Elevational gradients in oak defences and herbivory

Elevational gradients are powerful tools to study the co-evolution between plants and insect herbivores. Classical theory holds that higher abundance and diversity of herbivores at low elevations leads to stronger selection on plant defences. Inconsistency between studies have called into question the generality of this paradigm. With this thesis I explore potential sources of controversy. First elevational gradients could be contingent on plant life-history traits and developmental stage. Second, plants deploy multiple defensive strategies that are usually not simultaneously considered. And third, abiotic factors could influence elevational gradients in plant-herbivores interactions

Effects of insularity on insect leaf herbivory and chemical defences in a Mediterranean oak species

Aim Research on plant–herbivore interactions has shown that islands typically have low abundances and diversity of herbivores because of barriers to dispersal, isolation and reduced land area. Islands commonly have lower levels of herbivory relative to mainland regions, and, as a consequence, insular plants should exhibit lower levels of defences than their mainland counterparts. Despite these predictions, there are significant gaps in our understanding of insularity effects on plant–herbivore interactions. For instance, most work addressing the effects of insularity on plant–herbivore interactions have compared one or a few islands with a single mainland site. In addition, studies have measured herbivory or plant defences but not both, and the influence of abiotic factors has been neglected. Location Mediterranean Basin (from Spain to Greece). Taxon Quercus ilex L. Methods We conducted a large‐scale study to investigate whether insect leaf herbivory and plant chemical defences in holm oak (Quercus ilex L.) differ between insular versus mainland populations. We further investigated mechanisms by which insularity effects on herbivory may take place by assessing the influence of defences and climatic variables on herbivory. Results We found that insular populations exhibited lower herbivory and higher defences (condensed tannins) than their mainland counterparts. Our analyses, however, suggest that these concomitant patterns of insect herbivory and plant defences were seemingly unrelated as island versus mainland differences in defences did not account for the observed pattern in herbivory. Furthermore, climatic factors did not explain insularity effects on either herbivory or plant defences. Main conclusions Overall, this study provides one of the most robust assessments to date on insularity effects on herbivory and builds towards a better understanding of the ecology and evolution of plant–insect interactions in insular ecosystems.info:eu-repo/semantics/acceptedVersio

Effects of insularity on insect leaf herbivory and chemical defences in a Mediterranean oak species

Aim Research on plant–herbivore interactions has shown that islands typically have low abundances and diversity of herbivores because of barriers to dispersal, isolation and reduced land area. Islands commonly have lower levels of herbivory relative to mainland regions, and, as a consequence, insular plants should exhibit lower levels of defences than their mainland counterparts. Despite these predictions, there are significant gaps in our understanding of insularity effects on plant–herbivore interactions. For instance, most work addressing the effects of insularity on plant–herbivore interactions have compared one or a few islands with a single mainland site. In addition, studies have measured herbivory or plant defences but not both, and the influence of abiotic factors has been neglected. Location Mediterranean Basin (from Spain to Greece). Taxon Quercus ilex L. Methods We conducted a large‐scale study to investigate whether insect leaf herbivory and plant chemical defences in holm oak (Quercus ilex L.) differ between insular versus mainland populations. We further investigated mechanisms by which insularity effects on herbivory may take place by assessing the influence of defences and climatic variables on herbivory. Results We found that insular populations exhibited lower herbivory and higher defences (condensed tannins) than their mainland counterparts. Our analyses, however, suggest that these concomitant patterns of insect herbivory and plant defences were seemingly unrelated as island versus mainland differences in defences did not account for the observed pattern in herbivory. Furthermore, climatic factors did not explain insularity effects on either herbivory or plant defences. Main conclusions Overall, this study provides one of the most robust assessments to date on insularity effects on herbivory and builds towards a better understanding of the ecology and evolution of plant–insect interactions in insular ecosystems.info:eu-repo/semantics/acceptedVersio

Can School Children Support Ecological Research? Lessons from the Oak Bodyguard Citizen Science Project

Peer reviewe

Herbivory on the pedunculate oak along an urbanization gradient in Europe : Effects of impervious surface, local tree cover, and insect feeding guild

Urbanization is an important driver of the diversity and abundance of tree-associated insect herbivores, but its consequences for insect herbivory are poorly understood. A likely source of variability among studies is the insufficient consideration of intraurban variability in forest cover. With the help of citizen scientists, we investigated the independent and interactive effects of local canopy cover and percentage of impervious surface on insect herbivory in the pedunculate oak (Quercus robur L.) throughout most of its geographic range in Europe. We found that the damage caused by chewing insect herbivores as well as the incidence of leaf-mining and gall-inducing herbivores consistently decreased with increasing impervious surface around focal oaks. Herbivory by chewing herbivores increased with increasing forest cover, regardless of impervious surface. In contrast, an increase in local canopy cover buffered the negative effect of impervious surface on leaf miners and strengthened its effect on gall inducers. These results show that – just like in non-urban areas – plant-herbivore interactions in cities are structured by a complex set of interacting factors. This highlights that local habitat characteristics within cities have the potential to attenuate or modify the effect of impervious surfaces on biotic interactions.Agence Nationale de la Recherche, Grant/Award Number: ANR-10--LABX-45; Fondation BNP Paribas.info:eu-repo/semantics/publishedVersio

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

International audienceAim: The strength of species interactions is traditionally expected to increase toward the Equator. However, recent studies have reported opposite or inconsistent latitudinal trends in the bottom-up (plant quality) and top-down (natural enemies) forces driving herbivory. In addition, these forces have rarely been studied together thus limiting previous attempts to understand the effect of large-scale climatic gradients on herbivory. Location: Europe. Time period: 2018–2019. Major taxa studied: Quercus robur. Methods: We simultaneously tested for latitudinal variation in plant–herbivore–natural enemy interactions. We further investigated the underlying climatic factors associated with variation in herbivory, leaf chemistry and attack rates in Quercus robur across its complete latitudinal range in Europe. We quantified insect leaf damage and the incidence of specialist herbivores as well as leaf chemistry and bird attack rates on dummy caterpillars on 261 oak trees. Results: Climatic factors rather than latitude per se were the best predictors of the large-scale (geographical) variation in the incidence of gall-inducers and leaf-miners as well as in leaf nutritional content. However, leaf damage, plant chemical defences (leaf phenolics) and bird attack rates were not influenced by climatic factors or latitude. The incidence of leaf-miners increased with increasing concentrations of hydrolysable tannins, whereas the incidence of gall-inducers increased with increasing leaf soluble sugar concentration and decreased with increasing leaf C : N ratios and lignins. However, leaf traits and bird attack rates did not vary with leaf damage. Main conclusions: These findings help to refine our understanding of the bottom-up and top-down mechanisms driving geographical variation in plant–herbivore interactions, and indicate the need for further examination of the drivers of herbivory on trees

Herbivory on the pedunculate oak along an urbanization gradient in Europe: Effects of impervious surface, local tree cover, and insect feeding guild

Urbanization is an important driver of the diversity and abundance of tree-associated insect herbivores, but its consequences for insect herbivory are poorly understood. A likely source of variability among studies is the insufficient consideration of intra-urban variability in forest cover. With the help of citizen scientists, we investigated the independent and interactive effects of local canopy cover and percentage of impervious surface on insect herbivory in the pedunculate oak (Quercus robur L.) throughout most of its geographic range in Europe. We found that the damage caused by chewing insect herbivores as well as the incidence of leaf-mining and gall-inducing herbivores consistently decreased with increasing impervious surface around focal oaks. Herbivory by chewing herbivores increased with increasing forest cover, regardless of impervious surface. In contrast, an increase in local canopy cover buffered the negative effect of impervious surface on leaf miners and strengthened its effect on gall inducers. These results show that-just like in non-urban areas-plant-herbivore interactions in cities are structured by a complex set of interacting factors. This highlights that local habitat characteristics within cities have the potential to attenuate or modify the effect of impervious surfaces on biotic interactions

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

AimThe strength of species interactions is traditionally expected to increase toward the Equator. However, recent studies have reported opposite or inconsistent latitudinal trends in the bottom‐up (plant quality) and top‐down (natural enemies) forces driving herbivory. In addition, these forces have rarely been studied together thus limiting previous attempts to understand the effect of large‐scale climatic gradients on herbivory.LocationEurope.Time period2018–2019.Major taxa studiedQuercus robur.MethodsWe simultaneously tested for latitudinal variation in plant–herbivore–natural enemy interactions. We further investigated the underlying climatic factors associated with variation in herbivory, leaf chemistry and attack rates in Quercus robur across its complete latitudinal range in Europe. We quantified insect leaf damage and the incidence of specialist herbivores as well as leaf chemistry and bird attack rates on dummy caterpillars on 261 oak trees.ResultsClimatic factors rather than latitude per se were the best predictors of the large‐scale (geographical) variation in the incidence of gall‐inducers and leaf‐miners as well as in leaf nutritional content. However, leaf damage, plant chemical defences (leaf phenolics) and bird attack rates were not influenced by climatic factors or latitude. The incidence of leaf‐miners increased with increasing concentrations of hydrolysable tannins, whereas the incidence of gall‐inducers increased with increasing leaf soluble sugar concentration and decreased with increasing leaf C : N ratios and lignins. However, leaf traits and bird attack rates did not vary with leaf damage.Main conclusionsThese findings help to refine our understanding of the bottom‐up and top‐down mechanisms driving geographical variation in plant–herbivore interactions, and indicate the need for further examination of the drivers of herbivory on trees.</p

Dataset for: Interactive effects of tree species composition and water availability on growth and direct and indirect defences in Quercus ilex

[Methods] - Experimental design: This study was conducted in the ORPHEE experimental trial established in 2008 in South-West France (44°440 N, 00°460 W). The experimental design consisted of eight blocks and 32 plots within each block. Each plot represented a tree species composition treatment, corresponding to 31 possible combinations of one to five tree species (Betula pendula, Quercus robur, Q. pyrenaica, Q. ilex, and Pinus pinaster) and an additional plot replicate of the five species mixture. Each plot contained 10 rows of 10 trees planted 2 m apart (100 trees on 400 m²). Tree species mixtures were established according to a substitutive design, keeping tree density of tree neighbours equal across plots. Within plots, individual trees from different species were planted in a regular alternate pattern, such that a tree from a given species had at least one neighbour from each of the other species within a 2-m radius. From 2015 four out of the eight experimental blocks were allocated to an irrigation treatment consisting of sprinkling the equivalent of 3 mm precipitation from a 2 m height pole in the centre of each irrigated plot. Blocks were irrigated on a daily basis, at night, from May to October. The four remaining blocks were kept as controls. This datasets collects data for Q. ilex. In particular, we focused on Quercus ilex as target species and selected six blocks (three irrigated and three control) and four plots (tree species composition treatments) in each block corresponding to the monoculture of Q. ilex and its combinations with B. pendula and P. pinaster (Q. ilex + B. pendula, Q. ilex + P. pinaster and Q. ilex + B. pendula + P. pinaster). Therefore, a total of 24 experimental plots (4 tree species composition treatments × 2 irrigation treatments × 3 blocks) were included in the study. - Sampling and measurements: At the end of the growing season (September 2019), we haphazardly selected four Q. ilex trees in each of the 24 plots (N = 96 trees). Trees in the plot margins were not selected to avoid border effects. First, we estimated total height and basal diameter (± 30 cm aboveground) in all experimental trees with a tape-measure and a digital caliper respectively. After tree growth measurements, we collected VOCs for each tree. Briefly, we bagged one branch of each tree with a 1L nalophan bag and we trapped the compounds on a charcoal filter by pulling air through the filter using an air-sampling pump for 2 h at a rate of 250 ml min-1. Importantly, we sampled air VOCs in empty bags (one bag placed in the middle of each plot within each block) as controls, in order to identify compounds that may contaminate the blend of VOCs taken from the focal trees (e.g., VOCs emitted by neighbour species). After collecting the VOCs, we stored the filters at -80ºC until chemical analyses. Right after VOCs collection, we haphazardly collected 20 fully expanded and developed leaves throughout the tree’s canopy. Importantly, because Q. Ilex is an evergreen species, sampled leaves may have consisted of one to three cohorts of leaves (i.e. produced between 2017 and 2019; up to two-years old). For each leaf, we visually estimated the percentage of leaf area removed by insect herbivores (mostly leaf chewers) using the following scale: 0 = no damage; 1 = 1–5% damaged; 2 = 6–10% damaged; 3 = 11–25% damaged; 4 = 26–50% damaged; 5 = 51–75% damaged; 6 = >75% damaged (“leaf herbivory” hereafter). We averaged class values across all leaves to obtain a mean value per tree for statistical analyses. We selected a subset of 4-5 leaves with little or no evidence of herbivory for further chemical analyses of phenolic compounds. Leaves were oven-dried for 48 h at 40ºC. - Chemical analyses: Quantification of volatile organic compounds (VOCs). To analyse VOCs, we performed gas chromatography and mass spectrometry analyses. To extract the compounds from the charcoal traps, we first added 5 μl of naphthalene (20 ng ml−1) as an internal standard to the traps (Pellissier et al., 2016), and then eluted their contents with 400 μl of dichloromethane. We then injected 2 μl of the extract for each sample into a gas chromatograph (GC) coupled with a mass selective detector (MSD) fitted with a 30 m × 0.25 mm × 0.25 mm film thickness HP-5MS fused silica column. We operated the GC in splitless mode with helium as the carrier gas (constant flow rate 0.9 ml min−1). The GC oven temperature program was: 1 min hold at 40°C, and then 10°C min−1 ramp to 240°C. We identified individual volatile compounds (i.e., terpenes) using Kovats retention index from published work, the NIST Standard Reference Database 1A v17, and by comparison with commercial standards when available. Volatile emissions are reported as nanograms naphthalene equivalents. For subsequent analyses, we selected VOCs identified as either monoterpenes or sesquiterpenes. We quantified individual monoterpenes and sesquiterpenes relative to the internal standard and used for statistical analyses those exhibiting a relative abundance higher than 1%. Importantly, for those compounds present in both the samples and the corresponding control, we only consider those which intensity in the sample was at least double than in the control. Finally, we quantified the total concentration of VOCs as the sum of concentrations of all individual compounds. Quantification of phenolic compounds. We extracted phenolic compounds from 20 mg of dry leaf tissue with 1 ml of 70% methanol in an ultrasonic bath for 15 min, followed by centrifugation (Moreira et al., 2020) and transferred the extracts to chromatographic vials. To analyse the phenolic compounds, we performed chromatographic analyses using ultra-high performance liquid chromatography equipped with a Nexera SIL-30AC injector and one SPD-M20A UV/VIS photodiode array detector. The compound separation was carried out on a Kinetex 2.6 μm C18 82–102 Å, LC Column 100 × 4.6 mm, protected with a C18 guard cartridge. The flow rate was 0.4 ml min−1 and the oven temperature was set at 25°C. The mobile phase consisted of two solvents: water–formic acid (0.05%) (A) and acetonitrile–formic acid (0.05%) (B), starting with 5% B and using a gradient to obtain 30% B at 4 min, 60% B at 10 min, 80% B at 13 min and 100% B at 15 min. The injection volume was 15 μl. For phenolic compound identification, we used an ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry. We identified four groups of phenolic compounds: flavonoids, ellagitannins and gallic acid derivates (‘hydrolysable tannins’ hereafter), proanthocyanidins (‘condensed tannins’ hereafter) and hydroxycinnamic acid precursors to lignins (‘lignins’ hereafter). We quantified flavonoids as rutin equivalents, condensed tannins as catechin equivalents, hydrolysable tannins as gallic acid equivalents, and lignins as ferulic acid equivalents . The quantification of these was conducted by external calibration using the corresponding calibration curve at 0.25, 0.5, 1, 2 and 5 μg ml−1 for each of the four standards used (rutin, catechin, gallic acid and ferulic acid). We expressed phenolic compound concentrations in mg g−1 tissue on a dry weight basis.Plant diversity has often been reported to decrease insect herbivory in plants. Of the numerous mechanisms that have been proposed to explain this phenomenon, how plant diversity influences plant defences via effects on growth has received little attention. In addition, plant diversity effects may be contingent on abiotic conditions (e.g., resource and water availability). Here, we used a long-term experiment to explore the interactive effects of tree species composition and water availability on growth, direct (i.e. phenolics) and indirect (i.e. Volatile Organic Compounds – VOCs) defences and leaf herbivory in Quercus ilex. We quantified herbivory by chewing insects, phenolic compounds and VOCs in Q. ilex trees growing in stands differing in tree species composition (Q. ilex, Q. ilex + Betula Pendula, Q. ilex + Pinus pinaster and Q. ilex + B. pendula + P. pinaster) and water availability (irrigated vs control). Both direct and indirect defences were affected by tree species composition, but such changes were not mediated by changes in tree stem diameter. Q. ilex trees growing in stands with P. pinaster had the lowest concentration of both direct and indirect defences. Importantly, the effects of tree species composition on VOCs were exacerbated on irrigated blocks. Despite variation in defences, tree species composition did not affect herbivory in Q. ilex. Accordingly, we did not find any association between defences and insect herbivory. Our results suggest that changes in the micro-environment rather than growth-defence associations may mediate tree diversity effects on defences. In addition, reduced defensive investment in more diverse stands could negatively impact tree resistance masking the beneficial effects of species diversity at reducing insect herbivory.Consejo Superior de Investigaciones Científicas, Award: I-LINK12212018-2019.Peer reviewe

Interactive effects of tree species composition and water availability on growth and direct and indirect defences in Quercus ilex

Plant diversity has often been reported to decrease insect herbivory in plants. Of the numerous mechanisms that have been proposed to explain this phenomenon, how plant diversity influences plant defences via effects on growth has received little attention. In addition, plant diversity effects may be contingent on abiotic conditions (e.g. resource and water availability). Here, we used a long-term experiment to explore the interactive effects of tree species composition and water availability on growth, direct (i.e. phenolics) and indirect (i.e. volatile organic compounds – VOCs) defences and leaf herbivory in Quercus ilex. We quantified herbivory by chewing insects, phenolic compounds and VOCs in Q. ilex trees growing in stands differing in tree species composition (Q. ilex, Q. ilex + Betula Pendula, Q. ilex + Pinus pinaster and Q. ilex + B. pendula + P. pinaster) and water availability (irrigated versus control). Both direct and indirect defences were affected by tree species composition, but such changes were not mediated by changes in tree stem diameter. Quercus ilex trees growing in stands with P. pinaster had the lowest concentration of both direct and indirect defences. Importantly, the effects of tree species composition on VOCs were exacerbated on irrigated blocks. Despite variation in defences, tree species composition did not affect herbivory in Q. ilex. Accordingly, we did not find any association between defences and insect herbivory. Our results suggest that changes in the micro-environment rather than growth-defence associations may mediate tree diversity effects on defences. In addition, reduced defensive investment in more diverse stands could negatively impact tree resistance masking the beneficial effects of species diversity at reducing insect herbivory.We thank Xoaquín Moreira for funding field sampling through a ILINK-CSIC project (I-LINK12212018-2019). BC was funded by the ‘Diversity and Productivity of Trees in the context of Climate Change' project (DiPTiCC, Grant ANR-16-CE32-0003-01). CVG received financial funding from the MINECO-FPI program (BES-2016-076624) and the Xunta de Galicia-GAIN/Fulbright postdoctoral program (IN606B_2021_004)