KEYWORDS
Bursaphelenchus xylophilus , plant community structure, insect functional group, functional relationship
INTRODUCTION
With increase in the frequency of international trade, some invasive alien species are directly or indirectly introduced into new forest ecosystems. As an external disturbance factor, they inevitably affect the productivity, nutrient cycle, hydrological system, species diversity, and information transmission of the new ecosystem (Lovett et al., 2006; Ding et al., 2008; Westphal et al., 2008). The plant community structure and composition of the forest ecosystem can be significantly changed, and the succession rate of the community can be accelerated after the infestation of forestry invasive alien species (Castello et al., 1995; Spiegel & Leege, 2013; Gao et al., 2015). Consequently, the species composition and population structure of the specific animals, insects, and microorganisms that depend on the plant community may also undergo considerable changes (Karban, 2011; Li et al., 2016; Visakorpi et al., 2019).
As one of the most dangerous and destructive forest pests,Bursaphelenchus xylophilus (Steiner and Bhrer) Nickle (Nematoda: Aphelenchoididae) causes the occurrence of pine wilt disease (PWD) and has a strong destructive effect on pine forest ecosystems in China.Monochamus alternatus is the most efficient insect vector for spreading B. xylophilus . The average number of B. xylophilus carried by M. alternatus is 18,000 per beetle, and the maximum number is 289, 000 (Zhao, 2008). In China, B. xylophilus was first discovered in 40 black pines in September 1982 at Sun Yat-Sen’s Mausoleum in Nanjing (Wan et al., 2005; Zhao, 2008). Since then, the degree of damage inflicted by B. xylophilus is a continuous large-scale occurrence, which has caused huge economic and ecological losses around China (Wan et al., 2005; Zhao, 2008; Shi et al., 2013). By August 2020, PWD had spread to 18 provinces and 666 counties in China (The No.4 and No.8 bulletin of National Forestry and Grassland Administration in 2020). In addition, the extent of this disease in China has displayed explosive growth in the past two years, with 283 and 85 new counties affected with PWD epidemic added in 2018 and 2019, respectively. Moreover, the outbreak area of PWD has crossed the threshold of annual average temperature above 10°C, as proposed by the traditional theory (Roques et al., 2015; Firmino et al., 2017; Calvão et al., 2019). The western infection area of B. xylophilushas reached Xichang City, Sichuan Province, and the northern part has colonized Kaiyuan City, Liaoning Province, directly threatening the safety of pine forest resources in northern China (Figure 1).
Previously published works have documented that plant succession is a major factor that affects the insect community’s composition and structure (Siemann, 1998; June et al., 2006). As an important environmental indicator, insect species are usually linked to plant communities and are widespread in forest ecosystems (Siemann, 1998; Vandewalle et al., 2010; Cédric et al., 2013). Woody plant species provide much of the habitat and resource base for insects (Brown et al., 2001; Trotter et al., 2008). A greater diversity of plants provides a greater diversity of resources for insects; consequently, the increase in plant species richness also leads to an increase in insect species richness (Knops et al., 1999; Haddad et al., 2001). Furthermore, a higher diversity of herbivorous insects can provide more food sources for insect predators and parasitoids (Knops et al., 1999; Gao et al., 2008).
For the dominant tree species with a large biogeographic distribution, changes in the communities and population structure of this species are likely to affect many other tree or animal species at large landscape scales (Trotter et al., 2008). Masson pine is an important pine tree species in the Three Gorges Reservoir area, which is widely distributed from the bank of the Yangtze River to the top of the mountain (Gao et al., 2015). In this area, the main coniferous forest type was the Masson pine pure forest before the invasion of B. xylophilus in 2006. In the following years, this epidemic spread rapidly, killing Masson pine trees occupying a large area.
To date, little is known about the functional relationship between woody plants and insect communities in response to B. xylophilusinfestation. To address this issue, the research of woody plant and insect communities was conducted in the eastern part of the Three Gorges Reservoir region of China. The aim of this work was to address the following questions: after the invasion of B. xylophilus (i) How do woody plant species change? (ii) What impact does the change in woody plant structure have on the insect community structure and composition? (iii) How is the functional relationship between the woody plant community and insects impacted?
MATERIALS AND METHODS
2.1 Study sites
This study was carried out in Yiling District (latitude 30°32′–31°28′N, longitude 110°51′–111°39′E), which located in the eastern part of the Three Gorges Reservoir region and contained the demarcation point of the upper and middle reaches of the Yangtze River (Gao et al., 2019). In this area, PWD was first detected on Masson pine in 2006. Local forestry authorities removed all infected Masson pine trees in 2012, from then the coniferous forest regenerate naturally. The research was conducted on five Masson pine stand types, which were classified based on the duration of PWD infection from 2006 to 2012 (Table 1). Additionally, each Masson pine stand type had three repeated stands, and there were three permanent 15 m × 15 m Masson pine plots in each stand.