Geographical factors contribute to variations in EAB infestation levels
We found latitudinal distributions of ash trees were positively related to EAB infestation levels rather than their habitats, altitudinal distributions, ages, and sizes. Many susceptible non-Asian ash trees have been widely planted in northern China, but these exotic species have not been widely introduced and planted in southern China as they are not tolerant of high temperatures (Pan & You 1994). In North America, the three major species of ash including green, white and black ash are distributed as far north as central Manitoba and Cape Breton Island in Canada (up to 55°N latitude) and south to Northern Texas and Florida (28°N latitude). Average temperatures range from -18°C in the winter to 27° in the summer (mean annual temperature of 5.3°C) at northern sites, and from 10°C in the winter to 33°C in the summer (mean annual temperature of 23°C) at southern sites (Burns & Honkala 1990; DeSantis et al . 2013; USDA NRCS 2019b). The velvet ash is primarily distributed in the southwestern and southern U.S. - from northern California (42°N latitude) where average temperatures range from 4°C in the winter and 18°C in the summer (mean annual temperature of 11°C) to southern Texas (26°N latitude) with average temperatures of 11°C in the winter to 34°C in the summer (mean annual temperature of 24°C) (USDA NRCS 2019b). In China, green, white and velvet ash trees have been mainly planted in areas from Heilongjiang in the north (45-48°N latitude) to Zhejiang (27-31°N latitude) in the south. There were also a few white and velvet ash species planted in Yunnan Province (22-26°N latitude) (Yang 2000; Zhao et al . 2012), which is as far south as the most southern distribution of ash species in North America. Conversely, there are only a few ash trees distributed in southern China and most of them are native and highly resistant species (Wei et al . 2004). In addition, non-Asian ash trees were customarily planted as street trees in northern China, whereas many native and other introduced tree species, such as Cinnamomum camphora Presl, Ficus spp., Mangifera indica L., andPlatanus spp., are principally selected as street trees in southern China (Long et al . 2018). As a result, highly concentrated distributions of susceptible ash species in northern China might be the major reason for EAB outbreaks in these high latitude areas.