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.