Trade-off between plant growth and defense? A comparison of sagebrush populations

We used ecotypic variation in big sagebrush (Artemisia tridentata) to examine potential trade-offs between inherent growth rate and tolerance or resistance to herbivory. Seeds were obtained from seven geographic populations, and 1,120 seedlings were established in a common garden. In one set of plots, plants were subjected to five treatments: control, regular insecticide spray, moderate browsing, severe browsing, or moderate browsing plus insecticide. Plants in a second set of plots were all untreated, and were used to estimate ambient growth, flower production, and susceptibility to herbivorous insects. In the first growing season, population differences in relative growth rate produced approximately seven-fold variation in mean biomass. Two populations of basin big sagebrush (A. tridentata tridentata) and one population of mountain big sagebrush (A. tridentata vaseyana) grew fastest; those of Wyoming big sagebrush (A. tridentata wyomingensis) showed the slowest growth. Bi-weekly application of insecticide for two growing seasons had no effect on the growth of either browsed or unbrowsed plants. All populations showed compensatory growth (but not overcompensation) in response to browsing, but the degree of compensation was unrelated to inherent growth rate. Similarly, there was no consistent relationship between plant growth rate and flower production in the second growing season. Some insects colonized fast-growing populations more frequently than slow-growing ones, but patterns of insect colonization were species-specific. At the level of geographic populations and subspecies, we found little evidence of a built-in trade-off between inherent growth rate and the ability to tolerate or resist herbivory. Because population ranks for growth rate changed substantially between seasons, attempts to correlate growth and defense characters need to account for differences in the growth trajectories of perennial plants.     

Environmental context determines within- and potential between-generation consequences of herbivory

Plant tolerance to herbivory may depend on local environmental conditions. Models predict both increased and decreased tolerance with increasing resources. Transgenerational effects of herbivory may result in cross-generation tolerance. We evaluated within- and potential between-generation consequences of deer browsing in light-gap and understory habitats in the forest-edge herb, Campanulastrum americanum. Plants were assigned to deer-browsed, simulated-herbivory, and control (undamaged) treatments in the two light environments. In light gaps, plants were eaten earlier, more frequently, and had less vegetative recovery relative to uneaten plants than in the understory. As a result, browsed light-gap plants had a greater reduction in flowers and fruit than understory plants. This reduced tolerance was in part because deer browsing damaged plants in light gaps more than those in the understory. However, in the simulated herbivory treatment, where damage levels were similar between light habitats, plants growing in high-resource light gaps also had reduced tolerance of herbivory relative to those in the forest understory. C. americanum’s reproductive phenology was delayed by reduced light and the loss of the apical meristem. As a result, deer-browsed plants in the light gap flowered slightly later than uneaten plants in the understory. C. americanum has a polymorphic life history and maternal flowering time influences the frequency of annual and biennial offspring. The later flowering of deer-browsed plants in light gaps will likely result in a reduced frequency of high-fitness annual offspring and an increase in lower fitness biennial offspring. Therefore, additional between-generation costs of herbivory are expected relative to those predicted by fruit number alone.     

Effects of herbivory and genotype on growth and survivorship of sand-dune willow (Salix cordata)

Abstract.

• 1 The response of different clones of sand-dune willow, Salix cordata, to herbivory by a specialist herbivore, Altica subplicata, was studied in three glasshouse experiments. Plants were caged and exposed to three herbivory treatments: no beetles, low number of beetles, and high number of beetles.
• 2 Plants consistently had significantly higher growth rates in the absence of herbivory than under conditions of low or high herbivory (1.5–6 times higher).
• 3 Herbivore treatment influenced mortality from drought stress; more plants from the low and high herbivory treatments (40% and 80%) died from drought stress than did control plants (0%).
• 4 Clone genotype significantly influenced growth rates and the susceptibility of plants to drought stress. However, clones showed similar growth responses to herbivory, suggesting a lack of genetic variation in tolerance or resistance to herbivory.

     

No evolution of increased competitive ability or decreased allocation to defense in <Emphasis Type="Italic">Melaleuca quinquenervia</Emphasis> since release from natural enemies

If invasive plants are released from natural enemies in their introduced range, they may evolve decreased allocation to defense and increased growth, as predicted by the evolution of increased competitive ability (EICA) hypothesis. A field experiment using the invasive tree Melaleuca quinquenervia was conducted to test this hypothesis. Seeds were collected from 120 maternal trees: 60 in Florida (introduced range) and 60 in Australia (home range). Plants grown from these seeds were either subjected to herbivory by two insects from Australia that have recently been released as biological control agents or protected from herbivores using insecticides. Genotypes from the introduced range were initially more attractive to herbivores than genotypes from the home range, supporting EICA. However, genotypes from the introduced and home range did not differ in resistance to insects or in competitive ability, which does not support EICA. Plants from the introduced range had a lower leaf hair density, lower leaf: stem mass ratio, and a higher ratio of nerolidol: viridifloral chemotypes compared to plants from the native range. Plants with an intermediate density of leaf hairs and with high specific leaf area were more susceptible to herbivory damage, but there were no effects of leaf toughness or chemotype on presence of and damage by insects. Herbivory had a negative impact on performance of Melaleuca. Other than an initial preference by insects for introduced genotypes, there was no evidence for the evolution of decreased defense or increased competitive ability, as predicted by the EICA hypothesis. It does not appear from this study that the EICA hypothesis explains patterns of recent trait evolution in Melaleuca.     

Untangling individual variation in natural populations: ecological, genetic and epigenetic correlates of long-term inequality in herbivory

Individual variation in ecologically important features of organisms is a crucial element in ecology and evolution, yet disentangling its underlying causes is difficult in natural populations. We applied a genomic scan approach using amplified fragment length polymorphism (AFLP) markers to quantify the genetic basis of long‐term individual differences in herbivory by mammals at a wild population of the violet Viola cazorlensis monitored for two decades. In addition, methylation‐sensitive amplified polymorphism (MSAP) analyses were used to investigate the association between browsing damage and epigenetic characteristics of individuals, an aspect that has been not previously explored for any wild plant. Structural equation modelling was used to identify likely causal structures linking genotypes, epigenotypes and herbivory. Individuals of V. cazorlensis differed widely in the incidence of browsing mammals over the 20‐year study period. Six AFLP markers (1.6% of total) were significantly related to herbivory, accounting altogether for 44% of population‐wide variance in herbivory levels. MSAP analyses revealed considerable epigenetic variation among individuals, and differential browsing damage was significantly related to variation in multilocus epigenotypes. In addition, variation across plants in epigenetic characteristics was related to variation in several herbivory‐related AFLP markers. Statistical comparison of alternative causal models suggested that individual differences in herbivory are the outcome of a complex causal structure where genotypes and epigenotypes are interconnected and have direct and indirect effects on herbivory. Insofar as methylation states of MSAP markers influential on herbivory are transgenerationally heritable, herbivore‐driven evolutionary changes at the study population will involve correlated changes in genotypic and epigenotypic distributions.     

Effects of above- and belowground herbivory on growth,pollination, and reproduction in cucumber

Plants experience unique challenges due to simultaneous life in two spheres, above- and belowground. Interactions with other organisms on one side of the soil surface may have impacts that extend across this boundary. Although our understanding of plant–herbivore interactions is derived largely from studies of leaf herbivory, belowground root herbivores may affect plant fitness directly or by altering interactions with other organisms, such as pollinators. In this study, we investigated the effects of leaf herbivory, root herbivory, and pollination on plant growth, subsequent leaf herbivory, flower production, pollinator attraction, and reproduction in cucumber (Cucumis sativus). We manipulated leaf and root herbivory with striped cucumber beetle (Acalymma vittatum) adults and larvae, respectively, and manipulated pollination with supplemental pollen. Both enhanced leaf and root herbivory reduced plant growth, and leaf herbivory reduced subsequent leaf damage. Plants with enhanced root herbivory produced 35% fewer female flowers, while leaf herbivory had no effect on flower production. While leaf herbivory reduced the time that honey bees spent probing flowers by 29%, probing times on root-damaged plants were over twice as long as those on control plants. Root herbivory increased pollen limitation for seed production in spite of increased honey bee preference for plants with root damage. Leaf damage and hand-pollination treatments had no effect on fruit production, but plants with enhanced root damage produced 38% fewer fruits that were 25% lighter than those on control plants. Despite the positive effect of belowground damage on honey bee visitation, root herbivory had a stronger negative effect on plant reproduction than leaf herbivory. These results demonstrate that the often-overlooked effects of belowground herbivores may have profound effects on plant performance.     

Herbivory and plant competition reduce mountain beech seedling growth and establishment in New Zealand

The effect of herbivory and resource availability on the competitive ability of different plant species has been an area of intense debate amongst plant ecologists for at least two decades, but the interactive effects of herbivory and plant competition between woody and herbaceous plants are rarely studied and theory is poorly developed. This study used experimental manipulations on transplanted and naturally occurring mountain beech (Nothofagus solandri var. cliffortioides) seedlings to show the effects of deer browsing and competition from deer-induced, herbaceous turf communities on mountain beech regeneration in New Zealand. Differences in the species composition of these turfs had little effect on mountain beech seedling establishment, but turf removal increased seedling growth and survivorship, showing that competition with other plants had direct effects on mountain beech regeneration. Deer browsing reduced the establishment and growth of seedlings, but the size of this effect did not vary with light and nutrient availability. There was no immediate compositional response of turf communities following the removal of deer browsing. The addition of nutrients appeared to reduce the intensity of belowground competition (stem growth increased relative to root growth) and increase seedling mortality, but there was no effect of changing levels of light. These results showed simple and direct negative effects of deer browsing on mountain beech regeneration. Indirect negative effects on regeneration were caused by deer-induced turf communities. We found little evidence for interactive effects between herbivory, plant competition and the availability of light or nutrients on seedling regeneration, which suggests that these factors acted independently. Nomenclature: Beever et al. (1992); Parsons et al. (1995); Edgar and Connor (2000); and Brownsey and Smith-Dodsworth (2000). Raukaua simplex is described by Mitchell et al. (1997). Coprosma “taylorae” is referred to by Eagle (1986) and Halocarpus biformis, Phyllocladus alpinus, Podocarpus hallii and Podocarpus nivalis by Wilson and Galloway (1993).     

Above‐ and belowground herbivory jointly impact defense and seed dispersal traits in Taraxacum officinale

Plants are able to cope with herbivores by inducing defensive traits or growth responses that allow them to reduce or avoid the impact of herbivores. Since above‐ and belowground herbivores differ substantially in life‐history traits, for example feeding types, and their spatial distribution, it is likely that they induce different responses in plants. Moreover, strong interactive effects on defense and plant growth are expected when above‐ and belowground herbivores are jointly present. The strengths and directions of these responses have been scarcely addressed in the literature. Using Taraxacum officinale, the root‐feeding nematode Meloidogyne hapla and the locust Schistocerca gregaria as a model species, we examined to what degree above‐ and belowground herbivory affect (1) plant growth responses, (2) the induction of plant defensive traits, that is, leaf trichomes, and (3) changes in dispersal‐related seed traits and seed germination. We compared the performance of plants originating from different populations to address whether plant responses are conserved across putative different genotypes. Overall, aboveground herbivory resulted in increased plant biomass. Root herbivory had no effect on plant growth. Plants exposed to the two herbivores showed fewer leaf trichomes than plants challenged only by one herbivore and consequently experienced greater aboveground herbivory. In addition, herbivory had effects that reached beyond the individual plant by modifying seed morphology, producing seeds with longer pappus, and germination success.     

Compensatory response of a legume root-nodule system to nodule herbivory by Sitona hispidulus

A laboratory study was conducted to examine the hypothesis that herbivory of nitrogen-fixing root nodules on legumes causes an exact compensatory response in nodule growth. Plants of Medicago sativa (L.) were grown hydroponically in clear plastic growth pouches so that the number and biomass of root nodules could be estimated nondestructively before, and 10 and 18 days after, partial denodulation. For treatments, plants were subjected to 23% denodulation by first-instar larvae of Sitona hispidulus (F.) (a common herbivore of Medicago and Trifolium) or 50% nodule pruning; additional plants were left untreated. Results indicated that nodule herbivory and nodule pruning caused an overcompensatory response in number of nodules. This was also true for number of nodule units (an indirect measure of nodule biomass) per plant at 10 days after denodulation but had changed to an exact compensatory response by day 18. An inverse relationship between change in number of nodule units and initial number of nodules indicated that compensatory nodulation was regulated by a feedback mechanism. Shoot and root biomasses were not affected by denodulation in this study.     

Intraspecific variation in growth,defense and herbivory in Dialium guianense (Caesalpiniaceae) mediated by edaphic heterogeneity

Based on resource allocation theory, a negative correlation is predicted between resource availability and plant defense against herbivore attack. Plants growing in resource-limited environments should display lower growth and higher defense against herbivores than plants growing where resources are less limited. Interspecific comparisons generally support these predictions. We evaluated this hypothesis at the intraspecific level, for two sapling populations of the canopy tree Dialium guianense (Caesalpiniaceae) at the Lacandona rain forest in southeast Mexico. The two populations occur in nearby sites, adjacent to the Chajul Field Station, under the same climatic conditions and within the same vegetation type, but with considerable differences in soil quality. The Floodplain site, under the influence of the Lacantún River, has favorable conditions for plant growth, in terms of nutrient and water availability, whereas the Hills site, given its location and soil characteristics, provides more restricted conditions for plant growth. Plants in the Floodplain site had higher growth and lower concentration of phenolic compounds than plants in the Hills (a two-fold difference in leaf area production, 1.3 less total phenolics). These differences were correlated with differences in herbivore attack, as saplings from the Hills, with a higher defensive potential, had lower average levels of herbivory than Floodplain plants (3.86% ± 0.80 vs. 7.75% ± 1.43 of leaf area loss). The relationship between the concentration of phenolic compounds and leaf quality for herbivores was consistent with preference assays carried out under laboratory conditions using two species of generalist herbivores, the army worm Spodoptera fugiperda and the native katydid Orophus sp. In 63.8 and 81.3% of the cases, third-instar larvae of S. fugiperda and adults of Orophus, respectively, preferred leaflets from the Floodplain plants population. Moreover, on average, the adults of Orophus consumed 2.9 times more leaf area from the Floodplain than from the Hills. In addition, a reciprocal transplant experiment indicated that phenotypic plasticity is likely to be the mechanism by which the plants expressed differential growth and traits affecting herbivory levels. In this experiment, growth and herbivory levels were 1.6 and 1.7 times higher, respectively, in plants transplanted into a Floodplain experimental plot than those in a Hills plot. This work contributes to our understanding of how edaphic heterogeneity can determine intraspecific variation in the relationship of plants with their herbivores and evaluates the underlying mechanisms promoting such influence. This revised version was published online in June 2006 with corrections to the Cover Date.     

History of exposure to herbivores increases the compensatory ability of an invasive plant

Release from natural enemies is frequently cited as an important factor contributing to plant invasions. But such effects are likely to be temporary—native herbivores can form new plant-herbivore associations and co-evolved insects might reach the new range. While the potential effects of the initial enemy release have been well studied, the consequences of any resumption of herbivory are poorly understood. Alternanthera philoxeroides is one of the most widespread invasive plants in China and is attacked both by a specialist herbivore introduced from the native range, Agasicles hygrophila, and a native beetle Cassida piperata Hope which has formed a new association. However, these insects are not found throughout the invaded range. To test the effect of the history of population exposure to herbivory on compensatory ability, plants were cultured from 14 populations around China that differed in whether A. hygrophila or C. piperata were present. Treatment plants were exposed to herbivory by A. hygrophila for a week until 50% of the leaf area was defoliated, then grown for 80 days. Plants from populations with prior exposure to herbivory (of any kind) accumulated more root mass than populations without prior exposure, indicating that prior exposure to insects can stimulate plant compensation to herbivory. We would recommend that potential changes in plant tolerance in response to prior exposure to herbivory are considered in invasive plant management plans that employ bio-control agents.     

Tradeoff between height and relative growth rate in a dominant grass from the Serengeti ecosystem

We determined the relationship between plant height and whole-plant relative growth rate (g g^-1 day^-1) for ten genotypes of Sporobolus kentrophyllus collected from an intensively grazed site on the Serengeti Plains, Tanzania. Plants were grown for 7 weeks in a greenhouse in Syracuse, N.Y., and harvested weekly. Plants that received simulated bovine urine showed a negative relationship between plant height and growth rate, suggesting a genetic tradeoff between competitive ability if ungrazed (height) and ability to recover from grazing (growth rate). There was no height-growth rate relationship under nitrogen addition rates similar to field mineralization rates. In addition, faster-growing, shorter plants tended to have relatively higher above-ground growth rates than slower-growing, taller plants. These results suggest that natural selection has maintained a gradient of morphologies within this species ranging from short, rapidly growing genotypes adapted to intense grazing conditions to tall, slow-growing, grazer-susceptible genotypes that are superior light competitors in absence of herbivory.     

Defoliation and below-ground herbivory in the grass Muhlenbergiaquadridentata : Effects on plant performance and on the root-feeder Phyllophaga sp. (Coleoptera, Melolonthidae)

In this study we evaluated (1) the combined effects of simulated defoliation and below-ground herbivory (BGH) on the biomass and nitrogen content of tillers and roots of the bunchgrass Muhlenbergia quadridentata and (2) the effect of defoliation on the survival of third-instar root-feeder larvae of Phyllophaga sp. The experiment was performed in a pine forest area at an altitude of 3200 m above sea level. The grass and the root-feeder species were native and dominant in the understory and in the macroarthropod root-feeder communities, respectively. Plants were established in pots in the field and were subjected to the following treatments in a factorial design: simulated defoliation (three levels) and BGH (with or without root-feeder larvae) with ten replicates per treatment. Plants were defoliated three times at 2-month intervals. The interaction between defoliation and root herbivory was significant for all components of plant biomass. In every case, light defoliation with BGH decreased live above-ground, root and total plant biomass, and the number of live tillers by more than 50% with respect to the same defoliation level without root-feeders. Plants apparently did not compensate for the carbon drain by root-feeders when a high proportion of older leaves were not removed by defoliation. Plants under heavy defoliation were not affected by the presence of root-feeders and showed a greater live/dead above-ground biomass ratio than lightly defoliated and control plants. Defoliation and BGH did not change tiller and root N concentrations but root herbivores did decrease live-tiller N content in lightly defoliated plants. Root-feeders but not defoliation decreased the root/shoot ratio by 40% and the live/dead above-ground biomass ratio by 45% through increased tiller mortality. Survivorship and final biomass of Phyllophaga sp. larvae were not affected by defoliation treatments during the 6-month study period. Received: 17 May 1996 / Accepted: 1 November 1996     

Facing herbivory on the climb up: Lost opportunities as the main cost of herbivory in the wild yam Dioscorea praehensilis

Plants with simple architecture and strong constraints on their growth may offer critical insights into how growth strategies affect the tolerance of plants to herbivory. Although Dioscorea praehensilis, a wild yam of African forests, is perennial, both aerial apparatus and tuber are annually renewed. Each year, the tuber produces a single stem that climbs from the ground to the forest canopy. This stem bears no leaves and no branches until it reaches optimal light conditions. Once in the canopy, the plant's production fuels the filling of a new tuber before the plant dies back to the ground. We hypothesized that if deprived of ant defense, the leafless growth phase is a vulnerable part of the cycle, during which a small amount of herbivory entails a high cost in terms of loss of opportunity. We compared the growth of stems bearing ants or not as well as of intact stems and stems subjected to simulated or natural herbivory. Ants reduce herbivory; herbivory delays arrival to the canopy and shortens the season of production. Artificially prolonging the stem growth to the canopy increased plant mortality in the following year and, in surviving plants, reduced the stem diameter and likely the underground reserves produced. Tuber size is a key variable in plant performance as it affects both the size of the aerial apparatus and the duration of its single season of production. Aerial apparatus and tuber are thus locked into a cycle of reciprocal annual renewal. Costs due to loss of opportunity may play a major role in plant tolerance to herbivory, especially when architectural constraints interact with ecological conditions to shape the plant's growth strategy.     

Do <Emphasis Type="Italic">Polylepis australis</Emphasis> trees tolerate herbivory? Seasonal patterns of shoot growth and its consumption by livestock

Browsing is one of the main factors determining survival, growth rate, woodland structure, and distribution of the high mountain tree Polylepis australis. This species has a substantial regrowth capacity, which may function as a mechanism to tolerate herbivory, but it is unknown to what extent it may compensate for the impact of herbivory. In 15 low-density tree stands subject to exclusion, moderate, and heavy livestock pressure, we selected 12 P. australis individuals <2 m tall, tagged four new shoots per tree and measured shoot length every month during a year. At the stand and at the tree level, we analyzed monthly dynamics of growth and browsing, and the annual output in terms of total browsing and total gross and net growth (not discounting and discounting consumption, respectively). In addition, we assessed the influence of stand, tree and microsite characteristics on growth and browsing patterns. Polylepis australis fully compensated for herbivory in terms of shoot gross growth at moderate, but not at heavy livestock pressure. In terms of net growth, this species did not fully compensate for herbivory at any stocking rate. We found a strong coupling between browsing and growth along the year, suggesting that regrowth attracts browsing, and browsing promotes regrowth. At the stand level, annual gross growth was not affected by habitat characteristics, while at the tree level, annual gross growth decreased on more rocky microsites for browsed but not for unbrowsed trees. We concluded that stocking densities should be limited to allow for a reasonable annual net growth, as its nitrogen rich leaves are a valuable food resource and P. australis forests provide important ecosystem services.     

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.     

Hylobius abietis L. feeding on the novel host Pinus brutia Ten. increases emission of volatile organic compounds

Plants respond to feeding by herbivorous insects by producing volatile organic chemicals, which mediate interactions between herbivores and plants. Yet, few studies investigated whether such plant responses to herbivory differ between historical host and novel plants. Here, we investigated whether herbivory by the pine weevil Hylobius abietis causes a release of volatile organic chemicals from a novel tree Pinus brutia and compared the relative amounts of volatiles released from herbivore's historical hosts and P. brutia. We collected volatiles emitted from P. brutia seedlings that were either subjected to feeding by H. abietis or no feeding. Our results indicated that feeding increased emission of volatile compounds, composed of monoterpenes and sesquiterpenes, and that the emission was several fold higher in the damaged seedlings than in undamaged seedlings. In particular, emission of monoterpenes and sesquiterpenes increased by 4.4‐and 10‐fold in the damaged plants, respectively. Strikingly, individual monoterpenes and sesquiterpenes showed much greater dissimilarity between damaged and undamaged seedlings. Furthermore, several minor monoterpenes showed negative relationships with the weevil gnawed area. We discussed these results with the results of previous studies focused on historical host plants of H. abietis and hypothesized the ecological relevance and importance of our results pertaining relevance to the plant–herbivory interactions.     

Spatial correlations between browsing on balsam fir by white‐tailed deer and the nutritional value of neighboring winter forage

Associational effects, that is, the influence of neighboring plants on herbivory suffered by a plant, are an outcome of forage selection. Although forage selection is a hierarchical process, few studies have investigated associational effects at multiple spatial scales. Because the nutritional quality of plants can be spatially structured, it might differently influence associational effects across multiple scales. Our objective was to determine the radius of influence of neighbor density and nutritional quality on balsam fir (Abies balsamea) herbivory by white‐tailed deer (Odocoileus virginianus) in winter. We quantified browsing rates on fir and the density and quality of neighboring trees in a series of 10‐year‐old cutovers on Anticosti Island (Canada). We used cross‐correlations to investigate relationships between browsing rates and the density and nutritional quality of neighboring trees at distances up to 1,000 m. Balsam fir and white spruce (Picea glauca) fiber content and dry matter in vitro true digestibility were correlated with fir browsing rate at the finest extra‐patch scale (across distance of up to 50 m) and between cutover areas (300–400 m). These correlations suggest associational effects, that is, low nutritional quality of neighbors reduces the likelihood of fir herbivory (associational defense). Our results may indicate associational effects mediated by intraspecific variation in plant quality and suggest that these effects could occur at scales from tens to hundreds of meters. Understanding associational effects could inform strategies for restoration or conservation; for example, planting of fir among existing natural regeneration could be concentrated in areas of low nutritional quality.     

Interactive effects of clipping and nutrient availability on the compensatory growth of a grass species

Resource availability is an important factor affecting the capacity of compensatory growth after grazing. We performed a greenhouse experiment with Poa bulbosa, a small perennial grass of the Mediterranean and Central Asian grasslands, to test the importance of nutrient availability for compensatory growth after clipping. We also compared the results with predictions of the limited resource model (LRM). Plants were grown at low and high fertilization levels and subjected to a clipping treatment. Contrary to the LMR, we found that in Poa plants compensatory growth occurred under the high fertilization level, while it did not occur under the low level. The LMR predicts a higher tolerance for grazing in the stressful environment. Our plants showed a significant decrease in their relative growth rates (RGR) after clipping. Although the plants allocated a 32–188% greater fraction of the mass to lamina growth after clipping, this greater allocation to the leaves did not fully compensate for the initial reduction in leaf area ratio (LAR). A sensitivity analysis showed for the clipped plants under the high fertilization treatment, that changes in leaf allocation (f _lam) enabled the plants to compensate for a part of the potential loss caused by defoliation. Probably, the increased biomass allocation comes largely from the bulbs. We conclude that the inconsistency of the LRM with our results originates in the lack of compensatory mechanisms in the model. To better understand how environmental conditions affect tolerance to herbivory, the effects of compensatory growth should be taken into account.     

Plant‐mediated interactions between two herbivores differentially affect a subsequently arriving third herbivore in populations of wild cabbage

• Plants are part of biodiverse communities and frequently suffer from attack by multiple herbivorous insects. Plant responses to these herbivores are specific for insect feeding guilds: aphids and caterpillars induce different plant phenotypes. Moreover, plants respond differentially to single or dual herbivory, which may cascade into a chain of interactions in terms of resistance to other community members. Whether differential responses to single or dual herbivory have consequences for plant resistance to yet a third herbivore is unknown.
• We assessed the effects of single or dual herbivory by Brevicoryne brassicae aphids and/or Plutella xylostella caterpillars on resistance of plants from three natural populations of wild cabbage to feeding by caterpillars of Mamestra brassicae. We measured plant gene expression and phytohormone concentrations to illustrate mechanisms involved in induced responses.
• Performance of both B. brassicae and P. xylostella was reduced when feeding simultaneously with the other herbivore, compared to feeding alone. Gene expression and phytohormone concentrations in plants exposed to dual herbivory were different from those found in plants exposed to herbivory by either insect alone. Plants previously induced by both P. xylostella and B. brassicae negatively affected growth of the subsequently arriving M. brassicae. Furthermore, induced responses varied between wild cabbage populations.
• Feeding by multiple herbivores differentially activates plant defences, which has plant‐mediated negative consequences for a subsequently arriving herbivore. Plant population‐specific responses suggest that plant populations adapt to the specific communities of insect herbivores. Our study contributes to the understanding of plant defence plasticity in response to multiple insect attacks.