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.