Damage-induced changes in woody plants and their effects on insect herbivore performance: a meta-analysis

Simulated Herbivory Affects the Volatile Emissions of Oak Saplings, while Neighbourhood Affects Flavan-3-ols Content of Their Leaves

To what extent particular plant defences against herbivorous insects are constitutive or inducible will depend on the costs and benefits in their neighbourhood. Some defensive chemicals in leaves are thought to be costly and hard to produce rapidly, while others, including volatile organic compounds that attract natural enemies, might be cheaper and can be released rapidly. When surrounding tree species are more closely related, trees can face an increased abundance of both specialist herbivores and their parasitoids, potentially increasing the benefits of constitutive and inducible defences. To test if oaks (Quercus robur) respond more to herbivore attacks with volatile emission than with changes in leaf phenolic chemistry and carbon to nitrogen ratio (C: N), and whether oaks respond to the neighbouring tree species, we performed an experiment in a forest in Poland. Oak saplings were placed in neighbourhoods dominated by oak, beech, or pine trees, and half of them were treated with the phytohormone methyl jasmonate (elicitor of anti-herbivore responses). Oaks responded to the treatment by emitting a different volatile blend within 24 h, while leaf phenolic chemistry and C: N remained largely unaffected after 16 days and multiple treatments. Leaf phenolics were subtly affected by the neighbouring trees with elevated flavan-3-ols concentrations in pine-dominated plots. Our results suggest that these oaks rely on phenols as a constitutive defence and when attacked emit volatiles to attract natural enemies. Further studies might determine if the small effect of the neighbourhood on leaf phenolics is a response to different levels of shading, or if oaks use volatile cues to assess the composition of their neighbourhood.

A scarab beetle (Coleoptera: Scarabaeidae) causes tree mortality, delayed growth, and yield reduction in cacao with genotype-specific susceptibility to herbivory

BACKGROUND

This study explored the impact of Leucothyreus femoratus, a previously unreported folivorous pest in cacao cultivation, on cacao tree survival, development, and yield. The study was conducted in an experimental cacao plot in the Colombian plains, it featured 20 cacao genotypes in an agroforestry system, with plantain and Mexican sunflower providing temporary shade, and yopo offering permanent shade.

RESULTS

We found an infestation rate of 2.9 ± 0.3 adult beetles per cacao tree. L. femoratus larvae were discovered in association with the roots of all plants within the agroforestry arrangement; however, yopo and plantain exhibited the highest incidence of root-feeding larvae among these associated plants. Interestingly, male and female L. femoratus displayed distinct leaf consumption patterns in the laboratory, with females consuming more foliage relative to their body weight. Moreover, field observations highlighted the detrimental impact of L. femoratus herbivory on cacao tree survival and growth, leading to leaf skeletonization, reduced plant height, and stem diameter. Trees with over 50% leaf consumption suffered more than 20% mortality. Additionally, herbivory negatively affected cacao yield, correlating higher leaf surface damage with a decrease in harvested pods. The study also identified varying antixenotic resistance in different cacao genotypes, with some consistently displaying resistance while others showed variable levels during tree establishment and production stages.

CONCLUSION

This research underscores the significant role of L. femoratus as a cacao pest, emphasizing its adverse effects on cacao tree survival, development, and yield. Consequently, implementing effective control measures is vital for ensuring sustainable cacao cultivation. © 2023 Society of Chemical Industry.

Stem borer herbivory dependent on interactions of sugarcane variety, associated traits, and presence of prior borer damage

BACKGROUND

Herbivory risk is mediated by plant traits related to nutrition and defense that can vary within a species by genotype and age. Prior herbivore damage accrued by a plant can also interact with these traits to alter future herbivory potential by changing plant quantity or quality. Sugarcane (Saccharum spp.) is a perennial crop where aboveground biomass is harvested annually and with varieties differing in nutrition and defenses, making it conducive to evaluating varietal resistance mechanisms. Using data from 16 sugarcane varieties and 28 years, we assessed damage from the primary pest in Louisiana, the sugarcane borer (Diatraea saccharalis, SCB), relative to variety, crop year (ratoon), plant traits, and incidence of prior herbivory.

RESULTS

SCB damage differed among varieties but not crop year, mostly following previously established classifications of SCB resistance, and correlated with select nutritional and defense traits. Within a crop year, the probability of SCB damage increased with prior conspecific damage on the same stalk. However, the strength of this prior damage effect did not match known resistance patterns but still differed with variety.

CONCLUSIONS

Interactions of plant variety, traits, and prior pest damage but not age impacted sugarcane borer risk. Borer damage was associated with nutritional traits of fiber and sugar content, but not consistently with defensive traits like high stalk wax or hair density, indicating there may be additional resistance traits or indirect impacts of these traits on predators. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

Effects of leaf herbivory and autumn seasonality on plant secondary metabolites: A meta-analysis

Plant secondary metabolites (PSMs) are produced by plants to overcome environmental challenges, both biotic and abiotic. We were interested in characterizing how autumn seasonality in temperate and subtropical climates affects overall PSM production in comparison to herbivory. Herbivory is commonly measured between spring to summer when plants have high resource availability and prioritize growth and reproduction. However, autumn seasonality also challenges plants as they cope with limited resources and prepare survival for winter. This suggests a potential gap in our understanding of how herbivory affects PSM production in autumn compared to spring/summer. Using meta-analysis, we recorded overall production of 22 different PSM subgroups from 58 published papers to calculate effect sizes from herbivory studies (absence to presence) and temperate to subtropical seasonal studies (summer to autumn), while considering other variables (e.g., plant type, increase in time since herbivory, temperature, and precipitation). We also compared production of five phenolic PSM subgroups - hydroxybenzoic acids, flavan-3-ols, flavonols, hydrolysable tannins, and condensed tannins. We wanted to detect a shared response across all PSMs and found that herbivory increased overall PSM production in herbaceous plants. Herbivory was also found to have a positive effect on individual PSM subgroups, such as flavonol production, while autumn seasonality was found to have a positive effect on flavan-3-ol and condensed tannin production. We discuss how these responses might stem from plants producing some PSMs constitutively, whereas others are induced only after herbivory, and how plants produce metabolites with higher costs only during seasons when other resources for growth and reproduction are less available, while other phenolic PSM subgroups serve more than one function for plants and such functions can be season dependent. The outcome of our meta-analysis is that autumn seasonality changes some PSM production differently from herbivory, and we see value in further investigating seasonality-herbivory interactions with plant chemical defense.

Aphid and caterpillar feeding drive similar patterns of induced defences and resistance to subsequent herbivory in wild cotton

MAIN CONCLUSION

Our results indicate caterpillars and aphids cause similar levels of induced defences and resistance against caterpillars in wild cotton plants. These symmetrical effects are not consistent with patterns predicted by plant defensive signaling crosstalk and call for further work addressing the biochemical mechanisms underpinning these results. Plant-induced responses to attack often mediate interactions between different species of insect herbivores. These effects are predicted to be contingent on the herbivore's feeding guild, whereby prior feeding by insects should negatively impact subsequent feeding by insects of the same guild (induced resistance) but may positively influence insects of a different guild (induced susceptibility) due to interfering crosstalk between plant biochemical pathways specific to each feeding guild. We compared the effects of prior feeding by leaf-chewing caterpillars (Spodoptera frugiperda) vs. sap-sucking aphids (Aphis gossypii) on induced defences in wild cotton (Gossypium hirsutum) and the consequences of these attacks on subsequently feeding caterpillars (S. frugiperda). To this end, we conducted a greenhouse experiment where cotton plants were either left undamaged or first exposed to caterpillar or aphid feeding, and we subsequently placed caterpillars on the plants to assess their performance. We also collected leaves to assess the induction of chemical defences in response to herbivory. We found that prior feeding by both aphids and caterpillars resulted in reductions in consumed leaf area, caterpillar mass gain, and caterpillar survival compared with control plants. Concomitantly, prior aphid and caterpillar herbivory caused similar increases in phenolic compounds (flavonoids and hydroxycinnamic acids) and defensive terpenoids (hemigossypolone) compared with control plants. Overall, these findings indicate that these insects confer a similar mode and level of induced resistance in wild cotton plants, calling for further work addressing the biochemical mechanisms underpinning these effects.

Trait-mediated responses of caterpillar communities to spongy moth outbreaks and subsequent tebufenozide treatments

Outbreaks of the spongy moth Lymantria dispar can have devastating impacts on forest resources and ecosystems. Lepidoptera-specific insecticides, such as Bacillus thuringiensis var. kurstaki (BTK) and tebufenozide, are often deployed to prevent heavy defoliation of the forest canopy. While it has been suggested that using BTK poses less risk to non-target Lepidoptera than leaving an outbreak untreated, in situ testing of this assumption has been impeded by methodological challenges. The trade-offs between insecticide use and outbreaks have yet to be addressed for tebufenozide, which is believed to have stronger side effects than BTK. We investigated the short-term trade-offs between tebufenozide treatments and no-action strategies for the non-target herbivore community in forest canopies. Over 3 years, Lepidoptera and Symphyta larvae were sampled by canopy fogging in 48 oak stands in southeast Germany during and after a spongy moth outbreak. Half of the sites were treated with tebufenozide and changes in canopy cover were monitored. We contrasted the impacts of tebufenozide and defoliator outbreaks on the abundance, diversity, and functional structure of chewing herbivore communities. Tebufenozide treatments strongly reduced Lepidoptera up to 6 weeks after spraying. Populations gradually converged back to control levels after 2 years. Shelter-building species dominated caterpillar assemblages in treated plots in the post-spray weeks, while flight-dimorphic species were slow to recover and remained underrepresented in treated stands 2 years post-treatment. Spongy moth outbreaks had minor effects on leaf chewer communities. Summer Lepidoptera decreased only when severe defoliation occurred, whereas Symphyta declined 1 year after defoliation. Polyphagous species with only partial host plant overlap with the spongy moth were absent from heavily defoliated sites, suggesting greater sensitivity of generalists to defoliation-induced plant responses. These results demonstrate that both tebufenozide treatments and spongy moth outbreaks alter canopy herbivore communities. Tebufenozide had a stronger and longer lasting impact, but it was restricted to Lepidoptera, whereas the outbreak affected both Lepidoptera and Symphyta. These results are tied to the fact that only half of the outbreak sites experienced severe defoliation. This highlights the limited accuracy of current defoliation forecast methods, which are used as the basis for the decision to spray insecticides.

Elm tree defences against a specialist herbivore are moderately primed by an infestation in the previous season

The studies of the long-term effects of insect infestations on plant anti-herbivore defences tend to focus on feeding-induced damage. Infestations by an entire insect generation, including egg depositions as well as the feeding insects, are often neglected. Whilst there is increasing evidence that the presence of insect eggs can intensify plants' anti-herbivore defences against hatching larvae in the short term, little is known about how insect infestations, including insect egg depositions, affect plant defences in the long term. We addressed this knowledge gap by investigating long-term effects of insect infestation on elm's (Ulmus minor Mill. cv. 'Dahlem') defences against subsequent infestation. In greenhouse experiments, elms were exposed to elm leaf beetle (ELB, Xanthogaleruca luteola) infestation (adults, eggs and larvae). Thereafter, the trees cast their leaves under simulated winter conditions and were re-infested with ELB after the regrowth of their leaves under simulated summer conditions. Elm leaf beetles performed moderately worse on previously infested elms with respect to several developmental parameters. The concentrations of the phenylpropanoids kaempferol and quercetin, which are involved in egg-mediated, short-term effects on elm defences, were slightly higher in the ELB-challenged leaves of previously infested trees than in the challenged leaves of naïve trees. The expression of several genes involved in the phenylpropanoid pathway, jasmonic acid signalling, and DNA and histone modifications appeared to be affected by ELB infestation; however, prior infestation did not alter the expression intensities of these genes. The concentrations of several phytohormones were similarly affected in the currently challenged leaves of previously infested trees and naïve trees. Our study shows that prior infestation of elms by a specialised insect leads to moderately improved defences against subsequent infestation in the following growing season. Prior infestation adds a long-term effect to the short-term enhancer effect that plants show in response to egg depositions when defending against hatching larvae.

Bottom-up and top-down pressures mediate competition between two generalist insects

The effects of competition can have far-reaching consequences for individuals, populations, and communities and therefore we should strive toward a deeper understanding of competitive interactions. In some cases, dietary generalists may be predicted to experience weak competition effects because of their ability to use a wide range of host plants. However, competition between insects frequently occurs indirectly, which can hinder insects' abilities to avoid competitive interactions. Therefore, competition may be as strong among dietary generalists as among dietary specialists. Yet competition between insects that are dietary generalists is infrequently studied. We tested for evidence of competitive interactions between two common, temporally separated, generalist insects: the western tent caterpillar (Malacosoma californicum), which feeds early in the season, and the fall webworm (Hyphantria cunea), which feeds later in the season. Both species frequently use a common host plant species (chokecherry) as a preferred host at our field sites. We tested the relative strength of bottom-up effects resulting from competitive interactions between these two generalists with laboratory-rearing trials at the relevant time of year for each insect. We recorded three common fitness measures (development time, pupal mass, and survival) for caterpillars reared on chokecherry with no damage from either of our focal species, with tent caterpillar damage, and with fall webworm damage. To test the strength of top-down pressures on fall webworm larval fitness and any potential interactions with bottom-up effects, we reared larvae in the field either exposed to or protected from predators on host plants that either did or did not have tent caterpillars feeding on them earlier in the season. We found evidence of bottom-up fitness effects on tent caterpillars and top-down and bottom-up fitness effects on fall webworms confirming that tent caterpillars and fall webworms compete indirectly. Tent caterpillars had lower pupal mass when reared on leaves from shrubs damaged by fall webworms. Fall webworms had lower pupal mass and longer development time when reared on leaves from shrubs damaged by tent caterpillars. In field trials, fall webworms reared on shrubs damaged by tent caterpillars had a lower survival and pupal mass. We show evidence of indirect competition in temporally separated generalists through leaf quality (bottom-up effects) and natural enemies (top-down effects).

Effects of Nitrogen Supply on Induced Defense in Maize (Zea mays) against Fall Armyworm (Spodoptera frugiperda)

How nitrogen (N) supply affects the induced defense of plants remains poorly understood. Here, we investigated the impacts of N supply on the defense induced in maize (Zea mays) against the fall armyworm (Spodoptera frugiperda). In the absence of herbivore attack or exogenous jasmonic acid (JA) application, N supply increased plant biomass and enhanced maize nutrient (soluble sugar and amino acid) contents and leaf area fed by S. frugiperda (the feeding leaf area of S. frugiperda larvae in maize supplemented with 52.2 and 156.6 mg/kg of N was 4.08 and 3.83 times that of the control, respectively). When coupled with herbivore attack or JA application, maize supplemented with 52.2 mg/kg of N showed an increased susceptibility to pests, while the maize supplemented with 156.6 mg/kg of N showed an improved defense against pests. The changes in the levels of nutrients, and the emissions of volatile organic compounds (VOCs) caused by N supply could explain the above opposite induced defense in maize. Compared with herbivore attack treatment, JA application enhanced the insect resistance in maize supplemented with 156.6 mg/kg of N more intensely, mainly reflecting a smaller feeding leaf area, which was due to indole emission and two upregulated defensive genes, MPI (maize proteinase inhibitor) and PAL (phenylalanine ammonia-lyase). Hence, the optimal N level and appropriate JA application can enhance plant-induced defense against pests.

Water Availability Determines Tree Growth and Physiological Response to Biotic and Abiotic Stress in a Temperate North American Urban Forest

Warmer temperatures and frequent drought directly affect urban tree health. Both abiotic conditions also affect tree health via increased density of some insect pests. Warming is predicted to benefit urban trees by increasing carbon sequestration and allocation to biomass. However, increased drought and pests are rarely considered despite often co-occurring with heat. To determine the combined effects of these abiotic and biotic factors, we manipulated water availability for established urban red maple trees across a gradient of warming and pest density and measured leaf-level processes and tree growth over two years. We find that water availability is a major determinant of tree growth, physiological processes, and resilience to urban stress factors. Maples performed better with more water, which also made them resistant to effects of temperature and pest density. However, when drought became too severe, leaf-level processes declined with warming. Tree basal area growth was unaffected after two years, but stem elongation increased with increasing water, temperature, and pest density. We discuss potential mechanisms driving these responses and the implications in the context of urban forest management. Urban forest designs that reduce drought and align species adaptations to local conditions are critical for designing more resilient and productive urban forests.

Decline effects are rare in ecology

The scientific evidence base on any given topic changes over time as more studies are published. Currently, there is widespread concern about non-random, directional changes over time in the scientific evidence base associated with many topics. In particular, if studies finding large effects (e.g., large differences between treatment and control means) tend to get published quickly, while small effects tend to get published slowly, the net result will be a decrease over time in the estimated magnitude of the mean effect size, known as a "decline effect". If decline effects are common, then the published scientific literature will provide a biased and misleading guide to management decisions, and to the allocation of future research effort. We compiled data from 466 meta-analyses in ecology to look for evidence of decline effects. We found that decline effects are rare. Only ~5% of ecological meta-analyses truly exhibit a directional change in mean effect size over time arising for some reason other than random chance, usually but not always in the direction of decline. Most apparent directional changes in mean effect size over time are attributable to regression to the mean, consistent with primary studies being published in random order with respect to the effect sizes they report. Our results are good news: decline effects are the exception to the rule in ecology. Identifying and rectifying rare cases of true decline effects remains an important task, but ecologists should not overgeneralize from anecdotal reports of decline effects.

Species specific plant‐mediated effects between herbivores converge at high damage intensity

Plants are often exposed to multiple herbivores and densities of these attackers (or corresponding damage intensities) often fluctuate greatly in the field. Plant‐mediated interactions vary among herbivore species and with changing feeding intensity, but little is known about how herbivore identity and density interact to determine plant responses and herbivore fitness. Here, we investigated this question using Triadica sebifera (tallow) and two common and abundant specialist insect herbivores, Bikasha collaris (flea beetle) and Heterapoderopsis bicallosicollis (weevil). By manipulating densities of leaf‐feeding adults of these two herbivore species, we tested how variations in the intensity of leaf damage caused by flea beetle or weevil adults affected the performance of root‐feeding flea beetle larvae and evaluated the potential of induced tallow root traits to predict flea beetle larval performance. We found that weevil adults consistently decreased the survival of flea beetle larvae with increasing leaf damage intensities. In contrast, conspecific flea beetle adults increased their larval survival at low damage then decreased larval survival at high damage, resulting in a unimodal pattern. Chemical analyses showed that increasing leaf damage from weevil adults linearly decreased root carbohydrates and increased root tannin, whereas flea beetle adults had opposite effects as weevil adults at low damage and similar effects as them at high damage. Furthermore, across all feeding treatments, flea beetle larval survival correlated positively with concentrations of carbohydrates and negatively with concentration of tannin, suggesting that root primary and secondary metabolism might underlie the observed effects on flea beetle larvae. Our study demonstrates that herbivore identity and density interact to determine systemic plant responses and plant‐mediated effects on herbivores. In particular, effects are species‐specific at low densities, but converge at high densities. These findings emphasize the importance of considering herbivore identity and density simultaneously when investigating factors driving plant‐mediated interactions between herbivores, which advances our understanding of the structure and composition of herbivore communities and terrestrial food webs.

Cumulative meta-analysis: What works

To present time-varying evidence, cumulative meta-analysis (CMA) updates results of previous meta-analyses to incorporate new study results. We investigate the properties of CMA, suggest possible improvements and provide the first in-depth simulation study of the use of CMA and CUSUM methods for detection of temporal trends in random-effects meta-analysis. We use the standardized mean difference (SMD) as an effect measure of interest. For CMA, we compare the standard inverse-variance-weighted estimation of the overall effect using REML-based estimation of between-study variance τ 2 with the sample-size-weighted estimation of the effect accompanied by Kulinskaya-Dollinger-Bjørkestøl (Biometrics. 2011; 67:203-212) (KDB) estimation of τ 2 . For all methods, we consider Type 1 error under no shift and power under a shift in the mean in the random-effects model. To ameliorate the lack of power in CMA, we introduce two-stage CMA, in which τ 2 is estimated at Stage 1 (from the first 5-10 studies), and further CMA monitors a target value of effect, keeping the τ 2 value fixed. We recommend this two-stage CMA combined with cumulative testing for positive shift in τ 2 . In practice, use of CMA requires at least 15-20 studies.

Changes in Light Energy Utilization in Photosystem II and Reactive Oxygen Species Generation in Potato Leaves by the Pinworm Tuta absoluta

We evaluated photosystem II (PSII) functionality in potato plants (Solanum tuberosum L.) before and after a 15 min feeding by the leaf miner Tuta absoluta using chlorophyll a fluorescence imaging analysis combined with reactive oxygen species (ROS) detection. Fifteen minutes after feeding, we observed at the feeding zone and at the whole leaf a decrease in the effective quantum yield of photosystem II (PSII) photochemistry (Φ_PSII). While at the feeding zone the quantum yield of regulated non-photochemical energy loss in PSII (Φ_NPQ) did not change, at the whole leaf level there was a significant increase. As a result, at the feeding zone a significant increase in the quantum yield of non-regulated energy loss in PSII (Φ_NO) occurred, but there was no change at the whole leaf level compared to that before feeding, indicating no change in singlet oxygen (¹O₂) formation. The decreased Φ_PSII after feeding was due to a decreased fraction of open reaction centers (q_p), since the efficiency of open PSII reaction centers to utilize the light energy (Fv′/Fm′) did not differ before and after feeding. The decreased fraction of open reaction centers resulted in increased excess excitation energy (EXC) at the feeding zone and at the whole leaf level, while hydrogen peroxide (H₂O₂) production was detected only at the feeding zone. Although the whole leaf PSII efficiency decreased compared to that before feeding, the maximum efficiency of PSII photochemistry (Fv/Fm), and the efficiency of the water-splitting complex on the donor side of PSII (Fv/Fo), did not differ to that before feeding, thus they cannot be considered as sensitive parameters to monitor biotic stress effects. Chlorophyll fluorescence imaging analysis proved to be a good indicator to monitor even short-term impacts of insect herbivory on photosynthetic function, and among the studied parameters, the reduction status of the plastoquinone pool (q_p) was the most sensitive and suitable indicator to probe photosynthetic function under biotic stress.

Plant Mediated Interactions: Lower Sawfly Survival on Pines Previously Browsed by Moose

Insect herbivore performance and arthropod communities can be affected by mammalian grazing and browsing via altered plant communities and vegetation structure. Far less is known about whether changes to plant architecture can cause similar effects. Browsing generated changes to within plant architecture could potentially have large consequences for arthropod communities, herbivore survival and eventually damage to plants. This study investigates plant-mediated effects of ungulate browsing on arthropod predator communities and on the survival of herbivorous insects. More specifically we studied how different levels of ungulate browsing (1) influenced the arthropod predator community on Scots pine (Pinus sylvestris) and (2) affected the survival of the European pine sawfly (Neodiprion sertifer). We related these response variables to browsing-inflicted changes in pine architecture. An observational study of generalist arthropod predators on pine trees revealed a trend toward a quadratic response of ants to browsing intensity—i.e., a higher abundance of ants on moderately browsed trees and lower abundance on intensively browsed trees. A field survey of sawfly larvae revealed a 19% lower larval survival on browsed compared to un-browsed pines, but no difference in survival comparing pines with moderate and high intensity of browsing. A structural equation model revealed that moose generated changes to pine architecture had only a small effect on sawfly larval survival, suggesting additional mediating pine traits affected by browsing. We conclude that insect survival can be altered by ungulate browsing, which could affect damage levels.

Defoliation-induced tree growth declines are jointly limited by carbon source and sink activities

Defoliation resulting from herbivory, storm, drought, and frost may seriously impair tree growth and forest production. However, a comprehensive evaluation of defoliation impacts on tree carbon (C) assimilation and growth has not been conducted. We performed a meta-analysis of a dataset that included 1562 observations of 40 tree species from 50 studies worldwide, and evaluated defoliation impacts on photosynthetic capacity, C allocation, and tree growth. Our results showed that the reduced tree-level leaf area by defoliation outweighed the enhanced leaf-level photosynthesis, leading to a net reduction in tree C assimilation that was accompanied with decreases in nonstructural carbohydrates (NSCs) concentrations. The negative effects of defoliation on leaf NSCs decreased over time, but leaf production increased following defoliation, suggesting a shift in the C allocation towards shoots over roots. Defoliation intensity negatively affected tree growth, but post-defoliated recovery time did oppositely. The structure equation modelling showed that defoliation reduced tree growth mainly by indirectly reducing C assimilation (r = -0.4), and minorly by direct negative effect of defoliation intensity (r = -0.28) and positive effect of post-defoliated time (r = 0.33). These findings suggest that tree growth declines caused by defoliation are co-limited by C-source and sink activities, which provide a physiological basis of tree growth that is of significance in tree growth modelling and forest management under global changes.

Herbivory and leaf traits of Amazonian tree species as affected by irradiance

Herbivory is one of the major biotic stress factors that affect the establishment of plants. However, the main factors that drive herbivory in seedlings of Amazonian tree species are still not well understood. Here we investigated whether contrasting levels of irradiance influence herbivory according to different herbivory indicators and which leaf traits are most related to interspecific variation in herbivory under contrasting irradiance conditions. We measured the leaf area lost as a result of insect herbivory in five tree species planted in a silvicultural system of secondary forest enrichment according to two indicators, herbivore damage (accumulated since plant germination) and herbivory rate (measured over time), and two irradiance conditions, understorey PPFD 2.6 mol·m^-2 ·day^-1 ) and gap PPFD 33.1 mol·m^-2 ·day^-1 . Furthermore, we related the interspecific variation in herbivory to a set of leaf traits: SLA, RWC, sclerophylly, phenolic compound content, tannins, condensed tannins and non-structural carbohydrates. Herbivore damage was significantly affected by light availability and species, with the highest percentage variation observed in the Meliaceae (Carapa guianensis and Swietenia macrophylla). For the herbivory rate, only the interspecific variation was significant, with Bertholletia excelsa having the lowest rates. Chemical characteristics (phenolic compounds and tannins) were most related to herbivory rates, as well as highly influenced by light conditions. Non-structural carbohydrates (starch and sucrose) were also related to the interspecific variation in herbivory. The phenolic compounds and starch, as affected by light quantity, are species dependent. Thus, the selective pressure on herbivores may be driven by species-dependent responses to light conditions.

The Price of the Induced Defense Against Pests: A Meta-Analysis

Plants and phytophagous arthropods have co-evolved for millions of years. During this long coexistence, plants have developed defense mechanisms including constitutive and inducible defenses. In an effort to survive upon herbivore attack, plants suffer a resource reallocation to facilitate the prioritization of defense toward growth. These rearrangements usually end up with a penalty in plant growth, development or reproduction directly linked to crop losses. Achieving the balance to maximize crop yield requires a fine tune regulation specific for each host-arthropod combination, which remains to be fully elucidated. The purpose of this work is to evaluate the effects of induced plant defenses produced upon pest feeding on plant fitness and surrogate parameters. The majority of the studies are focused on specific plant-pest interactions based on artificial herbivory damage or simulated defoliation on specific plant hosts. In this meta-analysis, the relevance of the variables mediating plant-pest interactions has been studied. The importance of plant and pest species, the infestation conditions (plant age, length/magnitude of infestation) and the parameters measured to estimate fitness (carbohydrate content, growth, photosynthesis and reproduction) in the final cost have been analyzed through a meta-analysis of 209 effects sizes from 46 different studies. Herbivore infestation reduced growth, photosynthesis and reproduction but not carbohydrate content. When focusing on the analyses of the variables modulating plant-pest interactions, new conclusions arise. Differences on the effect on plant growth and photosynthesis were observed among different feeding guilds or plant hosts, suggesting that these variables are key players in the final effects. Regarding the ontogenetic stage of a plant, negative effects were reported only in infestations during the vegetative stage of the plant, while no effect was observed during the reproductive stage. In addition, a direct relation was found between the durability and magnitude of the infestation, and the final negative effect on plant fitness. Among the parameters used to estimate the cost, growth and photosynthesis revealed more differences among subgroups than reproduction parameters. Altogether, this information on defense-growth trade-offs should be of great help for the scientific community to design pest management strategies reducing costs.

Imidacloprid-mediated stress on non-Bt and Bt cotton, aphid and ladybug interaction: Approaches based on insect behaviour, fluorescence, dark respiration and plant electrophysiology

Plants and insects are parts of a complex system that involves interactions among many trophic levels, and it is important to understand the nature of such interactions. In the complex of interactions involving aphids and transgenic cotton expressing Bacillus thuringiensis, both the spraying of neonicotinoids and the occurrence of predatory coccinellids are common. However, there are gaps regarding the knowledge about possible impacts of neonicotinoids on physiological variables of the host plant and behavioural traits of the aphid (Aphis gossypii) and predator (Cycloneda sanguinea). Therefore, this study aimed to highlight the photosynthetic and electrical responses of the plant to the stress caused by the aphid attack combined with the stress generated by the use of imidacloprid in Bt and non-Bt cotton (Gossypium hirsutum L.) cultivars and to evaluate how this stress can influence the behavioural ecology of the predator and prey. Chlorophyll a fluorescence tests, dark respiration and electrophysiology on non-Bt and Bt cotton were carried out, the behaviour of the prey and predator was also evaluated with a video capture system. Our research is a study model that generates insights about possible impacts when using Imidacloprid without the occurrence of the pest on the plant, because the exposure of non-Bt and Bt cotton plants and the predator to imidacloprid unnecessarily, may result in stress on the physiology of the cotton plants and on the behaviour of the predator.

Clonal versus non-clonal milkweeds (Asclepias spp.) respond differently to stem damage, affecting oviposition by monarch butterflies

BACKGROUND

Oviposition decisions are critical to the fitness of herbivorous insects and are often impacted by the availability and condition of host plants. Monarch butterflies (Danaus plexippus) rely on milkweeds (Asclepias spp.) for egg-laying and as food for larvae. Previous work has shown that monarchs prefer to oviposit on recently regrown plant tissues (after removal of above-ground biomass) while larvae grow poorly on plants previously damaged by insects. We hypothesized that these effects may depend on the life-history strategy of plants, as clonal and non-clonal milkweed species differ in resource allocation and defense strategies.

METHODOLOGY/PRINCIPAL FINDINGS

We first confirmed butterfly preference for regrown tissue in a field survey of paired mowed and unmowed plots of the common milkweed A. syriaca. We then experimentally studied the effects of plant damage (comparing undamaged controls to plants clipped and regrown, or damaged by insects) on oviposition choice, larval performance, and leaf quality of two closely related clonal and non-clonal species pairs: (1) A. syriaca and A. tuberosa, and (2) A. verticillata and A. incarnata. Clonal and non-clonal species displayed different responses to plant damage, impacting the proportions of eggs laid on plants. Clonal species had similar mean proportions of eggs on regrown and control plants (≈35-40% each), but fewer on insect-damaged plants (≈20%). Meanwhile non-clonal species had similar oviposition on insect-damaged and control plants (20-30% each) but more eggs on regrown plants (40-60%). Trait analyses showed reduced defenses in regrown plants and we found some evidence, although variable, for negative effects of insect damage on subsequent larval performance.

CONCLUSIONS/SIGNIFICANCE

Overall, non-clonal species are more susceptible and preferred by monarch butterflies following clipping, while clonal species show tolerance to clipping and induced defense to insect herbivory. These results have implications for monarch conservation strategies that involve milkweed habitat management by mowing. More generally, plant life-history may mediate growth and defense strategies, explaining species-level variation in responses to different types of damage.

Insectivorous birds can see and smell systemically herbivore-induced pines

Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even when they cannot see or smell the actual herbivores or their feces. However, it often remained an open question whether birds are attracted by herbivore-induced changes in leaf odor or in leaf light reflectance or by both types of changes. Our study addressed this question by investigating the response of great tits (Parus major) and blue tits (Cyanistes caeruleus) to Scots pine (Pinus sylvestris) damaged by pine sawfly larvae (Diprion pini). We released the birds individually to a study booth, where they were simultaneously offered a systemically herbivore-induced and a noninfested control pine branch. In the first experiment, the birds could see the branches, but could not smell them, because each branch was kept inside a transparent, airtight cylinder. In the second experiment, the birds could smell the branches, but could not see them, because each branch was placed inside a nontransparent cylinder with a mesh lid. The results show that the birds were more attracted to the herbivore-induced branch in both experiments. Hence, either type of the tested cues, the herbivore-induced visual plant cue alone as well as the olfactory cues per se, is attractive to the birds.

Insect herbivory dampens Subarctic birch forest C sink response to warming

Climate warming is anticipated to make high latitude ecosystems stronger C sinks through increasing plant production. This effect might, however, be dampened by insect herbivores whose damage to plants at their background, non-outbreak densities may more than double under climate warming. Here, using an open-air warming experiment among Subarctic birch forest field layer vegetation, supplemented with birch plantlets, we show that a 2.3 °C air and 1.2 °C soil temperature increase can advance the growing season by 1–4 days, enhance soil N availability, leaf chlorophyll concentrations and plant growth up to 400%, 160% and 50% respectively, and lead up to 122% greater ecosystem CO₂ uptake potential. However, comparable positive effects are also found when insect herbivory is reduced, and the effect of warming on C sink potential is intensified under reduced herbivory. Our results confirm the expected warming-induced increase in high latitude plant growth and CO₂ uptake, but also reveal that herbivorous insects may significantly dampen the strengthening of the CO₂ sink under climate warming.

Warming is expected to increase C sink capacity in high-latitude ecosystems, but plant-herbivore interactions could moderate or offset this effect. Here, Silfver and colleagues test individual and interactive effects of warming and insect herbivory in a field experiment in Subarctic forest, showing that even low intensity insect herbivory strongly reduces C sink potential.

Responses in growth and phenolics accumulation to lateral bud removal in male and female saplings of Populus tremula (L.) under simulated climate change

The responses in growth and defense after tissue damage are highly variable in plants depending on species, damaged-tissue type and the intensity of damage. The prevailing abiotic conditions can also influence these responses. In this study, our aim was to examine how the removal of lateral vegetative buds affects the growth and accumulation of phenolics in saplings of the dioecious Populus tremula grown under simulated climate change. For three growing seasons, the saplings were grown under ambient conditions (control), elevated temperature (+2°C) and elevated UV radiation (30%) (UVB and UVA as its control), or a combination of these. In the fourth growing season, all saplings were grown under ambient conditions. The bud removal was performed twice - in summer and autumn - in the third year. The responses of growth and the accumulation of phenolics to the bud removal were measured at the end of the fourth growing season. Removal of 5% of the lateral buds resulted in higher leaf, stem and total plant biomass in both sexes of P. tremula saplings, compared to intact plants. The effects were greater in the temperature-treated plants, especially in the temperature-treated females. The concentrations of flavonoids and condensed tannins were higher in the bud-removed individuals. The concentration of condensed tannins was also higher in the males than in the females, opposite to the concentration of phenolic acids. There was no significant interaction between bud removal and UVB treatment on either growth or phenolics. Our results suggest that plants can allocate resources to both growth and defense simultaneously in response to tissue loss, and that global warming can modify the responses to some extent.

Variation in Wing Load of Female Spruce Budworms (Lepidoptera: Tortricidae) During the Course of an Outbreak: Evidence for Phenotypic Response to Habitat Deterioration in Collapsing Populations

Reproduction in female spruce budworms, Choristoneura fumiferana, entails sedentary oviposition early in life (gravid females with their heavy abdomen full of eggs are unable to sustain flight), followed by short- and long-range dispersal by females that have laid a portion of their eggs. Body size measurements (wing surface area and dry weight) of gravid females, spent females at death (after all eggs are laid), and inflight females captured at light traps were collected at one location (forest stands near Fredericton in New Brunswick) over multiple years, from the outbreak stage (1976-1979: peak budworm abundance) to late declining phase with collapsing populations (1988-1989, following near two-fold magnitude of decline in adult density after 1987). For both demographic phases, females rarely flew until having laid at least 40% of their eggs, in contradiction to the hypothesis that females in defoliated forest stands can fly upon emergence due to their light-weight abdomen. As expected, the weight and fecundity of females in 1988-1989 was significantly lower than early on; in terms of body size (wing surface area), however, females were larger in late outbreak phase. These trends suggest that females have evolved morphological adaptation to further dispersal from deteriorated habitats.

Changes in oak (Quercus robur) photosynthesis after winter moth (Operophtera brumata) herbivory are not explained by changes in chemical or structural leaf traits

Insect herbivores have the potential to change both physical and chemical traits of their host plant. Although the impacts of herbivores on their hosts have been widely studied, experiments assessing changes in multiple leaf traits or functions simultaneously are still rare. We experimentally tested whether herbivory by winter moth (Operophtera brumata) caterpillars and mechanical leaf wounding changed leaf mass per area, leaf area, leaf carbon and nitrogen content, and the concentrations of 27 polyphenol compounds on oak (Quercus robur) leaves. To investigate how potential changes in the studied traits affect leaf functioning, we related the traits to the rates of leaf photosynthesis and respiration. Overall, we did not detect any clear effects of herbivory or mechanical leaf damage on the chemical or physical leaf traits, despite clear effect of herbivory on photosynthesis. Rather, the trait variation was primarily driven by variation between individual trees. Only leaf nitrogen content and a subset of the studied polyphenol compounds correlated with photosynthesis and leaf respiration. Our results suggest that in our study system, abiotic conditions related to the growth location, variation between tree individuals, and seasonal trends in plant physiology are more important than herbivory in determining the distribution and composition of leaf chemical and structural traits.

Phytochemical Shift from Condensed Tannins to Flavonoids in Transgenic Betula pendula Decreases Consumption and Growth but Improves Growth Efficiency of Epirrita autumnata Larvae

Despite active research, antiherbivore activity of specific plant phenolics remains largely unresolved. We constructed silver birch (Betula pendula) lines with modified phenolic metabolism to study the effects of foliar flavonoids and condensed tannins on consumption and growth of larvae of a generalist herbivore, the autumnal moth (Epirrita autumnata). We conducted a feeding experiment using birch lines in which expression of dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS) or anthocyanidin reductase (ANR) had been decreased by RNA interference. Modification-specific effects on plant phenolics, nutrients and phenotype, and on larval consumption and growth were analyzed using uni- and multivariate methods. Inhibiting DFR expression increased the concentration of flavonoids at the expense of condensed tannins, and silencing DFR and ANR decreased leaf and plant size. E. autumnata larvae consumed on average 82% less of DFRi plants than of unmodified controls, suggesting that flavonoids or glandular trichomes deter larval feeding. However, larval growth efficiency was highest on low-tannin DFRi plants, indicating that condensed tannins (or their monomers) are physiologically more harmful than non-tannin flavonoids for E. autumnata larvae. Our results show that genetic manipulation of the flavonoid pathway in plants can effectively be used to produce altered phenolic profiles required for elucidating the roles of low-molecular weight phenolics and condensed tannins in plant–herbivore relationships, and suggest that phenolic secondary metabolites participate in regulation of plant growth.

Spatial distribution of damage affects the healing, growth, and morphology of coral

Many predators and herbivores do not kill their prey, but rather remove or damage tissue. Prey are often able to heal or regenerate this lost tissue. If the prey are modular organisms (e.g., some plants and cnidarians), regeneration is frequently influenced by other modules interconnected to damaged ones. For example, many coral predators remove tissue from colonies consisting of many polyps, and these polyps often share resources with their neighbors. Thus, the distribution of tissue loss on a coral colony could affect the coral's response. I hypothesized that spatially aggregated damage might be slow to heal due to competing demands on nearby polyps. To explore the spatial patterns of corallivory and their implications, I conducted: (1) field surveys documenting the spatial distribution of lesions on corals; (2) field experiments testing the effect of the distance between lesions on coral tissue healing, skeletal growth, and morphology; and (3) field surveys relating corallivore presence to coral growth and morphology. In the field surveys, lesions were aggregated at multiple spatial scales, and most lesions had other lesions within 2 cm. When lesions were near one another, coral tissue regeneration was depressed, although there was no effect on whole colony growth. After a year, however, linear extension was lower in the neighborhood of the lesions. Additionally, gastropod corallivores (Coralliophila violacea) with low movement decreased coral growth and increased coral topographical complexity. These results suggest that corallivores that create clusters of coral damage have a greater effect on coral growth and recovery from damage than corallivores that spread damage throughout the colony.

Trait-mediated indirect interactions: Moose browsing increases sawfly fecundity through plant-induced responses

Induced responses in plants, initiated by herbivory, create potential for trait-mediated indirect interactions among herbivores. Responses to an initial herbivore may change a number of plant traits that subsequently alter ecological processes with additional herbivores. Although common, indirect interactions between taxonomically distant herbivores, such as mammals and insects, are less studied than between taxonomically related species (i.e., insect-insect). In terms of mammal-insect interactions, effects on insect numbers (e.g., density) are relatively well studied, whereas effects on performance (e.g., fecundity) are rarely explored. Moreover, few studies have explored mammal-insect interactions on coniferous plants.The aim of this study was to investigate the effect of mammalian induced responses on insect performance. We specifically investigated the effect of moose (Alces alces) browsing on Scots pine (Pinus sylvestris) and subsequent effects on sawfly (Neodiprion sertifer) performance.Sawfly larvae were reared on browsed, clipped, and unbrowsed control pine trees in a controlled field experiment. Afterward, cocoon weight was measured. Needle C:N ratio and di-terpene content were measured in response to browsing.Sawfly performance was enhanced on trees browsed by moose. Cocoon weight (proxy for fecundity) was 9 and 13% higher on browsed and clipped trees compared to unbrowsed trees. Cocoon weight was weakly related to needle C:N ratio, and browsed trees had lower a C:N ratio compared to unbrowsed trees. Needle di-terpene content, known to affect sawfly performance, was neither affected by the browsing treatments nor did it correlate with sawfly weight.We conclude that mammalian herbivory can affect insect herbivore performance, with potential consequences for ecological communities and with particular importance for insect population dynamics. The measured plant variables could not fully explain the effect on sawfly performance providing a starting point for the consideration of additional plant responses induced by mammalian browsing affecting insect performance.

Trade-offs constrain the evolution of an inducible defense within but not between plant species

Inducible defense is a common form of phenotypic plasticity, and inducibility (change in defense after herbivore attack) has long been predicted to trade off with constitutive (or baseline) defense to manage resource allocation. Although such trade-offs likely constrain evolution within species, the extent to which they influence divergence among species is unresolved. We studied cardenolide toxins among genetic families in eight North American Asclepias species, spanning the full range of defense in the genus. Using common environment experiments and chemical assays, we report a consistent trade-off (negative genetic correlation) between concentrations of constitutive cardenolides and their inducibility within each species. However, no trade-off was found in a phylogenetic analysis across species. To investigate factors driving differences in defense allocation among species we used latitude as a proxy for growing season and herbivore pressure and found that divergence into lower latitudes resulted in evolution of higher cardenolides overall. Next we used an enzymatic assay of the cellular target of cardenolides (sodium-potassium ATPase) and confirm that higher cardenolides resulted in stronger toxicity to a sensitive species, but not to specialized monarch butterflies. Thus, plant speciation into biogeographic regions with alternative resources or pest pressure resulted in the macroevolution of cardenolide defense, especially against unspecialized herbivores. Nonetheless, trade-offs persist in the extent to which this defense is allocated constitutively or is inducible among genotypes within each species.

Photosynthetic Efficiency is Higher in Asymmetric Leaves than in Symmetric Leaves of the Same Plant

Symmetry pervades nature, but asymmetry is also rather common. Deviations from genetically programmed symmetry are usually associated with internal or external developmental disturbances and may therefore be related to imperfections in physiological processes. In this study, we test the hypotheses that the photosynthetic efficiency of individual leaves of a plant is negatively related to their asymmetry. We measured chlorophyll fluorescence in leaves of three woody species (Betula pubescens, Populus tremula and Salix caprea) in early and late summer in two localities situated ca. 1000 km apart, and we quantified the asymmetry of these leaves by a multivariate measure based on the relative positions of several landmarks. Contrary to our expectation, we found that the photochemical efficiency of photosystem II was positively correlated with leaf fluctuating asymmetry; this effect was weak but consistent across the studied plant species, localities and seasons. Our finding adds to limited evidence that within-plant variation in leaf asymmetry is associated with variation in leaf physiology. Irrespective of the underlying mechanisms, which remain unknown, the results suggest that trees may benefit even more from their asymmetric leaves, at least in terms of photosynthesis, than they do from their more symmetric leaves.

Population cycles: generalities, exceptions and remaining mysteries

Population cycles are one of nature's great mysteries. For almost a hundred years, innumerable studies have probed the causes of cyclic dynamics in snowshoe hares, voles and lemmings, forest Lepidoptera and grouse. Even though cyclic species have very different life histories, similarities in mechanisms related to their dynamics are apparent. In addition to high reproductive rates and density-related mortality from predators, pathogens or parasitoids, other characteristics include transgenerational reduced reproduction and dispersal with increasing-peak densities, and genetic similarity among populations. Experiments to stop cyclic dynamics and comparisons of cyclic and noncyclic populations provide some understanding but both reproduction and mortality must be considered. What determines variation in amplitude and periodicity of population outbreaks remains a mystery.

Herbivore resistance in congeneric and sympatric Nothofagus species is not related to leaf habit

PREMISE

Two fundamental hypotheses on herbivore resistance and leaf habit are the resource availability hypothesis (RAH) and the carbon-nutrient balance hypothesis (CNBH). The RAH predicts higher constitutive resistance by evergreens, and the CNBH predicts higher induced resistance by deciduous species. Although support for these hypotheses is mixed, they have rarely been examined in congeneric species.

METHODS

We compared leaf constitutive and induced resistance (as leaf polyphenol and tannin concentrations, and as damage level in non-choice experiments) and leaf traits associated with herbivory of coexisting Nothofagus species using (1) a defoliation experiment and (2) natural defoliation caused by an outbreak of a common defoliator of Nothofagus species.

RESULTS

In the defoliation experiment, polyphenol and tannin concentrations were similar between deciduous and evergreen species; regardless of leaf habit, polyphenols increased in response to defoliation. In the natural defoliation survey, N. pumilio (deciduous) had significantly higher herbivory, lower carbon/nitrogen ratio and leaf mass per area, and higher nitrogen and phosphorus concentrations than N. betuloides (evergreen); N. antarctica (deciduous) had intermediate values. Polyphenol concentrations and herbivore resistance indicated by the non-choice experiment were lower in N. pumilio than in N. antarctica and N. betuloides, which had similar values.

CONCLUSIONS

Higher herbivory in N. pumilio was associated with a higher nutritional value and a lower level of leaf carbon-based defenses compared to both the evergreen and the other deciduous species, indicating that herbivore resistance in Nothofagus species cannot be attributed to only leaf habit as predicted by the RAH or CNBH.

Insect community structure covaries with host plant chemistry but is not affected by prior herbivory

By feeding on plant tissue, insect herbivores can change several characteristics of their hosts. These changes have the potential to alter the quality of the plant for other herbivore species, potentially altering the structure of the community of species attacking the plant at a later point in time. We tested whether herbivory early in the season changes host plant performance, polyphenol chemistry, and the community structure of sessile herbivores later in the season. We experimentally manipulated densities of early-season moth caterpillars on a set of young oak trees and measured tree growth, reproduction, leaf chemistry, and the abundance and community composition of leafmining and galling species later in the season. The experimental manipulations of early-season herbivores did not affect late-season leaf chemistry or tree performance. Early-season herbivores had a weak negative effect on the abundance of gallers and a positive, tree-dependent effect on the overall diversity of late-season sessile herbivores. The chemical composition of leaves covaried with the species composition of the late-season leafmining and galling community. Both the chemical composition of the host tree and the late-season insect community structure were strongly affected by the growth location of the tree. Our results suggest that plant-mediated indirect effects between herbivores might play a limited role in this system, whereas the underlying variation in plant chemistry is an important factor structuring the associated insect community. Our results emphasize that factors other than prior herbivory can be important determinants of plant chemistry and the community composition of herbivores.

Phenological shifts in conifer species stressed by spruce budworm defoliation

Synchrony between host budburst and insect emergence greatly influences the time window for insect development and survival. A few alterations of bud phenology have been reported under defoliation without clear consensus regarding the direction of effects, i.e., advance or delay. Here, we compared budburst phenology between conifers in defoliation and control treatments, and measured carbon allocation as a potential mechanistic explanation of changes in phenology. In a 2-year greenhouse experiment, saplings of balsam fir, black spruce and white spruce of two different provenances (north and south) were subjected to either control (no larvae) or natural defoliation treatment (larvae added) by spruce budworm. Bud and instar phenology, primary and secondary growth, defoliation and non-structural carbohydrates were studied during the growing season. No differences were observed in bud phenology during the first year of defoliation. After 1 year of defoliation, bud phenology advanced by 6-7 days in black spruce and balsam fir and by 3.5 days in white spruce compared with the control. Because of this earlier bud break, apical and shoot growth exceeded 50% of its final length before mature instar defoliation occurred, which decreased the overall level of damage. A sugar-mediated response, via earlier starch breakdown, and higher sugar availability to buds explains the advanced budburst in defoliated saplings. The advanced phenological response to defoliation was consistent across the conifer species and provenances except for one species × provenance combination. Allocation of carbon to buds and shoots growth at the expense of wood growth in the stem and reserve accumulation represents a shift in the physiological resources priorities to ensure tree survival. This advancement in bud phenology could be considered as a physiological response to defoliation based on carbohydrate needs for primary growth, rather than a resistance trait to spruce budworm.

Gender Dimorphism Does Not Affect Secondary Compound Composition in Juniperus communis After Shoot Cutting in Northern Boreal Forests

Due to a difference in plant resource allocation to reproduction, the males of dioecious plants may be more growth-orientated, whereas females may allocate more resources for synthesizing secondary compounds. This mechanism is considered to cause gender-specific differences in the plant responses to the loss of plant biomass. Here, we tested gender dimorphism in the responses of common juniper (Juniperus communis) to shoot cutting in four juniper populations located in northern boreal forests in Finland. We collected shoots from uncut junipers and from junipers subjected to shoot cutting in the previous year, and analyzed them for their shoot growth as well as phenolic and terpenoid concentrations. There were no differences in foliar phenolic or terpenoid concentrations between the males and the females. Shoot cutting increased phenolic but not terpenoid concentrations, similarly, in both males and females. Our study reveals that the nature of gender dimorphism may differ among species and locations, which should be considered in theories on plant gender dimorphism. Given the similar phenolic and terpene concentrations in both genders, the different sexes in the northern juniper populations might experience equal levels of herbivory. This lack of gender dimorphism in biotic interactions could result from the high need of plant secondary metabolites (PSM) against abiotic stresses, which is typical for juniper at high latitudes.

Small-scale indirect plant responses to insect herbivory could have major impacts on canopy photosynthesis and isoprene emission

Insect herbivores cause substantial changes in the leaves they attack, but their effects on the ecophysiology of neighbouring, nondamaged leaves have never been quantified in natural canopies. We studied how winter moth (Operophtera brumata), a common herbivore in temperate forests, affects the photosynthetic and isoprene emission rates of its host plant, the pedunculate oak (Quercus robur). Through a manipulative experiment, we measured leaves on shoots damaged by caterpillars or mechanically by cutting, or left completely intact. To quantify the effects at the canopy scale, we surveyed the extent and patterns of leaf area loss in the canopy. Herbivory reduced photosynthesis both in damaged leaves and in their intact neighbours. Isoprene emission rates significantly increased after mechanical leaf damage. When scaled up to canopy-level, herbivory reduced photosynthesis by 48 ± 10%. The indirect effects of herbivory on photosynthesis in undamaged leaves (40%) were much more important than the direct effects of leaf area loss (6%). If widespread across other plant-herbivore systems, these findings suggest that insect herbivory has major and previously underappreciated influences in modifying ecosystem carbon cycling, with potential effects on atmospheric chemistry.

Multiannual effects of induced plant defenses: Are defended plants good or bad neighbors?

Defenses induced by herbivore feeding or phytohormones such as methyl jasmonate (MeJA) can affect growth, reproduction, and herbivory, not only on the affected individual but also in its neighboring plants. Here, we report multiannual defense, growth, and reproductive responses of MeJA-treated bilberry (Vaccinium myrtillus) and neighboring ramets. In a boreal forest in western Norway, we treated bilberry ramets with MeJA and water (control) and measured responses over three consecutive years. We observed the treatment effects on variables associated with herbivory, growth, and reproduction in the MeJA-treated and untreated ramet and neighboring ramets distanced from 10 to 500 cm. MeJA-treated ramets had fewer grazed leaves and browsed shoots compared to control, with higher effects in 2014 and 2015, respectively. In 2013, growth of control ramets was greater than MeJA-treated ramets. However, MeJA-treated ramets had more flowers and berries than control ramets 2 years after the treatment. The level of insect and mammalian herbivory was also lower in untreated neighboring ramets distanced 10-150 cm and, consistent with responses of MeJA-treated ramets, the stronger effect was also one and 2 years delayed, respectively. The same neighboring ramets had fewer flowers and berries than untreated ramets, indicating a trade-off between defense and reproduction. Although plant-plant effects were observed across all years, the strength varied by the distance between the MeJA-treated ramets and its untreated neighbors. We document that induced defense in bilberry reduces both insect and mammalian herbivory, as well as growth, over multiple seasons. The defense responses occurred in a delayed manner with strongest effects one and 2 years after the induction. Additionally, our results indicate defense signaling between MeJA-treated ramets and untreated neighbors. In summary, this study shows that induced defenses are important ecological strategies not only for the induced individual plant but also for neighboring plants across multiple years in boreal forests.

Population-level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

Grazing-induced changes in plant quality have been suggested to drive the negative delayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesis that reciprocal feedback between vole grazing pressure and the induction of anti-herbivore silicon defenses in grasses drives observed population cycles in a large-scale field experiment in northern England. We repeatedly reduced population densities of field voles (Microtus agrestis) on replicated 1-ha grassland plots at Kielder Forest, northern England, over a period of 1 year. Subsequently, we tested for the impact of past density on vole life history traits in spring, and whether these effects were driven by induced silicon defenses in the voles' major over-winter food, the grass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole density had been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations on vole body mass, spring population growth rate, or mean date for the onset of spring reproduction the following year. These findings show that grazing by field voles does induce increased silicon defenses in grasses at a landscape scale. However, at the vole densities encountered, levels of plant damage appear to be below those needed to induce changes in silicon levels large and persistent enough to affect vole performance, confirming the threshold effects we have previously observed in laboratory-based studies. Our findings do not support the plant quality hypothesis for observed vole population cycles in northern England, at least over the range of vole densities that now prevail here.