Invasive Plant Species Distribution

Diversity 2022, 14, 167 4 of 16 We collected the corresponding environmental factors of each site, including mean (1970-2000) annual temperature, mean (1970-2000) annual precipitation, elevation, and SOC (Table 1) [58,59]. We also collected traffic network data (including railways, express- ways, national roads, and country roads) from the GIM-Cloud (geographical information monitoring cloud platform, http://www.dsac.cn/, accessed on 1 January 2022), and the dis- tance from each sampling site to the closest road was calculated using ArcMap (version 10.8, ESRI, Redlands, CA, USA). Their Pearson correlations with plant invasion intensity (namely the number and biomass of invasive species) were calculated using IBM SPSS Statistics (version 19, IBM). Table 1. Possible environmental and traffic network factors affecting invasive plant distribution. Item Mean annual temperature Mean annual precipitation Elevation SOC Traffic Network Source WorldClim Data version 2.1 (https://www.worldclim.org/, accessed on 1 January 2022) ASTER GDEM (https://www.gscloud.cn/, accessed on 1 January 2022) Harmonized World Soil Database version 1.21 (http://webarchive.iiasa.ac.at/Research/LUC/External-World- soil-database/, accessed on 1 January 2022) Geographical Information Monitoring Cloud Platform (http://www.dsac.cn/, accessed on 1 January 2022) 3. Results 3.1. Catalogue of Invasive Plants Overall, 49 invasive plant species, belonging to 15 families and 41 genera, were found within the 373 grassland sampling sites (Table 2). Asteraceae (19 species), Poaceae (6 species), Amaranthaceae (5 species), and Fabaceae (4 species) were the main families, with Ipomoea L. (3 species), Trifolium Tourn. ex L. (2 species), Sonchus L. (2 species), Ambrosia L. (2 species), Crassocephalum Moench (2 species), Solanum L. (2 species), and Veronica L. (2 species) being the main genera (Table 2). Trifolium repens L., Bidens Pilosa, and Ageratina adenophora appeared in more than 20% (41.29, 39.14, and 23.06%) of the sampling sites. Cyperus rotundus L. and Praxelis clematidea (Griseb.) R.King & H.Rob. ap- peared in 10-20% (15.01 and 14.75%) of the sampling sites, whereas Erigeron sumatrensis, Sonchus wightianus DC., Crassocephalum crepidioides (Benth.) S.Moore, Solanum quitoense Lam., Chromolaena odorata, and Avena fatua L. were present in 5-10% of the sampling sites. Trifolium pretense L., Alternanthera philoxeroides, and Ageratum conyzoides, along with 10 other species, were found in 1-5% of the sampling sites. As relatively rare species, Euphorbia dentate Michx., Dysphania ambrosioides (L.) Mosyakin & Clemants, Solanum aculeatissimum, Ambrosia trifida, Ambrosia artemisiifolia L., and Solidago Canadensis, along with other species, appeared in less than 1% of the sampling sites. There was no significant difference in the average frequencies among the five invasive levels. Invasive plants were found in over 90% (90.35%) of the grassland sampling sites, whereas malignant invasive plants were detected in about 60% (59.79%) of all sites. Fur- thermore, in about 30% (29.23%) of the sampling sites, more than one malignant invasive species coexisted. There may be two (57 sites, 15.28%), three (39 sites, 10.46%), four (12 sites, 3.22%), or five (1 site, 0.27%) malignant invasive plant species within one site. Overall, 15 invasive plants, including Chromolaena odorata, Erigeron sumatrensis, Bidens pilosa, and others, were classified as malignant invasive plants (Level 1), whereas 12 species, including Trifolium pratense L., Avena fatua, and others, were classified as seriously invasive plants (Level 2). Five species, namely Pennisetum purpureum Schumach., Paspalum dilatatum Poir., Oenothera rosea Aiton, Veronica persica Poir., and Euphorbia dentate, were local invasive plants (Level 3). Ten species, including Medicago sativa L., Sida acuta Burm. fil., and others, were classified as general invasive species (Level 4), and seven species, including

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