2.2 Field surveys
From July to August 2012, the woody plants in the arboreal layer were
investigated in each of the 45 plots. For each plot, the basic
environmental factors, including elevation, slope, and canopy density,
were recorded. The woody plants with a diameter at breast height (DBH) ≥
2.5 cm were investigated using the method of “Tally” which includes
species name, height, DBH, and crown.
2.3 Insect sampling and specimen identification
For each plot, insect sampling was conducted once every 7 days from June
to August in 2013 and 2014. Insect specimens were collected by means of
“sweep net sampling”, “Malaise trap sampling”, and “window trap
sampling”. For the “sweep net sampling” method, a muslin insect swept
net was used for sweeping in each corner of the 45 plots, for a total of
more than 200 times net was swept for each plot. For the “Malaise trap
sampling”, a Malaise trap was placed in the center of the 45 plots and
was mainly used to trap small insects. For this method, the collected
insect specimens could be directly stored in 75% alcohol, which ensured
the necessary quality for subsequent molecular identification.
Additionally, five window traps (29.5cm×19.0cm×0.2cm) were placed at the
center and the four end points according to the “five-point sampling”
method. The window traps were suspended at a height of approximately 3
m, and the collection device was filled with 75% alcohol in order to
preserve the collected insect species.
According to relevant professional books and references, insect
individuals were identified as exactly as possible to genus and species.
The sampled insect specimens were divided into five groups: herbivorous
insects (He), predatory insects (Pr), parasitic insects (Pa), omnivorous
insects (Om), and detritivorous insects (De). Moreover, the dominant
insect group was the one whose individual number accounted for more than
10% of the total collected insects, common groups accounted for
1.0%–10% of the total number, and the rare groups accounted for less
than 1% of the total number (Wang and Wang, 2010).
2.4 Data analysis
One-way ANOVA and Fisher’s least significant difference (LSD) test with
an alpha value of P < 0.05 were used to compare the
changes in woody plant and insect community structure. Principal
component analysis (PCA) was conducted using CANOCO 5.0 (Microcomputer
Power, Ithaca, NY, USA) to determine the plant community structure in
different Masson pine stands. In CANOCO 5.0, redundancy analysis (RDA)
based on a constrained linear model was used to analyze the ordination
relation of woody plant species and insect species. All statistical
analyses were performed using GraphPad Prism 6.0 (GraphPad Software, La
Jolla, CA, USA) and SPSS 22.0 for Windows (SPSS Inc., Chicago, IL, USA).
RESULTS
3.1 Changes in woody plant community structure
The total number of P. massoniana in the healthy ecosystem (ST1)
was higher than that in the infected stand types. With the increase in
the infestation duration of PWD, the number of P. massonianabegan to decrease sharply, and the statistical difference between
different stand types reached a significant level (F =
6.99,P < 0.01). The total number of other tree species
in the arboreal layer (except P. massoniana ) increased slowly
with the increase in the degree of PWD infestation in the ecosystem.
Additionally, the difference in the total number of the remaining
species, ST4 and ST5, reached a significant level when compared with the
healthy Masson pine ecosystem (P < 0.05).
The PCA ordination biplot of woody plant species with an importance
value over 0.5 showed that there was a significant change in the
structure of the tree layer in different Masson pine forest stands
(Figure 2). P. massoniana was the most important tree in the tree
layer in ST1, with an importance value of up to 67.23%. With the
increase in the duration of B. xylophilus infestation, the
importance value of P. massoniana in the corresponding stand
types decreased sharply to 53.61%, 50.24%, 42.65%, 29.24% in ST2-5.
On the contrary, the importance value of Cinnamomum camphora ,Quercus aliena , Quercus variabilis , and Loropetalum
chinensis showed a rapid upward trend. After 7 years of continuousB. xylophilus infestation, C. camphora had surpassedP. massoniana to become the most important tree species in the
tree layer in ST5.
3.2 Insect community structure
In this study, we collected and identified 7,188 individual insects,
representing 510 species from 15 orders and 152 families (Table 3).
There were significant differences in the levels of order, family, and
individual among the collected insects. Overall, the dominant insect
communities were Hemiptera, Coleoptera, Hymenoptera, Diptera, and
Lepidoptera, which made up more than 10% of the total at the levels of
family, species, and individuals. Other insect communities accounted for
a relatively small proportion of the total insect population.
There was an upward trend in the insect population at the levels of
order, family, species, and individual among different stand types
(Table 4). However, there was little variation in insect community
composition at the order, family, and species levels among different
stand types, and the difference between them were not significant.
Conversely, the individuals were significantly
different among different stand types (F =204.20, P <0.01),
and the difference between ST1-2 and ST3-5 was significant.
Overall, the orders of Hemiptera, Coleoptera, Hymenoptera, and Diptera
were the dominant insect groups in different Masson pine forest stand
types, and each of their relative abundance was all greater than 10%
(Table 5). The dominant insect groups in Hemiptera were Cicadellidae
(17.42%), Aphidoidea (17.07%), Pentatomidae (15.36%). The dominant
insect groups in Coleoptera were Scarabaeidae (19.08%). The dominant
insect groups in Hymenoptera were Formicidae (37.29%), Ichneumonidae
(14.62%), and Vespidae (11.26%). The dominant insect groups in Diptera
were Tipulidae (33.91%) and Cecidomyiidae (14.92%).
3.3 Insect functional groups
The structure of insect functional groups had changed over the course ofB. xylophilus infestation (Table 6). The magnitude of relative
abundance for insect functional groups in the control stand type (ST1)
was He > Om > Pr > Pa
> De, and after infection, the magnitude of the relative
abundance for insect functional groups had changed (ST2-ST5: He
> Pa > Om, De > Pr). After the
invasion of B. xylophilus , each of the relative abundance of He,
Pa, and De was increased by varying degrees, with the values of Pa and
De increasing from 7.86% to 16.69% and from 2.10% to 11.16%,
respectively. Meanwhile, the values of the relative abundance of Pr and
Om showed a downward trend.
Figure 3 shows the difference in insect functional groups at the levels
of species and individuals in different Masson pine forest stands. In
terms of He, the number of insect species and individuals increased with
the increase in invasion duration of B. xylophilus , and the
difference in individuals among different stand types reached a
significant level (Figure 3a). For Pa and De, the number of individuals
increased significantly, but the variation range of species was not
large (Figure 3b and Figure 3e). There was little variation in the
species and individuals of Pr among different stand types (Figure 3c).
Both the number of species and individuals of Om decreased when the
Masson pine forest was first infested by B. xylophilus , but the
number of insect species and individuals demonstrated an increasing
trend, and the difference in individual numbers among different stand
types reached a significant level (Figure 3d).
The PCA ordination biplot of insect functional groups in different
Masson pine forest stands is shown in Figure 4, which intuitively
reflects the distribution of the insect community in each stand type.
Pr-S was positively associated with ST1 and ST2 but were negatively
associated with ST3-ST5. The most important insect functional groups in
ST3 and ST5 were Pa-S, Pa-I, Om-S, Om-I, and He-I. However, the He-S,
De-S, De-I, and Pr-I groups played essential roles in ST4.
3.4 Functional relationship between woody plants and insects
The RDA ordination biplot with woody plant species and insect functional
groups along the first two axes is presented in Figure 5. As revealed by
the ordination biplot, Pr-S was positively associated with the
distribution of P. massoniana and Ilex cornuta in ST1 and
ST2. De-S and Pr-I were positively associated with the growth of some
broad-leaved tree species, including Q. aliena , Q.
variabilis , Loropetalum chinensis , Celtis bungeana ,Rhus typhina , and Symplocos caudatahe . The distribution ofTrachycarpus fortune , Cotinus coggygria , andDalbergia hupeana may promote an increase in the numbers of He-S,
He-I, Om-S, and De-I. Moreover, there was a significant negative
correlation between parasitic insects (Pa-S and Pa-I) and the
distribution of P. massoniana , but the parasitic insects were
positively associated with some broad-leaved tree species, includingC. camphora , T. fortune , C. coggygria , D.
hupeana , Rhus chinensis , Litsea cubeba , Symplocos
paniculate , and Albizia kalkora .
DISCUSSION
The present study provides new insights into the functional relationship
between woody plants and insect communities after the invasion ofB. xylophilus . This study mainly examined the community structure
and abundance of woody plants and insects in response toB.xylophilus infestation in the eastern part of the Three Gorges
Reservoir region of China. The research was conducted on five Masson
pine stand types, which were classified based on the duration of PWD
infection from 2006 to 2012. Local forestry authorities removed all
infected Masson pine trees in 2012, after which the coniferous forest
regenerated naturally.
Under natural conditions, B. xylophilus infestation will cause
serious damage and loss to a healthy pine forest ecosystem, but with the
continuous succession and natural recovery of the ecosystem, the species
diversity of the plant community will increase significantly, and the
pine forest will evolve and develop in a more advanced direction (Zhao,
2008; Spiegel & Leege, 2013; Shi et al., 2013). The results of this
study show that the invasion of B. xylophilus can affect the
structure composition of woody plants and significantly affect the
spatial and structural distribution of plant species in the arboreal
layer. Specifically, the number of stems for all woody tree species
measured in five Masson pine forest plots were infected for different
periods by B. xylophilus . With the increase in the infestation
duration of B. xylophilus , the number of P. massonianabegan to decrease sharply. Meanwhile, the total number of other tree
species in the arboreal layer (except P. massoniana ) increased
gradually with an increase in the degree of B. xylophilusinfestation in the ecosystem. Similar results were found in many
previous studies, which showed that the invasion of B. xylophiluscan lead to the succession of the pure P. massoniana forest
ecosystem into a mixed coniferous and broadleaf forest ecosystem or even
a broadleaf forest ecosystem (Shi et al., 2007; Wang et al., 2014; Gao
et al., 2015). By comparing and analyzing the niche indices of the main
woody plant species, the vegetation communities in the woody plant layer
did not degenerate in the direction of the shrub layer after the
invasion of the Masson pine ecosystem by B. xylophilus . On the
contrary, plant species and diversity were greatly increased in the new
ecosystem (Wu et al., 2005; Spiegel & Leege, 2013). In general, the
structure of new forest ecosystems formed by succession tends to be more
stable and has a strong ability to resist forest pests and diseases
(Humphrey et al., 1999; Hambäck et al., 2000; Jobidon et al., 2004; Li
et al., 2012).
To study the effect of the invasion of B. xylophilus on insects,
insect communities were investigated and sampled from five Masson pine
stand types with different durations of B. xylophilus infection.
In this study, a total of 7,188 individual insects was collected and
identified, representing 510 species from 15 orders and 152 families.
Overall, the orders of Hemiptera, Coleoptera, Hymenoptera, and Diptera
were the dominant insect groups in different Masson pine forest stand
types, each with a relative abundance greater than 10%. This finding is
consistent with that of a previous study in which the insects of the
dominant orders belonged to Coleoptera, Hymenoptera, and Diptera in a
coniferous forest ecosystem affected by B. xylophilus in Zhejiang
province (Gao et al., 2013). Additionally, the insect population showed
an upward trend in Masson pine forest stand types with increasing PWD
infection periods, which demonstrated that the insect community was
significantly affected by the invasion of B. xylophilus . Previous
studies have indicated that a greater diversity of woody plant species
provides much more habitats and resources for insects (Root et al.,
1973; Siemann, 1998; Brown et al., 2001; Trotter et al., 2008;
Vandewalle et al., 2010; Cédric et al., 2013). Therefore, through this
study, we found that the invasion of B. xylophilus can affect the
community structure and composition, change the spatial and structural
distribution of woody plant species, and further affect the distribution
of insect community structure and individual populations.
As a crucial part of the biological communities in forest ecosystems,
the population dynamics and spatial distribution of insect functional
groups are significantly restricted by woody plant species (Root et al.,
1973; Vandewalle et al., 2010; Cédric et al., 2013; Bezemer et al.,
2014; Tchakonté et al., 2015). The structure of insect functional groups
can reflect the development and health status of a specific forest
ecosystem (Haddad et al., 2001; June et al., 2006; Karban, 2011;
Visakorpi et al., 2019). In this study, the magnitude of relative
abundance for insect functional groups in the control stand type (ST1)
was He > Om > Pr > Pa
> De, whereas after infection, the magnitude of relative
abundance for insect functional groups was altered (ST2-ST5: He
> Pa > Om, De > Pr). This result
indicated that the structure of insect functional groups changed over
the course of B. xylophilus infestation in the Masson pine
forests. Moreover, the number of insect species and individuals of
herbivorous insects increased with an increase in the invasion duration
of B. xylophilus , and the difference of individuals reached a
significant level among stand types. This result is consistent with the
“Resource Concentration Hypothesis” which states that plant diversity
affects the structure of insect functional groups and can significantly
increase the species and individual numbers of herbivorous insect
populations (Root, 1973). In order to control the population of
herbivorous insects and enhance degradation of plant and insect residues
(Haddad et al., 2001; June et al., 2006; Trotter et al., 2008; Visakorpi
et al., 2019), the relative abundance of parasitic insects and
detritivorous insects in this study also increased by varying degrees
after the invasion of B. xylophilus , with values increasing from
7.86% to 16.69% and from 2.10% to 11.16%, respectively.
For a more intuitive understanding of the complicated functional
relationship between woody plant community and insect functional groups,
a redundancy analysis was conducted using 16 woody plant species with an
importance value of over 1, and five insect functional groups with the
numbers of species and individuals. As revealed by the RDA ordination
graph, the population spatial distribution of insect functional groups
was significantly restricted by woody plant species. The numbers of
individuals and species of parasitic insects were significantly
associated with the distribution of P. massoniana , but were
positively associated with the distribution of some broad-leaved tree
species, which indicated that broad-leaved tree species may provide more
resources and ecological niches for parasitic insects. This result is
consistent with a previous study showing that the number and diversity
index of parasitic insects increased with an increase in the proportion
of broad-leaved tree species in a coniferous forest ecosystem in
Zhejiang province [36]. Plant diversity restricts the population of
parasitic insects and the increase in plant species diversity can
provide more habitats and food resources, which can significantly
increase the species and individuals of parasitic insects (Knops et al.,
1999; Taki et al., 2010; Gao et al., 2013; Gao et al., 2018). As the
population of parasitic and predatory insects continues to increase, the
number of herbivores, including M. alternatus , which is the
vector of B. xylophilus , may be limited.
5. CONCLUSIONS
The invasion of B. xylophilus can affect the composition of woody
plant communities and change the spatial and structural distribution of
woody plant species. A total of 7,188 insect specimens (representing 510
species from 15 orders and 152 families) was obtained and the insect
community was significantly affected by the invasion of B.
xylophilus . The structure of insect functional groups changed from He
> Om > Pr > Pa > De
in the control stand to He > Pa > Om, De
> Pr over the course of B. xylophilus infestation in
the Masson pine forests. Moreover, the redundancy analysis ordination
bi-plots reflected the complicated functional relationship between the
woody plant community and insects after the invasion of B.
xylophilus , and the invasion of B. xylophilus can promote the
population growth of herbivorous insects, parasitic insects, and
detritivorous insects.