Non-native plant cover and functional trait composition of urban temperate grasslands in relation to local- and landscape-scale road density

Abstract

Globally, many natural grasslands are becoming increasingly urbanised, with non-native plants invading as local and regional environmental attributes are altered. Using multiple linear regression models, we examined the functional trait composition of non-native plants in 69 native urban grasslands in Melbourne, Australia, against the spatial attributes of remnant patches and the surrounding road network at multiple spatial scales. We predicted that (1) urbanisation favours non-native plants with evolved characteristics conferring resistance to urban environments, (2) high road density in the immediate vicinity of remnants drives high non-native functional diversity, and (3) non-native plants that spread and become abundant do so through an investment in competitive traits. Non-native plants contributed to 31% of total grassland cover, with regionally widespread species dominant within sites. Non-native perennial grass cover, rather than annual grass cover, was positively associated with road density across the urban landscape, and community-weighted Specific Leaf Area was negatively associated with road density. At local scales, non-native plant functional diversity was positively associated with road density in the immediate vicinity of grasslands. Urbanisation favoured non-native plants invested in persistence over resource acquisition, potentially in response to the urban climate and declines in fire frequency. Re-introducing historic fire regimes may therefore have the potential to be a key strategy in reducing competition from non-native plants. High non-native functional diversity in areas of high local road density was the expected response to high propagule pressure. Urban planning that protects grasslands from road encroachment may therefore reduce invasion.