Intraspecific diversity of multiple plant species shows no change across an urbanization gradient

There are well documented differences in species diversity along urban-rural gradients, but the effects of urbanization on diversity within species are less well known. Nevertheless, intraspecific diversity is an important element of biodiversity that allows for adaptation and can affect community and ecosystem function. The amount of intraspecific diversity may differ along an urbanization gradient if urban areas filter for a very narrow range of traits or, alternatively, if selection is weaker or more spatially variable in urban areas and allows for a broader range of traits. To test the relationship between urbanization, trait means and intraspecific diversity in forested environments, we measured two traits in eight herbaceous understory plant species from nine populations across an urbanization gradient in Baltimore, Maryland, USA. We found that while impervious cover, soil moisture and soil pH were associated with changes in mean trait values of plants in different locations, there was little consistent effect of these abiotic conditions on the amount of intraspecific diversity in traits. This suggests that while urban environments may select for different trait values than non-urban areas, the strength of environmental filtering is similar across the urbanization gradient.