Although documenting the impact of urbanization on biodiversity has been an important research topic in many developed countries, research on this topic is rare in the developing countries. The aim of this study is to investigate the changes in indigenous plant diversity along an urbanization gradient and the relationship between indigenous plant species and the soil factors in their habitat. In Langfang City, a fast-urbanizing city in northern China, six sites with size of 20 m×20 m each were sampled along central urban, urban, suburban and rural gradient, respectively. In each site, 2 plots with size of 2 m×2 m each for shrub species and 3 plots with size of 1 m×1 m each for herbaceous species were selected for plant community survey. Species richness, diversity, and composition of indigenous plants were recorded and analyzed at each site.
Our results suggested that urbanization processes have a significant effect on the number, diversity, proportion, and composition of indigenous plant species. A total of 11 species were recorded in the central urban, 14 species in urban, 46 species in suburban, and 95 species in rural areas. Species diversity reflected by the Shannon-Wiener index increased from central urban to rural areas, with relatively low standard deviations in central urban but relatively high standard deviations in rural area. Evenness increased from urban to rural areas, with high values in central urban spaces. The standard deviation of evenness values was relatively high for all sampling sites, with a significant difference according to the Pearson-test (average 0.91 at P=0.05) along the urbanization gradient. There were six families in central urban areas, ten families were recorded in urban areas (mainly Gramineae), 19 families in suburban spaces and 45 families in rural areas. In central urban and urban areas, most indigenous plants belonged to the Gramineae and Chenopodiaceae families, but in the suburban areas species compositions were mixed with members from Gramineae, Compositae, Leguminosae, Chenopodiaceae and Asclepiadaceae. In central urban areas, the main indigenous plant species were Eleusine indica, Digitaria sanguinalis, and Setaria viridis. The urban area communities were dominated by Digitaria sanguinalis, Eleusine indica, Chloris virgata, Setaria viridis and Chenopodium album. Amaranthus retroflexus is present in suburban areas. In rural areas, Amaranthus retroflexus becomes replaced by Kochia scoparia.
The similarity index showed that most of the species distributed in the urban area were also found in the suburban, but not in the rural areas. With increase of distance to central urban, the Jaccard index decreased. Most indigenous plant species growing in rural or suburban areas were also likely to grow in central urban than in urban areas. The DCCA analysis showed that total soil nitrogen is the main factor influencing the spatial distribution of indigenous plant species under urbanization. Soil conductivity and total organic matter, and active potassium have a relatively lower effect, and soil pH has the lowest effect on the sample ordination in the DCCA. The plant species surviving in urban areas had high tolerance to tramping, soil compaction and mowing, most of them belong to the Gramineae and Chenopodiaceae families. Thus, urbanization has eliminated most local species and only most common species are left. This result suggests that, in general, urbanization facilitates the homogeneity of plant species distributions worldwide.