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.