Composition, time of introduction and spatial-temporal distribution of naturalized plants in east China.
Background: Naturalization is the pre-invasion stage. Detailed research on naturalized plants is a fundamental first step for scientific management of alien species. This research is necessary for predicting and hence preventing such invasion caused by alien plants. Regional studies of naturalized plants are important for local governments to manage and control of such plants. As an important area for foreign exchanges, East China is facing pressures from imported alien plants being naturalized and then invading. This study aims to enrich the basic information about naturalized plants of East China, discussed the current deficiencies in the study of naturalized plants, and summarized the direction of future research. Methods: We conducted a comprehensive survey of naturalized plants in East China (Anhui, Fujian, Jiangsu, Jiangxi, Shandong, Shanghai and Zhejiang provinces) from 2014 to 2019 using literature reports, specimen information, and taxonomic research. We analyzed the species composition and distribution pattern of naturalized plants in East China and conducted a comprehensive analysis of their first detected locations, introduced route, and the time when species were discovered. Results: We found that 299 species belonging to 181 genera and 62 families were naturalized in East China. The four most dominant families were Compositae (49 species), Leguminosae (35 species), Gramineae (28 species) and Amaranthaceae (25 species). The four most dominant genera were Amaranthus (16 species), Euphorbia(10 species), Ipomoea (9 species) and Solanum (9 species). There was a large diversity in species identified as naturalized, with a high proportion of plants coming from the Americas, and concentration in introduction routes. The Americas are the primary geographical origin of naturalized alien plant species, with more species originating from North America rather than South America. The route of introduction analysis revealed that 143 naturalized plant species were introduced unintentionally due to human activity and 156 species were introduced intentionally. Ninety-five frequencies were introduced as ornamental plants, which take up 55.2% of all the frequencies introduced intentionally. This suggests that introductions of plants for the purpose of ornamental use is the most common way for species naturalization to occur. A comparative study of the first detected locations show that the largest number of naturalized species were firstly introduced to South China (84 species) which mostly went to the Guangdong Province (33 species) and Hong Kong District (33 species). This was followed by East China (74 species), the Taiwan Province (72 species), Southwest China (19 species, mainly from Yunnan), Northeast China (15 species, mainly from Liaoning), Northwest China (14 species, mainly from Xinjiang) and North China (14 species, mainly from Beijing). There were only two naturalized species found in Central China. Jiangsu, Fujian and Zhejiang provinces were the main importing areas for East China. This may be due to many factors such as international communication, population density, climate similarity, disturbance levels, biological characteristics unique to species and investigation degree. Spatially, the species diversity of the Fujian Province (236 species) is significantly higher than that of other provinces, but little difference has been detected among other provinces. There are 57 species that are only distributed in Fujian (19.1%), which is the most when compared with other provinces. The amount of foreign exchange and climate similarity between native habitat and naturalized place may be the primary driver of current diversity and distribution pattern. Historically, the naturalized plants in East China showed an exponential growth trend after 1850. The growth rate is 1.5 species per year, and is currently in a period of rapid growth. In the 21st century, more than 80% of naturalized plants have been unintentionally introduced. Therefore, special attention should be placed on preventing the introduction of unintentional species. Proposal: Constructing an effective risk assessment system of alien plants based on an accurate and complete database of naturalized plants is one effective way to eliminate the risk of invasions from alien plant species.