Effects Of Simulated Herbivory Research Articles

Effects of Simulated Herbivory on the Vegetative Reproduction and Compensatory Growth of Hordeum brevisubulatum at Different Ontogenic Stages.

The response of plant vegetative reproduction and compensatory growth to herbivory has been widely discussed in biological and ecological research. Most previous research has supported the idea that both vegetative reproduction and compensatory growth are affected by their ontogenic stage. However, in many studies, the effects of foraging at different ontogenic stages was often confounded with the effects of foraging at different phenological periods for perennials. Our experiment was conducted in a natural meadow with a perennial grass, Hordeum brevisubulatum, and four ontogenic stages were chosen as our experimental objects. Three different clipping intensities during three phenological periods were implemented to explore the effects of simulating animal foraging on vegetative reproduction and compensatory plant growth. The results indicated that there were significant effects of ontogenic stage, phenological period, and clipping intensity on vegetative reproduction and compensatory growth. Moderate clipping intensities significantly increased the number of vegetative tillers, the total number of juvenile tillers and buds, and the aboveground biomass at early phenological periods for individuals at early ontogenic stages. Our results suggested that moderate clipping intensities could induce only an over-compensation response in perennial grasses at both the early ontogenic stage and phenological period, and the ability of compensatory growth gradually decreased with the progression of the ontogenic stage. This is of great significance to the primary production of grasslands subjected to herbivory.

Effect of simulated herbivory on bellyache bush (Jatropha gossypiifolia L.) growth and implications for biological control

Understanding the ability of invasive plants to compensate for herbivory damage is crucial for their management, but this has been poorly studied in weed biological control. Bellyache bush (Jatropha gossypiifolia L.) is an invasive weed and a target for biological control in Australia. To understand the response of bellyache bush to herbivory, we simulated herbivory damage, under field conditions, at a site in north Queensland. Two size classes, that is, seedlings and mature plants were subjected to either defoliation, shoot damage or a combination of both, and the damage was inflicted once, twice or thrice at 6–7 weeks interval. All herbivory types significantly reduced growth and biomass when herbivory was inflicted twice or thrice, whereas the single application had limited impact. Shoot damage alone and defoliation + shoot damage had a greater negative impact than defoliation treatment on plant height, basal stem diameter, the number of leaves, root length and the number of shoot tips produced while increasing the number of dead shoot tips. Defoliation alone and defoliation + shoot damage had a greater negative impact on the number of flowers and fruit pods produced than shoot damage. The greater negative effects noticed when herbivory was inflicted twice or thrice suggest that multivoltine biological control agents that can inflict shoot damage and defoliation should be sought for the biological control of bellyache bush.

Does mycorrhizal status alter herbivore-induced changes in whole-plant resource partitioning?

Both mycorrhizae and herbivore damage cause rapid changes in source-sink dynamics within a plant. Mycorrhizae create long-term sinks for carbon within the roots while damage by leaf-chewing herbivores causes temporary whole-plant shifts in carbon and nitrogen allocation. Thus, induced responses to herbivory might depend on the presence or absence of mycorrhizae. We examined the effects of mycorrhizal presence on induced resource partitioning in tomato (Solanum lycopersicon) in response to cues from a specialist herbivore Manduca sexta. Differences in plant size, growth and in the concentrations of carbon-based (soluble sugars and starch) and nitrogen-based (protein and total nitrogen) resources in three tissue types (apex, stem and roots) were quantified. Both mycorrhizae and simulated herbivory altered the concentrations of carbon- and nitrogen-based resources. Mycorrhizae promoted plant growth, altered sugar and starch levels. Simulated herbivory resulted in lower concentrations of most resources (sugar, starch and protein) in the rapidly growing apex tissue, while causing an increase in stem protein. There was only one interactive effect; the effects of simulated herbivory were much stronger on the sugar concentration in the apex of non-mycorrhizal plants. This clearly demonstrates that both mycorrhizal colonization and herbivore cues cause shifts in carbon- and nitrogen-based resources and further shows there is little interference by mycorrhizae on the direction and magnitude of plant responses to herbivory. Overall, our results suggest that herbivore cues, regardless of mycorrhizal status, reduce allocation to the growing apex while inducing protein storage in the stem, a possible mechanism that could increase the tolerance of plants to damage.

Drought and simulated deer herbivory reduce growth in Atlantic white cedar seedlings

White-tailed deer are overabundant in much of eastern North America and may be suppressing regeneration in some tree species. Herbivory initially reduces plant biomass, but it has the potential to stimulate compensatory growth. However, stressful environmental conditions may reduce recovery from herbivory. Seedlings of Atlantic white cedar, Chamaecyparis thyoides (L.) Britton, Sterns & Poggenb., are frequently browsed by white-tailed deer and also experience both flooded and dry soils in natural habitats. We conducted a growth chamber experiment to test for a potential interaction between simulated deer browsing and water availability treatments on seedling height, stem diameter, and above- and belowground biomass. Of the 180 study plants, half were subjected to an initial clipping treatment, and all plants were then exposed to high, intermediate, or low water availability for 90 days. We found that, over the short term, seedlings in favorable growing conditions could not compensate for lost biomass after simulated browsing. Drought reduced shoot biomass, and flooded soils reduced root but not shoot growth. Water stress and herbivory did not interact to reduce growth more than the effect of each treatment separately. In general, the effect of water stress on plant size was greater than the effect of simulated herbivory.

Effect of defoliation treatment on Mimosa pigra L. seedling survivability and resilience

Current advancements in the study of the theoretical basis of species interactions are helping scientists understand the basic parameters governing the dynamics of the interactions between generalist herbivores and their target plants. In practice, however, both inter- and intra-specific interactions between plants (as well as between herbivores and plants) within multispecies systems that are under the influence of interrelated biotic and abiotic variables are difficult to predict. Here, we discuss our findings on the effect of simulated herbivory on Mimosa pigra L. leaves on seedling survivability. In Malaysia, M. pigra, a semi-aquatic invasive plant introduced from the South American region, is already creating an ecological problem, especially in wetland habitats. To better understand the impact of herbivores on the M. pigra population, a simulated experiment of the herbivory effect on Mimosa seedlings was conducted. This experiment combined two treatments of simulated herbivory on the leaves of established Mimosa seedlings, that is, a two-level intensity treatment (50 and 100 % defoliation) and a seven-level frequency treatment (one to seven defoliations). The data suggest that Mimosa is highly resilient against herbivory. This plant was able to compensate for repeated losses, thus suggesting that the introduction of herbivores in an effort to totally eradicate the Mimosa population is unlikely to be successful.

Fruit mass variability in Vaccinium myrtillus as a response to altitude, simulated herbivory and nutrient availability

Variability in fruit mass may respond to environmental conditions, in addition to the genetic effect that has been reported in several reproductive features. We first analyzed fruit traits in 15 localities along an altitudinal gradient (300–2000ma.s.l.) in the berries of Vaccinium myrtillus. We then tested whether changes in natural conditions, such as nutrient availability and herbivory by ungulates, affected fruit features in this species. Therefore, nitrogen (N) addition and clipping were applied in plots situated in six of the 15 localities. In these plots, we later collected fruits in two consecutive years in order to obtain fruit mass, seed number and initial number of ovules per flower ovary. Data along the altitudinal gradient showed that fruit mass was determined by both ovule number and seed number, although these factors only explained approximately half of the variation. Fruit mass and number of ovules and seeds followed a similar pattern with altitude, increasing until reaching a maximum at around 1700m. Regarding the effect of the experimental treatments, fertilized non-clipped plants produced the heaviest fruits but clipping alone had no significant effect on fruit mass. Moreover, high soil pH had a positive influence on fruit mass. In addition, whereas the original number of ovules per ovary was not affected by the experimental treatments, the number of seeds per fruit increased and the abortion rate decreased in fertilized non-clipped plots, which means that the effect of simulated herbivory on these fruit traits depended on N addition. In conclusion, fruit mass was clearly affected by nutrient availability and this effect is attributed to an increase in the production of seeds rather than to an increase in module size and number of ovules per ovary. This outcome suggests that fruit mass is a plastic trait influenced by environmental factors like nutrient availability.

Effect of simulated herbivory on growth of the invasive weed Hygrophila polysperma: Experimental and predictive approaches

Herbivory simulation studies, through mechanical removal of leaf tissue, provide valuable insight about plant compensation and tolerance to defoliation. A mesocosm experiment was conducted to examine the effects of defoliation on growth and biomass accumulation of Hygrophila polysperma and thereby determine the critical level of herbivory necessary to achieve significant reduction in growth of this invasive plant. The data collected during the experiment were used to develop an empirical plant growth model to examine the usefulness of a model-based approach for a priori understanding of plant response to defoliation. The results of the mesocosm experiment showed that defoliation significantly influenced growth and biomass accumulation of hygrophila. The empirical plant growth model accurately simulated plant growth response to herbivory across treatments. Based on the results of the mesocosm experiment, an insect defoliator that causes complete defoliation of hygrophila at least at monthly intervals may be able to reduce biomass and growth of hygrophila. The ability of the mathematical model to predict the effects of defoliation on hygrophila suggest that it could be a useful tool for the selection of effective biological control agents.

The effect of simulated herbivory on growth and nutrient status of focal and neighbouring early successional woody plant species

Defoliation through herbivory is well known to affect target plants and their associated belowground properties, but the response of plants and their soil environment to defoliation of their neighbours is less well understood. We performed a controlled shade-house experiment involving three plant species that colonize New Zealand floodplains during primary succession, i.e. a palatable N2-fixing shrub (Carmichaelia odorata), a palatable deciduous small tree (Fuchsia excorticata) and a less palatable evergreen tree (Weinmannia racemosa). All species were grown in large pots for 40 months both singly and in two species pairs, and either one or both of the species grown in pairs were clipped to simulate herbivory. Responses of growth and foliar nutrient status to clipping varied strongly among species, with Carmichaelia having the largest response and Fuchsia having the smallest. Carmichaelia also enhanced soil microbial biomass and activity, and foliar N concentrations of Weinmannia. However, this did not translate to a net positive effect; instead Carmichaelia competitively reduced growth and foliar P concentrations of both other species. Most effects of Carmichaelia on the soil microflora, and growth and nutrient status of its neighbours, disappeared when Carmichaelia was clipped. Further, the effect of clipping Carmichaelia had a stronger impact on growth, soil activity and nutrient status of the other two species than did the clipping of those species. These results contradict expectations that N2-fixing plants should promote growth of other species in pioneer communities or that defoliation of N2-fixers exacerbate positive effects; in our study, defoliation of Carmichaelia merely mitigated the negative effects that it had on other species. They also suggest that interplay of competition and differential herbivory among coexisting plants has important implications for soil microflora and processes, relative nutrient acquisition and stoichiometry of coexisting plant species, and potentially plant community development.

Induced responses in the subtropical evergreen, broad-leaf tree Schima superba: Effects of simulated herbivory on leaf quality and subsequent insect attack during leaf expansion

Induced responses in the subtropical evergreen, broad-leaf tree Schima superba: Effects of simulated herbivory on leaf quality and subsequent insect attack during leaf expansion

Interactive Effects of Simulated Herbivory and Interspecific Competition on Survival of Blackbrush (Coleogyne ramosissima) Seedlings

Abstract I investigated the individual and joint effects of simulated herbivory and interspecific competition on survival of Coleogyne ramosissima Torr. (blackbrush) seedlings. Seeds of C. ramosissima and Bromus rubens L. (red brome grass) were collected at mid-elevations (1220 to 1770 m) of the Spring Mountains in southern Nevada. A pot trial experiment was conducted for six months (27 wk) in a controlled environmental greenhouse. This trial experiment, consisting of a 2 × 2 factorial design with simulated rodent herbivory and interspecific competition with B. rubens as the main effects, resulted in four treatments. Herbivory on C. ramosissima by heteromyid and non-heteromyid species of rodents was simulated by clipping the top 3 cm of young shoots. Significant interaction was detected between herbivory and competition for C. ramosissima seedling survival. When herbivory and competition were examined independently, both factors had significant adverse effects on C. ramosissima seedling survival, with the...

Effects of Simulated Herbivory on Defensive Compounds in Forage Plants of Norwegian Alpine Rangelands

A field study on the effects of current grazing practices on plants in central Norway found no increase in either phenolic compounds or proteinase inhibitors in plants subjected to grazing by sheep. This could either reflect insufficient damage to the plants due to low grazing intensity or a lack of a long-term response of the plants to grazing. In this study, we tested the hypothesis that damage to forage plants used by sheep and rodents in Norwegian alpine rangelands can stimulate a long-term (at least 2-week) increase in levels of defensive compounds. We used clipping experiments to manipulate the severity and timing of damage to eight species of common plants used by herbivores in Norway. Under greenhouse conditions (i.e., climate-controlled), we subjected mature plants to one of four clipping treatments: control (0% leaf tissue removed), low (10-15% leaf tissue removed), high (70-75% leaf tissue removed), or sustained (15% of leaf tissue removed every other day up to a total removal of 75%, i.e., five clippings over 9 days). Samples were collected 2 weeks after final clipping and analyzed for concentrations of total phenolics, proteinase inhibitors, ratio of total phenolics to soluble proteins, and ratio of proteinase inhibitors to soluble plant proteins. As expected, the different species of plants responded differently to simulated herbivory, but most plants either showed no response to mechanical wounding and tissue loss or had reduced defensive compounds. Thus, our results do not support the hypothesis that herbivory induces a long-term increase in defensive compounds in alpine rangelands of Norway, a result consistent with those from field studies.

Effects of simulated herbivory and resource availability on the invasive plant, Alternanthera philoxeroides in different habitats

In biological control programs, the insect natural enemy’s ability to suppress the plant invader may be affected by abiotic factors, such as resource availability, that can influence plant growth and reproduction. Understanding plant tolerance to herbivory under different environmental conditions will help to improve biocontrol efficacy. The invasive alligator weed ( Alternanthera philoxeroides) has been successfully controlled by natural enemies in many aquatic habitats but not in terrestrial environments worldwide. This study examined the effects of different levels of simulated leaf herbivory on the growth of alligator weed at two levels of fertilization and three levels of soil moisture (aquatic, semi-aquatic, and terrestrial habitats). Increasing levels of simulated (manual) defoliation generally caused decreases in total biomass in all habitats. However, the plant appeared to respond differently to high levels of herbivory in the three habitats. Terrestrial plants showed the highest below–above ground mass ratio (R/S), indicating the plant is more tolerant to herbivory in terrestrial habitats than in aquatic habitats. The unfertilized treatment exhibited greater tolerance than the fertilized treatment in the terrestrial habitat at the first stage of this experiment (day 15), but fertilizer appears not to have influenced tolerance at the middle and last stages of the experiment. No such difference was found in semi-aquatic and aquatic habitats. These findings suggest that plant tolerance is affected by habitats and soil nutrients and this relationship could influence the biological control outcome. Plant compensatory response to herbivory under different environmental conditions should, therefore, be carefully considered when planning to use biological control in management programs against invasive plants.

Effects of simulated herbivory on photosynthesis and N resorption efficiency in Quercus pyrenaica Willd. saplings

We examined the effects of simulated folivory by caterpillars on photosynthetic parameters and nitrogen (N) resorption efficiency in Quercus pyrenaica saplings. We analyzed the differences between intact leaves in control plants, punched leaves in damaged plants, and intact leaves in damaged plants. We then established two levels of simulated folivory: low (≈13% of the leaf area of one main branch removed per plant) and high (≈26% of the leaf area of one main branch removed per plant) treatments. No differences were found in net assimilation rate and conductance between either leaf type or treatment during the most favourable period for photosynthesis. However, the N content was lower in punched than in intact leaves, and as a result PNUE was higher in damaged leaves from treated trees. In leaf-litter samples, N mass was significantly higher in punched than in intact leaves in treated plants, and LMA was significantly higher in damaged than in intact leaves of both the treated and control plants. Consequently, N resorption efficiency was around 15% lower in damaged leaves as compared with intact leaves from treated and control plants. Mechanical injury to leaves not only triggered no compensatory photosynthetic response to compensate a lower carbon uptake due to leaf area loss, but also affected the resorption process that characterizes leaf senescence.

Effects of simulated herbivory on nitrogen enzyme levels, assimilation and allocation in Thalassia testudinum

Recent estimates suggest that turtlegrass ( Thalassia testudinum) can persist while being heavily grazed by smaller herbivores (e.g., sea urchins and bucktooth parrotfish) in many areas of the western North Atlantic. Based on studies conducted in terrestrial and aquatic ecosystems, we hypothesized that seagrasses compensate for grazer-induced losses of aboveground production by (1) increasing the uptake of growth-limiting nitrogen from the surrounding environment and (2) translocating this newly acquired nitrogen to grazed tissues. We tested our hypotheses by comparing the activity levels of nitrogen metabolizing enzymes (nitrate reductase (NR) and glutamine synthetase (GS/T)) and the distance of nitrogen translocation, in the roots and leaves of mechanically damaged and undamaged plants. These experiments were repeated along a seasonal productivity gradient (in mid summer, early fall, and late fall). Significantly greater enzyme activity (GS/T) was measured in the roots of clipped plants in all months. NR and GS/T activity was also significantly higher in partially clipped leaves than in unclipped outer leaves in two of three months. The degree of leaf damage had no impact on the enzyme activity of inner (i.e., younger) leaves. Nitrogen-15, as either ammonium or nitrate, was translocated up to 50 cm away from the application point within 48 h. Defoliation, however, did not lead to increased 15 N accumulation in clipped segments, suggesting that turtlegrass does not preferentially translocate nitrogen toward grazed areas.

Effects of simulated herbivory and resources on Chinese tallow tree (Sapium sebiferum, Euphorbiaceae) invasion of native coastal prairie

Trade-offs associated with maintaining herbivory resistance and herbivory tolerance are frequently inferred in plant life histories. Invasive success for many non-native plants is often attributed to novel resistance that repels native herbivores. Chinese tallow tree (Sapium sebiferum, Euphorbiaceae) is a non-native invader that threatens ecosystems throughout the southeastern United States, including imperiled coastal prairie regions. Low herbivore loads due to costly resistance are generally assumed to give Sapium a competitive advantage over native plants. We predicted that if Sapium experienced higher damage levels it would show significant decreases in growth and reduced ability to compete with native prairie vegetation. We conducted full-factorial, paired greenhouse and field experiments designed to assess the effects of simulated leaf herbivory on Sapium growth in the presence of prairie vegetation at different levels of light and nitrogen. Contrary to our expectations, neither low-intensity, chronic defoliation nor high-intensity, acute defoliation negatively affected Sapium seedlings in any resource combination in either experiment. These studies reveal that Sapium possesses considerable phenotypic plasticity, and herbivory tolerance is a newly appreciated trait that likely contributes to its invasive potential.

Within‐Alga Integration and Compensation: Effects of Simulated Herbivory on Growth and Reproduction of the Brown Alga, Fucus vesiculosus

We investigated the level of within‐alga integration, the consequent ability to compensate for simulated herbivory, and the effect of nutrient availability on the ability to compensate in Fucus vesiculosus (L.). Although F. vesiculosus lacks vascular connections, resource translocation occurs on a local scale. Vegetative apical parts and reproductive structures were dependent on the resources provided by the mature thallus. Thus, damage from herbivory has the potential to decrease the growth and reproductive success of F. vesiculosus. In contrast to vascular plants, F. vesiculosus did not show any plasticity in response to different types of damage. Removal of an apical part terminated the growth of the focal branch but had no effect on the growth of the neighboring branch. Removal of the older part of the thallus had no effect on the branching of the apical meristems. Thus, the within‐alga growth pattern remained similar whether the old thallus or an apical part was damaged. Increased resource availability did not enhance the ability of algae to compensate for lost thallus. The strong dependence of the apical part on the older thallus and the low plasticity in the type of response to damage indicate that in F. vesiculosus the tolerance of herbivory may be low.

Effects of simulated herbivory and resource availability on native and invasive exotic tree seedlings

The introduced tree, Sapium sebiferum, is a serious invader throughout the southeastern United States. Low herbivore loads, commonly attributed to high herbivore resistance, are assumed to give Sapium a competitive advantage over native vegetation. Because herbivory resistance is considered energetically expensive, we predicted that if Sapium experienced higher levels of damage its growth would significantly decrease. We examined the effects of different levels of simulated leaf herbivory on the growth of Sapium and Celtis laevigata, a native tree, at three levels of nitrogen and three levels of light availability. Potted seedlings were grown in a complete factorial design. Stem height growth rate, diameter growth rate and plant mass of both Sapium and Celtis were significantly affected by shade and nitrogen treatments. Nitrogen increased the growth of both species and seedlings in shaded conditions grew larger than those in ambient sunlight. Simulated leaf herbivory had a significantly negative impact on diameter growth rate and plant mass of Celtis. Surprisingly, neither Sapium height growth rate, diameter growth rate, nor any measure of plant mass were significantly affected by simulated leaf herbivory. Rather, Sapium exhibited considerable phenotypic plasticity and was able to compensate for leaf damage in all resource conditions. Our findings suggest herbivory tolerance is a previously unappreciated trait of Sapium that, in conjunction with phenotypic plasticity and low levels of herbivory in its introduced range, likely contributes to its invasiveness. Der eingeführte Baum, Sapium sebiferum, ist ein bedeutender Einwanderer in den gesamten südöstlichen Vereinigten Staaten. Es wird angenommen, dass eine geringe Herbivoren-Belastung Sapium einen Konkurrenzvorteil gegenüber der einheimischen Vegetation verschafft, die im Allgemeinen der hohen Herbivoren-Resistenz zugeschrieben wird. Weil Herbivoren-Resistenz als energetisch aufwändig betrachtet wird, sagten wir voraus, dass das Wachstum signifikant abnimmt, wenn Sapium einen höheren Grad der Beschädigung erfährt. Wir untersuchten die Auswirkungen unterschiedlicher Grade simulierter Herbivorie auf das Wachstum von Sapium und Celtis laevigata, einem einheimischen Baum, bei drei Niveaus der Stickstoff- und drei Niveaus der Lichtverfügbarkeit. Eingetopfte Keimlinge wurden in einem durchgehend faktoriellen Design kultiviert. Die Wachstumsraten der Stammhöhe und des Durchmessers sowie die Pflanzenmasse wurden bei Sapium und Celtis signifikant durch die Schatten- und Stickstoffbehandlungen beeinflusst. Stickstoff erhöhte das Wachstum bei beiden Arten und die Keimlinge wuchsen unter Schattenbedingungen höher als bei vollem Sonnenlicht. Simulierte Blattherbivorie hatte bei Celtis eine signifikant negative Einwirkung auf die Durchmesser-Wachstumsrate und die Pflanzenmasse. éberraschenderweise wurden bei Sapium weder die Höhen- und Durchmesser-Wachstumsrate, noch irgendein ein Maß für die Pflanzenmasse durch die simulierte Blattherbivorie signifikant beeinflusst. Stattdessen zeigte Sapium eine beachtliche phänotypische Plastizität und war in der Lage Blattbeschädigungen unter allen Ressourcen-Bedingungen zu kompensieren. Unsere Ergebnisse legen nahe, dass Herbivorietoleranz ein bisher nicht beachtetes Merkmal von Sapium ist, das wahrscheinlich in Verbindung mit der phänotypischen Plastizität und den geringen Graden der Herbivorie im neuen Verbreitungsgebiet zu seiner Invasivität beiträgt.

Effects of simulated herbivory in three old field Compositae with different inflorescence architectures.

The effects of simulated herbivory (early or late defoliation and cutting of the flowering shoot) on the growth and reproduction of three species of monocarpic composite forbs (Crepis pulchra, Picris hieracioides and C. foetida) with different inflorescence architectures were studied in experimental plots. For the three species studied, early defoliation had no significant effect on subsequent growth. In contrast, late defoliation, occurring at the start of the season of drought, had a negative effect on growth and reproduction in the two Crepis species, particularly C. foetida, but had less effect on P. hieracioides. Sexual biomass was more clearly affected by late defoliation than was vegetative biomass, although the effects differed markedly among species possibly as a result of differences in phenology. Clipping the flowering shoot removed about 3 times less biomass than late defoliation and had little effect on vegetative biomass. It had much greater effects on the sexual biomass in P. hieracioides and C. pulchra, and resulted in the production of many shoots sprouting from the rosette, allowing the treated plants to regain a vegetative biomass close to that of control plants. Clipping did however lead to the production of shorter shoots and a reduction in the number of capitula formed. In C. foetida, much branching occurred even when the main shoot was not cut; the architecture of individual plants was therefore only slightly changed by clipping the apical bud and the sexual biomass of this species was not affected by ablation of the flowering shoot. Overcompensation was found in only two families of C. pulchra for vegetative biomass. No over-compensation was found for sexual biomass, despite an increase in the number of flowering shoots in C. pulchra and P. hieracioides following clipping. However situations close to compensation for the vegetative biomass in the three species and in P. hieracioides for the sexual biomass were recorded. The response of the three study species to simulated herbivory were related to their architecture and to the time of defoliation.

The effect of natural and simulated insect herbivory, and leaf age, on the process of infection of Rumex crispus L. and R. obtusifolius L. by Uromyces rumicis (Schum.) Wint.

summaryThe development of uredinia of Uromyces rumicis (Schum.) Wint. was studied on Rumex crispus L. and R. obtusifolius L. plants inoculated in the laboratory. Fewer uredinia developed on leaves injured by simulated insect herbivory, those fed upon by Gastrophysa viridula Degeer (Coleoptera: Chrysomelidae), and young, incompletely expanded leaves, than on uninjured, fully expanded control leaves. The effect of simulated herbivory and leaf age on the process of U. rumicis infection was investigated using Calcofluor staining and epifluorescence microscopy. Simulated insect herbivory reduced significantly the proportion of sporelings producing appressoria and the proportion of sporelings with appressoria that entered the substomatal cavity. In R. obtusifolius, leaf injury also reduced the proportion of penetration hyphae that formed substomatal vesicles. The proportion of sporelings that produced intercellular hyphae in injured leaves was reduced by 58 % in R. crispus and 89 % in R. obtusifolius. G. viridula feeding induced similar resistance in R. obtusifolius leaves.Simulated insect herbivory reduced both uredinial density and the area occupied by intercellular hyphae by between 65% and 89%. Injury had a greater effect than leaf age on sporelings in the pre‐haustorial stages of development. However, the total area occupied by intercellular hyphae in young leaves was reduced by between 42% and 78% compared to mature leaves, owing to a lower uredinial density and, in R. obtusifolius, also to a significant reduction in the size of individual uredinia.

Effects of Simulated Herbivory and Intraspecific Competition on the Compensatory Ability of Birches

To improve our knowledge regarding plant responses to herbivory we subjected juvenile birches (Betula pubescens) to different types of simulated herbivory, i.e., removal of the top (1 cm) of the main stem or defoliaiton (including removal of main stem top), and to different levels of intraspecific competition (i.e., three stand densities). Treatments were also repeated during the following year. Birch responses to these treatments were measured as changes in plant height and dry mass. Defoliated plants showed reduced growth irrespective of the level of intraspecific competition. Topped plants in the low—density plots showed enhanced growth, i.e., over—compensation, whereas topped plants growth in the high— and medium—density plots were unaffected by the treatments. Our results indicate that we should expect a continuum of plant responses to herbivory. Thus, there is little value in asking whether herbivory has a positive or negative effect on plant growth. Instead, emphasis should be placed on identifying conditions under which we would expect different responses. Further, we suggest that overcompensation should not generally be considered as an adaptive response that has evolved as a consequence of natural selection due to herbivory, but rather as an indirect consequence of selection for competitive ability. Because apical dominance (assumed to be an adaptation to competition) restricts growth, the growth rate will increase if apical dominance is removed.