The Impact Of Stand Types And Site Conditions On Po Pulation Dynamic Of Kunyushan Web-spinning Sawfly (Cephalcia Kunyushanica)

Relationships between herbivorous insect and plant diversity are a central topic in biodiversity-functioning research in forest ecosystem, and mechanism under this relationship is the most important theoretical foundation for sustainable management of forest pests. The near-natural conditions, in which the host of Kunyushan web-spinning sawfly (Cephalcia kunyushanica) ranges a proportion from 0% to 100% in Kunyushan region naturally formatted away from anthropogenic interference, have the advantages of providing a herbivorous insect-host system with a multitude of interactions that might not have developed in the same way in artificial experiments.This study was conducted in the 40 permanent plots set by Kunyushan Ecosystem Station and to use Kunyushan web-spinning sawfly-host system as a model. Fisrt, forest community and plant diversity of Kunyushan ecosystem was analyzed. Based on this, the interactions between site conditions, plant community factors and larval density of C. kunyushanica were analyzed to explain factors and structures of forest community mediating C. kunyushanica population fluctuations. Furthermore, the functions of herbivory by C. kunyushanica on growth and mortality of Pinus densiflora was detected. The study could further improve and supply knowledge of forest system mediating pests and ecologically based pest management technologies. It also has important therotical significance for promoting development of forest protection and forest ecology. The main results are as follows:1. Principal coordinate analysis revealed that plant communities at Kunyushan Forest Reserve could be grouped into six stand types. Species indicator values (IndVals) analysis suggested the each stand type had its own unique indicators. No statistically significant difference was found among the six stand types in their abundance and species richness, while the diversity indices of Fisherα, Shannon–Wiener and Simpson were significantly different among these stand types. The highest Fisherαindex and Shannon–Wiener index were found in broadleaved stand types, whose Simpson index was the lowest. Fisherαand Shannon–Wiener indexes showed no statistically significant differences among the other stand types, i.e. P. thunbergii stands, P. densiflora stands, Larix kaempfer/Cunninghamia lanceolata stands, conifer-Quercus acutissima mixed stands and conifer-broadleaved mixture. Rényi diversity profiles were more intuitive to display some detailed differences of species diversity in communities than that which can not be distinguished by other analysis approaches. ANOVA analysis suggested that shrub and herb diversity indices under various elevations, slope positions, slope gradients and soil depths were not significantly different, except that Shannon-Winner index (F=4.665, P=0.002)and Simpson index(F=2.465, P=0.044) were significantly different in various stand types. However, Rényi diversity profiles indicated that, by having consistently higher profiles than other stand types, broadleaved stands and conifer-broadleaved mixed stands appeared to have higher species diversity, while the other five stand types could not be ordered in diversity with one another as they had corresponding diversity profiles that intersected. On the other hand, site conditions had profound effect on diversity. Plots at lower elevation, lower slope position and gradient, soil thickness > 5cm and ranging 1-3 cm, had lower species diversity. The mean tree number density decreased from 13000 trees/hm2 in 1996 to 2377 trees/hm2 in 2008, which indicated that stands in this region have entered the stem exclusion stage. Shannon diversity index (H) and evenness index (JS) of shrub and herbage plant communities under P. densiflora pure forest was 2.50 and 0.79 in 2008, lower than (H) 2.69 and (JS) 0.85 in 1996, respectively. In P. thunbergii mixed forest, Shannon diversity index (H) and evenness index (JS) of shrub and herbage plant communities was 2.27 and 0.79 in 2008, higher than (H) 1.85 and (JS) 0.77 in 1996, respectively. Species turnover rate (T) was 125.9%, which implied the change of species over 13 years in Kunyushan forest communities. The formation of diverse forest stand types mainly resulted from anthropogenic disturbance about 30 years ago, when new species were introduced and many trees were planted. 2. Larval density of C. kunyushanica differed significantly among year 2006, 2008 and 2009, which it peaked in 2006 and maintained at the lowest level in 2009. In 2006, larval density was significantly positive related to elevation, slope, slope aspect, the host proportions, and Shannon-winner index. The significant positive relation between larval density and host proportion corresponded with“resource concentration hypothesis”. Microclimate triggered by site conditions directly affected the occurrence of C. kunyushanica in 2008. However, larval density in 2009 positively related to stand age, which suggested that the trend of host selection changed along with larval density. Thses results revealed that mechanism for herbivorous insect population outbreak is far more complex in landscape scale than in stand level.3. Path analysis permitted insight into the causal mechanism by making a variety of predictions about the strength of direct and indirect interactions causing web-spinning sawfly occurrences in Kunyushan region. 21 significant paths were extracted from total 402 paths, which 381 paths were not significant. The residual paths with 5.22% in proportion of total paths explained 33.29% information of original model. The results indicated that direct interaction to affect web-spinning sawfly outbreak in 2006 included elevation, slope, slope position, host proportion, and Shannon-Winner index. All these factors had a significant positive effect on sawfly population outbreak. All stand features were directly affected by site conditions, and these stands features further affect other factors and all these combining factors together affected sawfly population. Shannon-Winner index had a direct effect on sawfly population, while it was affected by both site conditions and stand features.4. The study indicated that no difference of herbivory to host trees was found among different forest types. Almost all forest types performed strong“associational susceptibility”to Kunyushan web-spinning sawfly, except that weak“associational resistance”was observed in conifer-Quercus acutissima mixed stands in 2008, while this resistance only existed other than in 2008. The result confirmed that taxonomically dissimilar plants may be more likely to interact in ways that lead to“associational resistance”, while the strength, consistency, and relative impact of“associational susceptibility”or“associational resistance”on plant fitness also are likely to vary temporally and spatially as environmental conditions and herbivore and plant abundance change. Further, this was probably the first study to prove that site conditions might be contributors for landscape level“associational resistance”which led to continious low larval density in sites at low elevation and shady slope.5. Plant diversity played a main role for Kunyushan web-spinning sawfly occurrence. Sawfly larval density increased positively with increasing of plant diversity index values, which the diversity performance differed with the change of forest types. This conclusion is in contrast to many studies reporting a decrease in herbivory or herbivorous insect abundance with increasing plant diversity both in forests and in other systems.6. Current year shoot length under different feeding modes was significantly different (F1,7=2.422,p=0.02) and so did the growth rate (F1,7=2.439,p=0.019). Growth of current year shoot was not affected by current year feeding while was affected by last year’s feeding. Current year shoot suffered significant length reduction when it had previously experienced high intensities of two successive herbivore attacks last year and the year before. However, increment of DBH in various stands under different sawfly severities showed no significant difference. This indicated that infestations of sawfly may have obvious impact on shoot growth while it might have little affection on persistence of whole pine stands. We found a significant higher mortality rate (average 13.35%) in severely infested stands whose sawfly larval density was > 25 larvae/tree than that in slightly infested stands (p=0.003). Meanwhile, a positive correlation was found between tree’s mortality and sawfly larval density and canopy cover. Growth of diameter at breast height (DBH) of severely infested stands was slightly higher than that of slightly infested stands while there was no significant difference between them. Sawfly disturbance was most likely the promoter for reverse“J”shape of diameter distribution of stands. This indicated that higher tree mortality caused by sawfly attack could accelerate adjustment of stand density in severely infested stands compared to the same course by population self-thinning in slightly infested stands, however, the attack of sawfly did not inhibited tree growth promisingly. C. kunyushanica population dynamics is governed by nonlinear processes, while the impact of site and stand features on its population has a continious effect. The results suggested that future monitoring of sawfly population could be limited in key sites and forest types, which would be effective for accurate prediction of future sawfly occurrence. Keywords: Kunyushan Forest Ecosystem Station, forest ecosystem, Cephalcia kunyushanica,...