Study site
The study was conducted in a wild population of V. negundo var.heterophylla (30 × 30 m, population density of 5.3 plants per 100m2) in Qiangu Mountain (36°49′ N, 120°73′ E, 210-220 m above sea level) in Qingdao, Shandong Province, China. The study area belongs to the warm temperate monsoon continental climate, and also has significant maritime climate characteristics, which is characterized by humid air, abundant rainfall, moderate temperature, and four distinct seasons. The hottest month is August, with an average temperature of 25.3 ℃; the coldest month is January, with an average temperature of minus 0.5 ℃. The average annual temperature is about 12 ℃, and an average annual rainfall of about 662 mm. According to our measurements, the soil pH was 5.3, and the content of total nitrogen, total phosphorus, and organic matter in soil during the summer were 0.15 (g/kg), 0.24 (g/kg), and 50.8 (g/kg), respectively. In our study sites,V. negundo var. heterophylla was identified as the dominant species (1-20 cm in diameter) in the study area, and the subdominant species was Ziziphus jujuba var. spinosa(Rhamnaceae).