High richness of exotic trees in tropical urban green spaces: Reproductive systems, fruiting and associated risks to native species

Urban ecosystems provide green landscapes that are important to global biodiversity conservation. In tropical urban ecosystems, exotic plants are widely used in landscaping and this can have several negative effects on the native plant and pollinator communities. Here, we aimed to investigate the origin (native or exotic to Brazil, and native or exotic to the phytogeographic dominium where the urban space is inserted), sexual and reproductive systems and fruiting of tree species occurring in 17 urban green spaces in the city of Recife, Pernambuco, located in the northeastern Brazilian Atlantic forest. A total of 114 tree species were registered in the urban green spaces, 78 out of them were observed for fruiting. Overall, 49.1 % of the species are native to Brazil and 50.9 % are exotic, while 86 % do not occur naturally in the northeastern Brazilian Atlantic forest. In terms of abundance of individuals, these values were 37 % native, 63 % exotic and 80.5 % not occurring in the northeastern Brazilian Atlantic forest. Most of the species, native or exotic, are hermaphrodite and self-incompatible (i.e. obligatory cross-pollinated). From the 78 species observed for fruiting, 77 set fruits, 43.7 % had sub-annual pattern; all exotic species (43 spp.) set fruits, even the 13 exotic species that are self-incompatible. Our results reveal that factors such as the high use of exotic species that successfully set fruits in tropical urban green spaces may indicate risks for plant-animal interactions (e.g. pollination; dispersion), threaten the reproductive success of native plant species, increase the risk of biological invasion, and consequently impair the maintenance of biodiversity. We suggest that decision makers pay attention and consider the impacts of using exotic plant species in the dynamic of tropical urban ecosystems.