Increased rate of introduction of non-native plants in China since 2002 driven by intentional introductions


 The history of plant introductions in China is inextricably associated with globalization and trade. There have been two periods where China has opened to the world, punctuated by periods of isolation. Here, we compare historical rates of introduction from 1840–2011 and ask how the rate of introduction of non-native plants has changed and how intentional and non-intentional introductions compare. The highest rate of introductions occurred between 1900 and 1940, where mean annual introduction was 8.7 plant species. Plant introductions post 2002 occurred at the second highest rate, but this was still little over half the earlier period at 4.4 plant species per year. In both periods, intentional introductions contributed the greatest proportion, and intentional and non-intentional introductions shared similar temporal trajectories.


Abstract
The history of plant introductions in China is inextricably associated with globalization and trade. There have been two periods where China has opened to the world, punctuated by periods of isolation. Here, we compare historical rates of introduction from 1840-2011 and ask how the rate of introduction of nonnative plants has changed and how intentional and non-intentional introductions compare. The highest rate of introductions occurred between 1900 and 1940, where mean annual introduction was 8.7 plant species. Plant introductions post 2002 occurred at the second highest rate, but this was still little over half the earlier period at 4.4 plant species per year. In both periods, intentional introductions contributed the greatest proportion, and intentional and non-intentional introductions shared similar temporal trajectories.

Plant Introductions In The Chinese Context
China has one of the highest species diversities of any country in the world, but many species are already facing a high extinction risk (He, 2009;Lu et al., 2018). Biological invasion is among the leading threats and is being exacerbated by rapid changes in climate and land use (Ma et al., 2013). The temporal dynamics of plant introductions are thought to re ect China's trade history (Ding et al., 2008;Essl et al., 2011). In the rst half of the 20 th century, China gradually became less isolated, expanding trade with other countries (Ding et al. 2008), increasing the potential for introduction of non-native plants. National policy limiting trade with the western world then dominated from 1950s to 1970s, presumably reducing the rate of introduction over this period. Since the 1970s reforms, trade and economic activity have increased dramatically, facilitating plant introductions in China again.
However, little is known on how the relative rate of introduction in the early 20 th century compares with the years since re-opening post 1970s. Here, we report historical trends in introduction of non-native plants to China from 1840-2011, which we obtained from the comprehensive inventory of naturalized plants in China compiled by Jiang et al. (2011). Species lists were modi ed following review of the primary sources (Ma et al. 2013;Ma 2014). We excluded native species and 'casual' alien species (i.e., those without established populations in China) yielding 538 naturalized plants. The date of introduction for each species was estimated based on its rst reported time in the literature and/or the collection date for the rst herbarium specimen present in the Chinese Virtual Herbarium (http://www.cvh.org.cn/), which recorded the collection time and location of the specimens of each species. Further details on data processing are in Ni et al. (2021). The data for rst record time and introduction pathway are provided in supplementary material. We calculated a cumulative total of introductions by year of rst record, and source of introduction (intentional or unintentional). We then applied segmented linear regression to identify changes in the rate of introduction and report the slope for each identi ed segment. Segmented regression used R-package 'segmented' (Muggeo, 2003), with starting values for breakpoints set as 1900, Page 3/6

Historical And Contemporary Rates Of Introduction Of Non-native Plants In China
Segmented regression tted the introduction data well (R 2 adj > 0.99) and identi ed four periods where the rate of introduction differed: 1840-1905, 1905-1935, 1935-2002 and 2002-2011 (Table 1, Fig. 1). There were 51 non-native plant species found in China before 1840, most of which were intentionally introduced as food or medicinal plants. The number of introduced species then increased only slowly from 1840 to 1905. After this time however, a steep rise in the rate of introductions saw the majority (45%) of contemporary non-native species introduced between 1905 and 1935, at an average rate of 8.7 species per year (Fig. 1, Table 1). After 1935, the rate of plant introductions again decreased, before signi cantly increasing again after 2002, attaining a mean rate of 4.4 species per year ( Fig. 1; Table 1). Most nonnative plants were intentionally introduced (68%), but both intentionally and unintentionally introduced species shared a similar temporal trajectory.

Signi cance
Many have highlighted the need to manage the growing bio-invasion risk due to the rapid economic growth and dramatic landscape changes occurring in China (Ding et al., 2008;He, 2009;Early et al, 2016, Huang et al., 2016. Indeed, China faces future invasion risks that rank among the highest in the world (Early et al, 2016;Liu et al., 2019). Interestingly, here we show that rates of introduction are only around half the historical maximum, which was observed during the early 20 th century. However, our data do not extend beyond 2011, so we are unable to give a rate for the last decade. Irrespective, there has been a clear increase in the rate of introductions post 2002 and -as was the case in the early 20 th centuryintentional introductions account for the majority of this. While unintentionally introduced plants have a higher risk of creating severe ecological or environmental impacts (Ni et al., in press), it is important that the risk that intentionally introduced species become invasive is not overlooked. Table   Table 1 Identi ed break points and estimated slope (Estimate) and standard error ( Figure 1