investigated to what extent tree bases along streets contribute to maintaining biodiversity in cities by acting as ecological corridors between larger urban green spaces. To do so, we introduced a new estimation framework to assess whether a plant species can survive and spread through urban tree bases, given presence/absence data (such as floristic inventories). The novelty of our approach is to combine two factors previously identified as particularly relevant in a urban environment: selection against dispersion, and the potential presence of a seed bank of dormant seeds in the soil. We applied our estimation framework to a dataset of yearly floristic inventories carried out in 1324 tree bases in Paris, France. We identified several plant traits making it possible to use urban tree bases as ecological corridors. Our study sheds light on how urban tree bases are integrated into the urban ecosystem. Our findings can be used to design improved management strategies for urban tree bases, that will contribute to biodiversity preservation in urban environments. [ABSTRACT FROM AUTHOR]