Phylogenetic and functional structures of plant communities along a spatiotemporal urbanization gradient: Effects of colonization and extinction

Question: Urbanization has remarkable impacts on the phylogenetic and functional structures of plant communities. Both temporal and spatial comparisons along urbanization gradients are widely used in related studies, but there has been a lack of consistency in the results. Moreover, there is a need for studies that determine species assembly mechanisms through immigration and extinction. Therefore, two questions were addressed: (a) How do the phylogenetic and functional structures of ruderal species respond to urbanization, and do their shifts follow a similar pattern along temporal and spatial urbanization gradients? (b) What are the key underlying processes, i.e., either extinction- or colonization-caused clustering, that determine the phylogenetic and functional structures of ruderal species under urbanization?. Study site: Two metropoles (Shanghai and Harbin) experiencing rapid urbanization in China. Methods: We collected occurrence data on ruderal species from 1955 and the present in two cities. Standardized effect sizes of mean pairwise phylogenetic distance and of mean pairwise functional distance values (MPD_SES and MFD_SES, resp.) were calculated to test whether there was phylogenetic and/or functional structure clustering along spatial or temporal urbanization gradients. β-MPD_SES and β-MFD_SES values were used to quantify the similarities among colonists, extinct species, and residents. Results: Along both the spatial and temporal gradients, the MPD_SES values in each city decreased from significantly positive to significantly negative with increasing urbanization. Inconsistently, along the temporal gradients, the β-MPD_SES values of the colonists/extinct species to the residents were significantly negative; along the spatial gradients, the β-MPD_SES values of extinct species to residents were significantly positive with increasing urbanization. Conclusions: We found there was a clear phylogenetic clustering of ruderal species with increasing spatial and temporal urbanization gradients. Our analysis showed that the changes across the urban–rural gradient are mainly driven by species going extinct that are phylogenetically dissimilar to the resident species. The temporal dynamics are, however, primarily driven by colonist species that are phylogenetically similar to the resident species.