Competitive Interactions between Hydrilla (Hydrilla verticillata) and Vallisneria (Vallisneria americana) as Influenced by Insect Herbivory

Two experiments (winter and summer) were conducted in outdoor tanks using addition-series methods to evaluate the impact of specialized feeding by two biological control agents,Hydrellia pakistanaeDeonier andBagous hydrillaeO'Brien, on competitive interactions between hydrilla [Hydrilla verticillata(L.f.) Royle] and vallisneria (Vallisneria americanaMichx). Competitive abilities of each plant species were determined using the reciprocal-yield model of mean plant weight. In the absence of the biocontrol agents, intraspecific competition from hydrilla on itself was 8.3 times stronger than interspecific competition from vallisneria.Hydrellia pakistanaeinterfered with hydrilla canopy formation by removing as much as 80% of the plant biomass in the top 30 cm of the water column. Damage byH. pakistanaealso caused a 43% reduction in hydrilla tuber production during the winter experiment. Similarly,B. hydrillaecaused up to a 48% reduction in hydrilla plant weight in the summer experiment. Neither insect species damaged vallisneria. As a result, there were significant shifts in the competitive balance between hydrilla and vallisneria due to selective insect feedings. In the presence ofH. pakistanae, hydrilla intraspecific competition was nearly equal to interspecific competition from vallisneria, indicating that hydrilla had lost its competitive edge over vallisneria.Bagous hydrillaealso produced similar, but smaller, shifts in the relative competitive abilities of hydrilla and vallisneria. These results indicate that biological control agents can disrupt the competitive balance between plant species in favor of native species, thus adding another element to the weed biological control strategies.     

Hydrellia spp. [Diptera: Ephydridae] attackingHydrilla verticillata in south India

The leaf-mining fliesHydrellia sp. andH. pakistanae Deonier were observed attacking the aquatic weedHydrilla verticillata (L.f.) L.C. (Hydrocharitaceae) in south India. Their life histories were very similar. The duration of the immature stages ofH. pakistanae was 25 to 29 days. The number of leaves ofH. verticillata mined by a single larva varied from 8 to 12. In the laboratory, adults feeding on 50% aqueous solution of honey lived for 6 to 21 days and laid an average of 26 eggs. Several overlapping generations occurred in the field except during the period of drought. BothHydrellia sp. andH. pakistanae were heavily parasitized by the braconidsAdemon sp. nr.decrescens Nees andChaenusa sp.      

Tree seedling establishment under insect herbivory: edge effects and inter- annual variation

As the density and species composition of insects may change in relation to distance from the forest edge, the role of herbivory in tree establishment may also change across edges. To determine the importance of insect herbivory in tree establishment, insect densities were experimentally altered at different distances from the forest edge. Plots were established at three distances from the edge, with plots located in forest, edge, and field habitats. In half of each plot, insect densities were reduced by insecticide application. Seeds of two tree species, Acer rubrum and Fraxinus americana, were planted into each plot in 1995. The experiment was repeated in 1996 with the addition of Quercus palustris and Quercus rubra.Distance from the forest edge was the most important factor in determining seedling emergence and mortality. Overall seedling performance increased from field to edge to woods, although responses varied among species. In 1995, a drought year, insect removal increased emergence and decreased mortality of tree seedlings. In 1996, a year with normal precipitation, insect removal had much less effect on A. rubrum and F. americana. For the two Quercus species, mortality was reduced by insect removal. The tree species differed in their susceptibility to insect herbivory, with Acer rubrum the most susceptible and Fraxinus americana the least. Herbivory by insects was shown to have the potential to affect both the composition and spatial pattern of tree invasions. Herbivore importance differed greatly between the two years of the study, making the interaction between insects and tree seedlings variable both in space and time.     

Trade-offs in plant responses to herbivory influence trophic routes of production in a freshwater wetland

Responses of aquatic macrophytes to leaf herbivory may differ from those documented for terrestrial plants, in part, because the potential to maximize growth following herbivory may be limited by the stress of being rooted in flooded, anaerobic sediments. Herbivory on aquatic macrophytes may have ecosystem consequences by altering the allocation of nutrients and production of biomass within individual plants and changing the quality and quantity of aboveground biomass available to consumers or decomposers. To test the effects of leaf herbivory on plant growth and production, herbivory of a dominant macrophyte, Nymphaea odorata, by chrysomelid beetles and crambid moths was controlled during a 2-year field experiment. Plants exposed to herbivory maintained, or tended to increase, biomass and aboveground net primary production relative to controls, which resulted in 1.5 times more aboveground primary production entering the detrital pathway of the wetland. In a complementary greenhouse experiment, the effects of simulated leaf herbivory on total plant responses, including biomass and nutrient allocation, were investigated. Plants in the greenhouse responded to moderate herbivory by maintaining aboveground biomass relative to controls, but this response occurred at the expense of belowground growth. Results of these studies suggest that N. odorata may tolerate moderate levels of herbivory by reallocating biomass and resources aboveground, which in turn influences the quantity, quality and fate of organic matter available to herbivores and decomposers.     

Additive and nonadditive effects of herbivory and competition on tree seedling mortality, growth, and allocation

The interaction between simulated cotyledon herbivory and interspecific competition was studied in a greenhouse experiment using two species of trees, Acer rubrum and Quercus palustris, which commonly invade abandoned agricultural fields. Herbivory treatments were applied as a gradient of cotyledon removal for A. rubrum with 0, 25, 50, 75, and 100% of cotyledon tissue removed. Cotyledons from Q. palustris were clipped and removed (control, early, and late removal) to create a gradient of seed reserve availability. The competition treatment consisted of plugs of old-field vegetation that filled the pots with perennial cover. Mortality of seedlings was higher with competition. There was a significant interaction between herbivory and competition with the highest mortality occurring with competition at the highest intensity of herbivory in both species. Herbivory reduced biomass for Q. palustris only, while competition reduced biomass in both species. Neither species showed an interaction between herbivory and competition for growth. There was a significant interaction between herbivory and competition on allocation patterns for both species, with greater allocation to roots with competition at the highest intensity of herbivory. This study demonstrates the potential for cotyledon herbivory and competition to interact, altering the invasion of tree seedlings into abandoned agricultural land.     

Reproduction and Development of the Biocontrol AgentHydrellia pakistanae(Diptera: Ephydridae) on Monoecious Hydrilla

Previous investigations of the laboratory biology and host range ofHydrellia pakistanae,a biological control ofHydrilla verticillata(hydrilla), used the dioecious hydrilla biotype common to Florida, Texas, and California. A monoecious biotype that is now spreading throughout the mid-Atlantic states, California, and Washington was not investigated during these original studies. We therefore compared the dioecious and monoecious hydrilla biotypes as hosts forH. pakistanae.FemaleH. pakistanaeaccepted the two biotypes equally as ovipositional substrates. Overall developmental success differed little: 42% of eggs oviposited on monoecious hydrilla produced adults compared to 39% of eggs oviposited on dioecious hydrilla. Fly development required about 33 days on both biotypes (at 22 ± 2°C), but larvae that completed development mined 1.6 times as many leaves on monoecious hydrilla as on dioecious plants. These data suggest thatH. pakistanaewould be a useful biocontrol agent of monoecious hydrilla, should this plant invade areas where it can grow as a perennial.     

The effects of nitrate loading on the invasive macrophyte Hydrilla verticillata and two common, native macrophytes in Florida

Rising nitrate concentrations in the water column and the spread of invasive, non-native macrophytes are two major threats to Florida's oligotrophic, freshwater ecosystems. We used a replicated mesocosm experiment to test the effects of elevated nitrate concentrations in the water on the growth of the invasive macrophyte Hydrilla verticillata and two common, native submerged macrophytes Vallisneria americana and Sagittaria kurziana. Results from this study indicate that nitrate concentrations of 1.0 mg L⁻¹ NO₃-N in the water increased the final dry-weight biomass of H. verticillata by 2.75 times, while having no statistical effect on the growth of the two native species. Additionally, H. verticillata grew at a faster rate than the two native species in the low nitrate treatments accounting for 82% of the total biomass, indicating that it may have the capacity to invade relatively pristine communities. In waters where nitrate concentrations continue to rise, the cost of control efforts for H. verticillata may substantially increase in the future.     

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

Effects of Zn<Superscript>2+</Superscript> on nodulation and growth of a South American actinorhizal plant, <Emphasis Type="Italic">Discaria americana</Emphasis> (Rhamnaceae)

Discaria americana is a xerophytic shrub which lives in symbiosis with an actinomycete of the genus Frankia. The objective of this paper was to investigate the effects of high soil Zn²⁺ concentrations on growth and nodulation on the association Discaria americana–Frankia with the aim of determining if this association is suitable for improving contaminated soils. Two experiments were performed in 1 dm³ pots containing soil and different Zn additions, from 0 to 2,000 mg Zn²⁺ kg⁻¹ dry soil, with or without N fertilization. Zn additions strongly delayed shoot and root growth, but once growth was initiated, the biomass production of the plants supplied with moderate Zn amounts did not differ from the control plants. Zn reduced the final nodule number, but not the total nodule biomass. At the end of the experiment only the highest Zn treatments showed a lower nodule weight than the control plants, while N addition completely inhibited nodulation. It is concluded than Zn reduces the number of Frankia infections, but once the actinomycete is inside the roots, nodules can continue growing according to plant demand for N, compensating the reduced nodule number with more biomass. On the other hand, there is a toxic effect of Zn itself on plants when present in very high concentrations.     

Swan foraging shapes spatial distribution of two submerged plants,favouring the preferred prey species

Compared to terrestrial environments, grazing intensity on belowground plant parts may be particularly strong in aquatic environments, which may have great effects on plant-community structure. We observed that the submerged macrophyte, Potamogeton pectinatus, which mainly reproduces with tubers, often grows at intermediate water depth and that P. perfoliatus, which mainly reproduces with rhizomes and turions, grows in either shallow or deep water. One mechanism behind this distributional pattern may be that swans prefer to feed on P. pectinatus tubers at intermediate water depths. We hypothesised that when swans feed on tubers in the sediment, P. perfoliatus rhizomes and turions may be damaged by the uprooting, whereas the small round tubers of P. pectinatus that escaped herbivory may be more tolerant to this bioturbation. In spring 2000, we transplanted P. perfoliatus rhizomes into a P. pectinatus stand and followed growth in plots protected and unprotected, respectively, from bird foraging. Although swan foraging reduced tuber biomass in unprotected plots, leading to lower P. pectinatus density in spring 2001, this species grew well both in protected and unprotected plots later that summer. In contrast, swan grazing had a dramatic negative effect on P. perfoliatus that persisted throughout the summer of 2001, with close to no plants in the unprotected plots and high densities in the protected plots. Our results demonstrate that herbivorous waterbirds may play a crucial role in the distribution and prevalence of specific plant species. Furthermore, since their grazing benefitted their preferred food source, the interaction between swans and P. pectinatus may be classified as ecologically mutualistic.     

Influence of migrant tundra swans (Cygnus columbianus) and Canada geese (Branta canadensis) on aquatic vegetation at Long Point, Lake Erie, Ontario

Numerous studies have shown that large, herbivorous waterfowl can reduce quantity of aquatic plants during the breeding or wintering season, but relatively few document herbivory effects at staging areas. This study was done to determine if feeding activities of tundra swans (Cygnus columbianus columbianus) and Canada geese (Branta canadensis) had a measurable additive influence on the amount of aquatic plants, primarily muskgrass (Chara vulgaris), wild celery (Vallisneria americana), and sago pondweed (Potamogeton pectinatus), removed during the fall migration period at Long Point, Lake Erie, Ontario. Exclosure experiments done in fall 1998 and 1999 showed that, as compared to ducks and abiotic factors, these two large herbivorous waterfowl did not have any additional impact on above or below ground biomass of those aquatic plants. As expected, however, there were substantial seasonal reductions in above-ground and below-ground biomass of aquatic plants in wetlands that were heavily used by all waterfowl. We suggest that differences in large- and small-scale habitat use, feeding activity, and food preferences between tundra swans and other smaller waterfowl as well as compensatory herbivory contributed to our main finding that large waterfowl did not increase fall reductions of Chara spp, V. Americana, and P. pectinatus biomass.     

凤眼莲入侵程度对金鱼藻和黑藻生长及种间关系的影响

湿地生态系统中凤眼莲（Eichhornia crassipes）入侵造成湿地植物群落结构退化及功能崩溃,直接影响沉水植物的生长繁殖及初级生产力。目前关于凤眼莲的入侵机制有一定的研究,而关于凤眼莲入侵程度对沉水植物金鱼藻（Ceratophyllum demersum）和黑藻（Hydrilla verticillate）生长及种间关系的影响相对缺乏。以外来入侵植物凤眼莲,沉水植物金鱼藻和黑藻为研究对象,设计凤眼莲入侵程度（无入侵,轻度入侵对应盖度25%,重度入侵对应盖度75%）交叉定植方式（黑藻单种模式、金鱼藻单种模式,金鱼藻和黑藻混种模式）的控制实验,探究凤眼莲入侵强度对沉水植物金鱼藻和黑藻生长及种间关系的影响。结果表明,凤眼莲入侵程度显著降低了金鱼藻的生物量、分枝数;黑藻的株高、分枝数和分节数。无凤眼莲入侵时,两种沉水植物生物量均最大,两者种间竞争关系较强;随凤眼莲入侵盖度增加,两种沉水植物的生物量先急剧降低后略微增加,种间关系经过微弱促进后又变为竞争作用,其中黑藻表现出明显的竞争优势。此外,凤眼莲入侵略微降低了水体中的总氮、总磷含量。结构方程模型分析结果表明凤眼莲入侵以及水体总氮、总磷等水体理化性质对沉水植物生长均有显著负向影响（P<0.05）,且水体理化性质对沉水植物生长的影响强于凤眼莲入侵。总之,凤眼莲入侵显著降低了金鱼藻和黑藻生长繁殖,随着凤眼莲入侵程度增加,两种沉水植物种间关系由竞争转变为促进再转变为竞争。研究结果为凤眼莲入侵有效控制及湿地沉水植被的恢复与重建提供了一定的理论依据和技术支撑。     

Changes in biomass and spatial distribution of <Emphasis Type="Italic">Elodea nuttallii</Emphasis> (Planch.) St. John,an invasive submerged plant,in oligomesotrophic Lake Kizaki from 1999 to 2002

Distribution of pure Elodea nuttallii vegetation was surveyed from 1999 to 2002, immediately after the most recent expansion of the species in Lake Kizaki, Japan. During 2001 and 2002, areas of E. nuttallii vegetation rapidly diminished and the summer plant height decreased wherever the vegetation remained. The organic matter content, total phosphorus, and extracted P of the sediment from the vegetation bed were measured. A linear relationship was observed between the extracted P in the sediment and the biomass. The extracted P significantly decreased in the shallow littoral vegetation bed, where the biomass clearly diminished. A fertilization experiment using the shallow littoral sediment collected in the vegetation bed was conducted in 2001. In this experiment, apical shoots of E. nuttallii were planted in pots with fertilized sediment (nitrogen, phosphorus, and potassium additions). The growth of E. nuttallii shoots was significantly enhanced by enrichment with phosphorus alone. The ecological implication of sediment phosphorus limitation is discussed in relation to the cause of decline in the E. nuttallii population in Lake Kizaki.     

Shoot herbivory on the invasive plant, Centaurea maculosa, does not reduce its competitive effects on conspecifics and natives

Herbivory can have negative, positive, or no effect on plants. However, insect biological control assumes that herbivory will negatively affect the weed and release natives from competition. Centaurea maculosa, an invader in North America, is tolerant to herbivory, and under some conditions, herbivory may increase its competitive effects on natives. Therefore, we investigated two hypotheses: 1) herbivory stimulates compensatory growth by C. maculosa, which increases its competitive effects, and 2) herbivory stimulates the allelopathic effect of C. maculosa. In the greenhouse, Trichoplusia ni shoot herbivory reduced C. maculosa biomass when shoot damage exceeded 40% of the total original leaf area. Conspecific neighbors had no effect on C. maculosa biomass, and the presence of the natives Festuca idahoensis and F. scabrella had a positive effect on C. maculosa. Neighbors did not alter the effects of shoot herbivory. More importantly, even intense shoot herbivory on C. maculosa did not benefit neighboring plants. In a field experiment, clipping 50% of C. maculosa aboveground biomass in the early summer and again in the late summer reduced final biomass by 40% at the end of the season; however, this clipping did not affect total biomass production or reproductive output. Festuca idahoensis neighbors did not increase the effects of clipping, and aboveground damage to C. maculosa did not release F. idahoensis from competition. In the greenhouse we used activated carbon to adsorb allelochemicals, which reduced the competitive effects of C. maculosa on F. idahoensis but not on F. scabrella or other C. maculosa. However, we found no increase in the allelopathic effects of C. maculosa after shoot herbivory. In summary, our results correspond with others indicating that exceptionally high intensities of herbivory are required to suppress C. maculosa growth and reproduction; however, even intense herbivory on C. maculosa does not insure that native bunchgrasses will benefit.     

Growth of Hydrilla verticillata (L.f.) Royle under controlled conditions

Aquatic macrophytes are important resources for the maintenance of trophic chains and in biogeochemical processes, but they can also be deleterious for several uses if present in excess. Hydrilla verticillata was found in the Paraná River (Brazil) after 2005, which requires monitoring owing to the invasive potential of this species. In this study, we measured the growth of H. verticillata under controlled conditions and compared the growth dynamics for the two development strategies (branch and tuber). We show that this species has great potential to develop in tropical (Brazilian) aquatic ecosystems. The parameters from the modelling of the growth kinetics indicated a doubling time of 19.8 days for H. verticillata growing from stems; however, the growth from tubers were much faster, with doubling times ranging from 2.5 to 11 days. The delay for the tubers to sprout caused a decrease in the number of branches of H. verticillata stems. From the growth parameters obtained from the experiments under controlled conditions, we concluded that the high temperature and light availability in most South American reservoirs (including the Porto Primavera Reservoir where it was first recorded) are suitable for H. verticillata to compete and probably displace other native aquatic macrophytes, such as Egeria najas, Egeria densa, and Cerathophyllum demersum. This is a matter of concern because these and other submersed species are commonly found in several natural and man-made South-American aquatic ecosystems, where they are key for biodiversity maintenance.     

Herbivory and competition slow down invasion of a tall grass along a productivity gradient

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced aboveground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.Plant nomenclature follows Van der Meijden et al. (1990)     

Competition between two grass species with and without grazing over a productivity gradient

Soil nutrient-level and herbivory are predicted to have opposing effects on the allocation pattern of the competitive dominant plant species. Lower stem and higher leaf allocation are favoured when plants are grazed, whereas a higher stem allocation is favoured at high nutrient levels. Grazing by hares and geese can prevent invasion of the tall Elymus athericus, into short vegetation of Festuca rubra, at unproductive stages of salt-marsh succession but not at more productive stages. We hypothesise that the negative effect of herbivory on Elymus decreases due to increasing soil nitrogen levels and shifts the competitive balance towards this species. We tested how simulated grazing and nitrogen availability affected the competitive balance between adult plants of both grass species in a greenhouse experiment. Elymus had a higher above-ground biomass production, invested relatively more in stem and root tissue and had a larger shoot length than Festuca. The above-ground relative yield of Elymus in mixtures of both species increased with increasing nitrogen levels. This indicates that Elymus was the superior competitor at high soil fertility. Although clipping removed relatively more biomass from Elymus than from Festuca and exceeded the observed biomass removal in field conditions, it did not change the competitive balance between both species. Decreasing effects of herbivory due to increasing nitrogen levels are not a likely explanation for the invasion of Elymus in productive marshes. The results suggest that once Elymus has established it can easily invade vegetation dominated by Festuca irrespective of grazing by herbivores such as hares and geese. Herbivory by small herbivores may mainly retard the invasion of this plant by influencing establishment itself.     

Defoliation of Centaurea solstitialis Stimulates Compensatory Growth and Intensifies Negative Effects on Neighbors

Efforts to arrest the spread of invasive weeds with herbivory may be hindered by weak effects of the herbivores or strong compensatory responses of the invaders. We conducted a greenhouse experiment to study the effects of defoliation and soil fungi on competition between the invasive weed Centaurea solstitialis and C. solstitialis and Avena barbata, a naturalized Eurasian annual grass, and Nassella pulchra, a native California bunchgrass. Surprisingly, considering the explosive invasion of grasslands by C. solstitialis, Avena and Nassella were strong competitors and reduced the invader’s biomass by 80.2% and 80.1% over all defoliation and soil fungicide treatments, respectively. However, our experiments were conducted in artificial environments where competition was probably accentuated. When fungicide was applied to the soil, the biomass of C. solstitialis was reduced in all treatment combinations, but reduction in the biomass of the invader had no corollary impact on the grasses. There was no overall effect of defoliation on the final biomass of C. solstitialis as the invader compensated fully for severe clipping. In fact, the directional trend of the clipping effect was +6.4% over all treatments after eight weeks. A significant neighbor × soil fungicide × clipping effect suggested that the compensatory response was the strongest without soil fungicide and when C. solstitialis was alone (+ 19%). Our key finding was that the compensatory response of C. solstitialis in all treatments was associated with an increase in the weed’s negative effects on Nassella and Avena – there was a significant decrease in the total biomass of both grasses and the reproductive biomass of Avena in pots with clipped C. solstitialis. Our results were obtained in controlled conditions that may have been conducive to compensatory growth, but they suggest the existence of mechanisms that may allow C. solstitialis, like other Centaurea species, to resist herbivory.     

Growth,reproduction and defence in nettles: responses to herbivory modified by competition and fertilization

We studied the effects of environmental factors on the ability to compensate for simulated herbivory in an annual (Urtica urens) and two perennial (U. dioica subspp. dioica and sondenii) nettle species in order to test the compensatory continuum hypothesis. Further, we studied the expression of costs of structural defence in different environmental conditions. Herbivory interacted significantly with density and fertilization for height growth, branching and reproductive traits. The negative effects of simulated herbivory on relative height growth were less detrimental for plants grown in low density and high fertilization level, which supports the compensatory continuum hypothesis. However, with respect to other traits measured, our results do not support the idea of compensatory continuum. The negative effects of apical excision on inflorescence biomass were relatively more prominent in plants growing at low density and receiving more fertilization than on those growing in worse conditions. In addition, branching was reduced by apical excision regardless of resource levels. The lack of compensation for herbivory is explained by the role which the damaged or removed tissue plays in plant development and function. In the perennial U. dioica, defensive responses to herbivory, measured as changes in trichome density, were stronger than in the annual U. urens. In the southern subspecies dioica, trichome density increased in newly emerged leaves after leaf clipping. In the northern subspecies sondenii, trichome density increased after apical excision. The differences in the defensive responses between the two subspecies of U. dioica may be due to differential natural herbivory pressures on subspecies inhabiting different geographical regions. We observed negative phenotypic correlations between structural defence and other plant traits, which suggests the existence of costs of defence. In addition to differences among the species studied, the expression of costs of defence was dependent on the resource levels.     

不同水分处理和密度配置对牛鞭草与狗牙根生长与种间竞争的影响

为合理利用三峡库区消落带优良草本植物进行退化植被恢复,并探索恢复过程中草本植物的最佳混植比例,选取三峡库区消落带适生先锋草本植物牛鞭草(Hemarthria compressa)(H)和狗牙根(Cynodon dactylon)(C)为研究材料,于2016年4月29日在盆栽控制条件下设置3种不同水分条件(对照组——CK组、浅淹组——SF组、全淹组——TF组)、7种配置比例,每盆牛鞭草与狗牙根株数分别按2株进行递增与递减,具体的配比分别为H0C12,H2C10,H4C8,H6C6,H8C4,H10C2、H12C0,比较研究混植条件下牛鞭草与狗牙根在水淹环境中的生长及二者的竞争作用。研究发现:(1)无论在单植还是二者混植条件下,水分胁迫均显著降低狗牙根与牛鞭草生物量,且牛鞭草对水淹胁迫的响应更敏感;(2)狗牙根和牛鞭草的生长均具有明显的密度制约效应,但狗牙根的反应更为强烈;(3)不同水分与密度条件下,混植体系总相对生物量均大于1。在CK组,狗牙根与牛鞭草表现出竞争关系;在SF组和TF组,二者之间的竞争作用减小,表现出一定的促进作用。综合分析本试验不同水分与密度条件下牛鞭草与狗牙根的总生物量、根冠比、竞争系数(相对总生物量),发现常规供水处理下牛鞭草和狗牙根的最佳配置比例为H2C10,而浅水淹和深水淹处理下最佳配置比例为H8C4。研究结果可以为三峡库区消落带不同海拔位草本植被的恢复及管理提供依据,也为生态类型相同或相似地区人工恢复草本植被提供理论参考。