Drivers of the distribution of spontaneous plant communities and species within urban tree bases

Abstract Many studies have shown that the quality of biodiversity influences the well-being of citizens. Nevertheless, the drivers that shape biodiversity in urbanized zones are poorly understood. Although tree bases present reduced surface areas, they occur in great numbers in a deliberate spatial arrangement and may play an important ecological role in urban environments by offering limited favorable spaces for the development of spontaneous flora. The purpose of this study was to identify the factors that influence the composition of plant communities harbored by tree bases in an urban district. We analyzed floristic inventory data collected in 2014 about plants growing at the bases of the 1474 trees on the 26 streets of the Bercy district in Paris (France). Our results indicated that the plant communities growing in the urban tree bases varied according to different factors. The abundance and distribution of these species were dependent on their biological traits (seed longevity in the soil bank) as well as the tree base characteristics (tree trunk diameter, equipment type around the tree bases, soil compaction, animal excrement, solar radiation, and urban tree species), the street orientation according to the air flow following the Seine River, and the geographic structure of the district (the influence of the presence of green spaces). The results of this study showed that the tree bases could be considered favorable stepping-stone habitats for certain species between more important green spaces such as parks and gardens. Thus, these areas actively participate in the enhancement of urban biodiversity.