Urban plant species patterns are highly driven by density and function of built-up areas

This paper aims to assess the relative importance of the type of built-up area in structuring plant species composition and richness in urbanised environments. The study was carried out in the city of Brussels where all vascular plant species were recorded in 189 grid cells of 1 km(2) each. The effect of urban land use type on species composition was investigated using first Canonical Correspondence Analysis. Densely built-up area was the most powerful predictor for species composition, followed by industrial built-up areas, half open or open built-up areas with plantations, and open built-up areas with much natural vegetation in the surroundings. Indicator species were found for each type of built-up area and a response curve to the amount of built land was produced using Generalised Additive Modelling. Various types of built-up areas had different effects on environmental conditions as inferred by Ellenberg's indicator values, as well as on the species richness, species rarity, number of exotic species and proportion of extinction-prone species. It is concluded that future ecological studies should not treat urban areas as homogeneous areas by combining all anthropogenic factors into one aggregated variable. Instead, the urban matrix should be categorised in subsystems as it is multidimensional and highly variable across space.