Defense Syndromes Of Young Leaves Against Insect Herbivores In Subtropical Evergreen Broad-leaved Forests

Plants and insect herbivores are integral to the forest ecosystem. They formed close relationships through a long term evolution process. They connect with primary production and consumption and play a major role on nutrient cycling and energy transfer in forest ecosystem. Young leaves are preferred by herbivores because they are more nutritious and tender in comparision with the mature leaves. The majority of damage occurs during the short window when leaves are young and expanding. Although young leaves are ephemeral, they are key for understanding the interaction relationship between plant and herbivore. We ask how the young leaves protect themselves from feeding by leaf-feeding insects?Before answering this problem, we need to address the following questions:(1) What are the herbivory patterns of young leaves? (2) Facing herbivory pressure, do young leaves present similar suits of traits that characterize defense sydromes and what are these syndromes? (3) What factors influence plant defense? (4) How do leaf defense syndromes affect insect herbivores? And how the insect herbivores adapt to it?Evergreen broad-leaved forest is the regional vegetation of subtropical area in eastern China, and differs from tropical and temperate forests in both species diversity and community structure. To understand the plant-insect interactions, the study was conducted in Mt. Meihuashan, Fujian province (116°45'25"-116°57'24" 25°15'24"-25°35'44") and Tiantong National Forest Park, Zhejiang province (29°48'N,121°47'E). Through field monitoring, traits analyzing, artificial simulating and leaf-feeding insect larvae feeding, herbivory pattern diversity, leaf phenology, leaf traits, insect diversity, larvae growth and defense traits were record. We analyzed the characteristics of leaf herbivory, plant defense syndromes and the effects of plant quality on leaf-feeding insects. The results and conclusions were as follows:1. The leaf damage of leaves in evergreen broad-leaved forest is between those in tropical rain forest and temperate deciduous forest. The majority of damage occurs during the short window when leaves are young and expanding. To determine the patterns of herbivory in subtropical evergreen broad-leaved forest, the study investigated herbivory rate and frequency on leaves of 76 evergreen plant species in Mt. Meihuashan, Fujian province and Tiantong National Forest Park, Zhejiang province. In this study, herbivory on leaves was 7.21% and herbivory frequency was 32.95%. Leaf herbivory rate in shrub species (7.52%) was significantly higher than that of tree species (7.02%). With herbivory frequency between 10% and 60%, herbivory rates of most species were less than 10%. Herbivory rate on leaves of companion species (7.23%) was higher than dominant species (6.94%), there were no significant difference between them (p>0.05). Over 60% of the lifetime damage occurs during the few weeks that leaves are expanding. More herbivory occurs at lower latitudes. The herbivory rate of young leaves in the same specie decrease along latitudinal gradient.The results indicated that, the leaf damage of leaves in evergreen broad-leaved forest is between those in tropical rain forest (11.1%) and deciduous forest (7.1%). Life form and dominance affected herbivory on leaves. Rates of leaf damage reduced with the rising of latitude and altitude gradient. The majority of damage occurs during the short window when leaves are young and expanding.2. Young leaves have traits against herbivory that may occur together and increase defense efficiency. The plant defense traits grouped into three syndromes for plants.To test whether there are defense syndomes in evergreen broad-leaved forest, we measured defense traits and leaf herbivory on 38 plants in Tiantong, Zhejiang province. Our research showed that most pairwise correlations of traits were complementary. The defense syndromes of these species clusters in evergreen broad-leaved forest are associated with either low nutritional quality, or a balance of higher nutritional quality coupled with defense or escape:(1) The small area species with lower leaf nutrients content and water content might employ'low nutritional quality'strategy to reduce leaf losses. (2) The large area species that leafed out later with higher leaf nutrients content, low chemical defense and toughness but high leaf expansion rate might employ'escape'strategy to reduce insect attack. (3) The species that leafed out with larger area, higher leaf nutrients content and higher defense material and lower leaf expansion rate, might employ'nutrition and defense'strategy to reduce leaf losses.The results suggested that, plant defense are composed of multiple traits, they might be organized into coadapted complexes. The defense syndromes of these species clusters in evergreen broad-leaved forest are associated with'low nutritional quality','nutrition and defense'or'escape'.3. Defense strategies of young leaves were influenced by ontogenetic stages, expansion stages and early-season insect attack.Many factors may influence the defense of young leaves against leaf-feeding insects. We carried out studies on the typical evergreen broad-leaved forest plants—Schima superba and Castanopsis fargesii to understand the effect factors of plant defense.(1) Leaf traits influencing plant quality as food and/or shelter for herbivores may change during plant ontogeny, and as a consequence, influence the amount of herbivory that plants receive as they develop. To assess plant ontogenetic differences in foliage quality as food for herbivores, nutritional and defensive traits were evaluated in saplings and reproductive trees of S. superba. Herbivore density was 1.52 times higher and herbivory was significantly greater in saplings than in reproductive trees (p<0.01). Accordingly, concentrations of total foliar phenols were higher in reproductive trees than in saplings (44%-April;29.7%-August), whereas leaf toughness, water and nitrogen concentration did not vary between ontogenetic stages. The results indicated that ontogenetic changes in traits influencing plant defense capability. With increasing defense capability, leaf damage was lower for reproductive trees than for saplings (p<0.01).(2) In this study, we measured damage throughout the whole leaf expansion to understand the herbivory pattern and the defense strategy on young leaves of C. fargesii. Leaf herbivory frequency and herbivory rate on young leaves in unfolded stage were significantly higher than in folded stage (F1,32=8.97, p=0.0054;F1,32=12.38, p=0.0014). The herbivory of leaves in folded stage consisted with large amount of low proportion herbivory, while the leaves in folded stage were mainly occupied by high proportion herbivory. Leaves were intensively grazed in the nighttime (t=2.51, p=0.017), which accounted for about 85% of the total leaf herbivory. Young leaves of C.fargesii might follow different strategies in the two expansion stages.(3) Induced defense is a principal response of plants to herbivory. Induced responses to herbivory are physical, nutritional, and chemical traits that change in plants following damage or stress, and that reduce the performance of herbivores. The result showed that young leaves that were damaged by Neospastos simaona in the beginning of the leaf expansion had higher toughness and concentration of tannins, but lower water content and nitrogen content when compared with the control leaves. As a result, the damaged shoots had lower rates of herbivory than control shoots. The results may imply that early-season herbivory on the leaves of S. superba reduced the nutritional quality of leaves and increased the amount of secondary compounds, therefore influencing later-season herbivory through the induction of plant responses that may act to reduce plant quality as food for herbivores.4. Diversity, growth and defensive traits of caterpillars were correlated with the nutritional and defensive traits of their hosts.A survey of leaf-feeding insect larvae feeding on plants was carried out in Tiantong, Zhejiang province. The results showed that:(1) There were temporal and spatial consistence on insects and young leaves. The greatest caterpillar density related to high leaf quality on spring, and maintained high density during summer. Then insect density decreased rapidly form autumn. (2) Growth and defensive traits of caterpillars were correlated with the nutritional and defensive traits of their hosts. Growth rates were faster on young than mature leaves (F1,43=13.50, p<0.001), reflecting the higher nitrogen and water content of the former. Growth was also positively correlated with leaf expansion rate (r2=0.51, p<0.001), because of higher nitrogen and water contents of fast-expanding young leaves. Specialists grew faster than generalists, with average growth of 1.8 times on young leaves and 4.2 on mature leaves. Generalists with low growth rate had more defense traits than specialists. Because slow growth for caterpillars increased their risk to natural enemies, generalists had more defense traits than specialists. These results suggested that diversity, growth and defensive traits of caterpillars were limited by leaf quality of host plants. For getting appropriate food resource, it is crucial for caterpillars to be synchronized with their host plants phenology. Growth and larva period of caterpillars were correlated with the nutritional and defensive traits of their hosts, which influenced the defensive traits against natural enemies indirectly.Innovations of this PhD dissertation are mainly:(1) Evergreen broad-leaved forest is the regional vegetation of subtropical area in eastern China, and differs from tropical and temperate forest in both species composition and community structure. We summarized the patterns of leaf herbivory in subtropical evergreen broad-leaved forest from space and time level.(2) Interactions between plant and leaf-feeding insects were studied systemically by field investigating, leaf traits analying, insects rearing and artificial simulating. The research system of multiple methods is rare.(3) Defense syndromes of young leaves were studied at the community level. It could be useful to add further insights to defense syndrome theory.(4) The factors which affect defense syndromes of young leaves were evaluated in the two typical evergreen broad-leaved forest plants—Schima superba and Castanopsis fargesii. Thus defense syndromes of young leaves in subtropical evergreen broad-leaved forests can be comprehensive known and understanded.(5) The relationships how host nutrition and defense traits affect in herbivore diversity, growth rates and defenses against natural enemies were determined. It is capable of deeper understanding on the interaction relationship between plant and leaf feeding insects...