Soil pollution with metals is a worldwide problem with negative effects on ecosystem health. Metals are persistent and can accumulate in the food web, with potential toxic effects for most organisms. Some metal-tolerant plants can accumulate metals in their aboveground parts, including the pollen and nectar of their flowers. Therefore, insects visiting these flowers are exposed to these toxic elements. They can also be influenced by the plant species turnover observed along pollution gradients. Surprisingly, the impact of soil metal contamination on pollinating insect diversity has rarely been studied. To fill in this gap, we carried out a study in a former mining valley in the French Pyrenees with metal contamination by Zn, Pb and Cd. We estimated the richness and community composition of insects visiting the flowers of metal-tolerant and metal-intolerant plant species in 96 plots along metal pollution gradients. Insect richness did not change along the pollution gradient and was similar between metal-tolerant and metal-intolerant plant species. On the contrary, insect community composition changed along the gradient and differed between plant types. We also found that insect species richness increased with the flower abundance of the focal plant. This study showed that flowers of metal-tolerant plants, including hyper-accumulator species, are visited by several insect species and harbour a distinct pollinator community. The ingestion of metal-rich nectar or pollen may have lethal or sub-lethal effects for these flower-visiting insects. Further experimental studies are needed to determine if metal-rich floral resources act as an ecological trap for some of these species.

Plant litter decomposition governs how much carbon soils store and emit, yet the microbial traits that shape ecosystem-scale decay remain unresolved. Metagenomes can quantify genes encoding plant cell-wall-degrading enzymes, but it is unclear whether ecosystem differences in decay reflect distinct enzymatic repertoires, and whether these data improve prediction beyond climate and soil properties. We paired standardized green and rooibos tea-bag decomposition assays across 3–24 months with 295 soil metagenomes from 264 global sites. Using 196 European plots for primary inference, we built a stage-resolved catalogue of 17.6 million carbohydrate-active enzyme (CAZyme) genes. Forest microbiomes decomposed tea faster than grasslands, but this was not explained by greater CAZyme family richness. Instead, ecosystems differed in CAZyme abundance, subfamily and protein-sequence variation, and allocation across biochemical stages of plant cell-wall decay, with evidence of ecosystem-specific selection. CAZyme profiles added explanatory power for 24-month mass loss and improved within-ecosystem prediction but generalized poorly across ecosystems and continents. By showing that ecosystem differences in decomposition arise from the stage-specific distribution of shared enzymatic functions rather than their presence alone, this work shifts microbial trait inference beyond gene inventories and provides a mechanistic genomic framework for carbon-cycle modelling within defined environmental limits.

In planted forest landscapes, semi-natural forest remnants constitute essential habitats for forest biodiversity. However, modifying forest cover to restore these stands and enhance the connectivity among them remains a logistical challenge for forest managers. While hedgerows have been widely studied for their role in promoting biodiversity in agricultural landscapes (i.e., “bocage”), it remains unclear to what extent these interstitial elements, would enhance biodiversity in planted forest landscapes. To address this question, we conducted our study in a homogeneous and monospecific pine plantation landscape in southwestern France, where we compared the diversity of six taxonomic groups in broadleaved hedgerows vs. pine stand edges. We also analysed the effect of the connectivity of hedgerows to semi-natural broadleaved stands and the proportion of broadleaved stands in the landscapes. Beyond species richness and community composition of each taxon, we calculated multidiversity indexes across all groups (using dominant, rare, or forest specialist species). Multidiversity was significantly higher in hedgerows than in pine stand edges. Hedgerows were home to communities with a distinct composition, including a greater abundance of rare species and forest specialist species. Increasing broadleaved cover in the landscape had a negative effect on multidiversity but altered community composition in three out of six groups. The connectivity of hedgerows to broadleaved stands had no significant effect on biodiversity. Preserving or planting broadleaved hedgerows therefore emerges as an effective and practical management method for enhancing biodiversity, particularly of forest specialist species, in pine plantation landscapes.

Broadleaved hedgerows provide important refuges for forest biodiversity within intensively managed pine plantation landscapes. However, little is known on the relationships between the vertical and horizontal structure of these hedgerows and the associated taxonomic biodiversity, as well as the use of these characteristics as indirect bioindicators. lidar-derived metrics are increasingly used in ecology, but few studies have evaluated their ability to capture structural drivers of multi-taxonomic biodiversity compared to traditional field measurements. We investigated whether structural metrics obtained from airborne lidar and from ground surveys could predict multi-taxonomic biodiversity and tree-related microhabitats (TreMs) profile in 22 broadleaved hedgerows located at the edge of maritime pine plantations. Six taxonomic groups (vascular plants, butterflies, carabid beetles, spiders, birds and reptiles) and TreMs were surveyed. Twelve lidar- and ground-based structural variables were quantified. lidar-derived horizontal variability in total cover (canopy + understorey vegetation) was the best predictor of overall multidiversity (R2 = 0.28), whereas mean canopy cover most strongly explained the multidiversity of forest-specialist species (R2 = 0.46). Both variables outperformed equivalent ground-based variables. By contrast, lidar metrics showed no predictive power for TreMs abundance or richness, while basal area measured in the field proved to be a good predictor (R2 = 0.47 and 0.43 for TreMs abundance and richness, respectively). Our findings highlight the complementarity of lidar- and ground-based indicators for biodiversity assessment. As high-resolution lidar data become increasingly available at broad scales, horizontal heterogeneity and canopy cover emerge as promising indirect indicators of multi-taxonomic biodiversity in hedgerows across large landscapes.

Xylella fastidiosa (Xf) is a xylem-limited bacterium that has been recorded in several European countries since its detection in 2013 in Apulia (Italy). Given the prominence of the wine industry in many southern European countries, a big threat is the development of Pierce's disease in grapevines. Yet, the insect-habitat and insect-plant interaction networks in which xylem feeders, possible vectors of Xf, are involved around European vineyards are largely unknown. Here we describe these networks in three key wine-growing regions of southern France (Provence-Alpes-Côte d'Azur, Occitanie, Nouvelle-Aquitaine) to identify primary xylem feeder habitats, and assess their specialization degree at the habitat, plant family, and plant species levels. A total of 92 landscapes (and 700 sites) were studied over three sampling sessions in the fall 2020, spring 2021, and fall 2021. Among the habitats sampled, meadows hosted the largest xylem feeder communities, followed by alfalfa fields. Vineyard headlands and inter-rows hosted slightly smaller xylem feeder communities, indicating that potential Xf vectors thrive near crops. Grapevines hosted few xylem feeders, suggesting rare but possible transfer to vulnerable crops. Philaenus spumarius, Aphrophora alni, Lepyronia coleoptrata, and Cicadella viridis were all similarly generalists at the habitat, plant family or plant species level. The only specialist was Aphrophora grp. salicina, which was restricted to riparian forests and to Salicaceae. Neophilaenus spp. were extremely specialist at the plant family level (Poaceae), but rather generalist at the habitat and plant species levels. All 1017 insects screened for the presence of Xf tested negative, showing that Xf is not widespread in the studied regions. Our study provides new basic ecological information on potential vectors of Xf, especially on their specialization and feeding preferences, as well as practical information that may be relevant for the design of epidemiological surveillance plans.

Mixed forests exhibit greater resistance to pests compared to monospecific forests. However, stand-level diversification is challenging for managers. An alternative approach is to enhance tree diversity at the landscape scale. Building on the agricultural landscapes with hedgerows, we hypothesized that in a pine plantation landscape broadleaved hedgerows along pine stands locally increase tree diversity, while broadleaved stands increase landscape level diversity. According to the associational resistance concept, both methods should reduce pest damage. We tested these hypotheses in a pine plantation landscape (dominated by Pinus pinaster), by counting nests of the pine processionary moth (Thaumetopoea pityocampa, hereafter “PPM”) along 36 pine edges adjacent or not to broadleaved hedgerows, in landscapes with low or high broadleaved cover. PPM bird and bat predators activities were assessed using acoustic devices. At the local level, PPM nests were 42

The number of invasive non-native pests is increasing rapidly in forests as a result of global change. It is therefore important to prevent their damage in order to preserve the integrity of forest ecosystems and the associated services. According to the biotic resistance hypothesis, species-rich communities are less likely to be invaded. The associational resistance hypothesis states that insect herbivores are more likely to colonise and exploit plants surrounded by conspecific neighbours than heterospecific, non-host plant species. More diverse forests would therefore be less damaged by non-native pests than tree monocultures. We tested these hypotheses by comparing the damage caused to seeds by the western conifer bug Leptoglossus occidentalis, , an invasive insect native to North America, in plots of pure maritime pine and mixed plots of maritime pine and birch. These plots were in two tree diversity experiments in Europe. Mixed pine plots differed in terms of pine density (1250 versus 625 pines/ha), proportion of pine and birch (25 %, 50 %, 75 %) and spatial mixing pattern (aggregated versus dispersed). We sampled 635 cones in 37 plots. Overall, the proportion of seeds damaged by the invasive bug was significantly lower in mixed plots than in pure pine plots at both experimental sites. There was no significant effect of pine density or relative proportion in the mixtures on seed damage. Aggregated pines in the mixed plots were significantly more damaged than dispersed pines, suggesting that bugs may have done less damage when they had more difficulty locating their host trees. This is consistent with the hypothesis of lower plant apparency. These results support the view that forest plantations of mixed species, which are known to be less vulnerable to attack by native insects, may also be more resistant to infestation by exotic insects. Row-wise, intimate mixtures of two species represent a promising option for designing tree plantations that are more resistant and easier to manage.

Aim: Climate is a major driver of large-scale variability in biodiversity, as a likely result of more intense biotic interactions under warmer conditions. This idea fuelled decades of research on plant-herbivore interactions, but much less is known about higher-level trophic interactions. We addressed this research gap by characterizing both bird diversity and avian predation along a climatic gradient at the European scale. Location: Europe. Taxon: Insectivorous birds and pedunculate oaks. Methods: We deployed plasticine caterpillars in 138 oak trees in 47 sites along a 19 degrees latitudinal gradient in Europe to quantify bird insectivory through predation attempts. In addition, we used passive acoustic monitoring to (i) characterize the acoustic diversity of surrounding soundscapes; (ii) approximate bird abundance and activity through passive acoustic recordings; and (iii) infer both taxonomic and functional diversity of insectivorous birds from recordings. Results: The functional diversity of insectivorous birds increased with warmer climates. Bird predation increased with forest cover and bird acoustic activity but decreased with mean annual temperature and functional richness of insectivorous birds. Contrary to our predictions, climatic clines in bird predation attempts were not directly mediated by changes in insectivorous bird diversity or acoustic activity, but climate and habitat still had independent effects on predation attempts. Main Conclusions: Our study supports the hypothesis of an increase in the diversity of insectivorous birds towards warmer climates but refutes the idea that an increase in diversity would lead to more predation and advocates for better accounting for activity and abundance of insectivorous birds when studying the large-scale variation in insect-tree interactions.

Next-generation biomonitoring proposes to combine machine-learning algorithms with environmental DNA data to automate the monitoring of the Earth's major ecosystems. In the present study, we searched for molecular biomarkers of tree water status to develop next-generation biomonitoring of forest ecosystems. Because phyllosphere microbial communities respond to both tree physiology and climate change, we investigated whether environmental DNA data from tree phyllosphere could be used as molecular biomarkers of tree water status in forest ecosystems. Using an amplicon sequencing approach, we analysed phyllosphere microbial communities of four tree species (Quercus ilex, Quercus robur, Pinus pinaster and Betula pendula) in a forest experiment composed of irrigated and non-irrigated plots. We used these microbial community data to train a machine-learning algorithm (Random Forest) to classify irrigated and non-irrigated trees. The Random Forest algorithm detected tree water status from phyllosphere microbial community composition with more than 90% accuracy for oak species, and more than 75% for pine and birch. Phyllosphere fungal communities were more informative than phyllosphere bacterial communities in all tree species. Seven fungal amplicon sequence variants were identified as candidates for the development of molecular biomarkers of water status in oak trees. Altogether, our results show that microbial community data from tree phyllosphere provides information on tree water status in forest ecosystems and could be included in next-generation biomonitoring programmes that would use in situ, real-time sequencing of environmental DNA to help monitor the health of European temperate forest ecosystems.

Litter decomposition is a key ecosystem function in forests and varies in response to a range of climatic, edaphic, and local stand characteristics. Disentangling the relative contribution of these factors is challenging, especially along large environmental gradients. In particular, knowledge of the effect of management options, such as tree planting density and species composition, on litter decomposition would be highly valuable in forestry. In this study, we made use of 15 tree diversity experiments spread over eight countries and three continents within the global TreeDivNet network. We evaluated the effects of overstory composition (tree identity, species/mixture composition and species richness), plantation conditions (density and age), and climate (temperature and precipitation) on mass loss (after 3 months and 1 year) of two standardized litters: high-quality green tea and low-quality rooibos tea. Across continents, we found that early-stage decomposition of the low-quality rooibos tea was influenced locally by overstory tree identity. Mass loss of rooibos litter was higher under young gymnosperm overstories compared to angiosperm overstories, but this trend reversed with age of the experiment. Tree species richness did not influence decomposition and explained almost no variation in our multi-continent dataset. Hence, in the young plantations of our study, overstory composition effects on decomposition were mainly driven by tree species identity on decomposer communities and forest microclimates. After 12 months of incubation, mass loss of the high-quality green tea litter was mainly influenced by temperature whereas the low-quality rooibos tea litter decomposition showed stronger relationships with overstory composition and stand age. Our findings highlight that decomposition dynamics are not only affected by climate but also by management options, via litter quality of the identity of planted trees but also by overstory composition and structure.

Early detection of insect infestation is a key to the adoption of control measures appropriated to each local condition. The use of remote sensing was recommended for a quick scanning of large areas, although it does not work well with signals bearing low intensity or items that are difficult to detect. Unmanned Aerial Vehicle (UAV, or drone) may help in getting closer to individual trees and detect atypical signals of small dimensions. The larvae of the pine processionary moth (PPM, Thaumetopoea pityocampa (Denis & Schiffermüller, 1775, Lepidoptera, Notodontidae) build conspicuous silk nests on the external parts of the host plants at the beginning of the winter and their early detection may prompt managers to adopt management techniques. This work aims at testing two deep learning methods (Region-based Convolutional Neural Network - R-CNN and You Only Look Once - YOLO) to detect the nests under three different conditions of host plant species and forest stands in southern Europe. YOLO algorithm provided better results and it allowed us to achieve F1-scores as high as 0.826 and 0.696 for the detection of presence / absence and the individual nests, respectively. The detection of all the nests that can be present on a tree is not achievable with either UAV scanning or traditional ground observation, therefore the integration of the methods may allow the complete efficiency of the surveillance. The use of UAV combined with Artificial Intelligence (AI) image analysis is recommended for further use in forest and urban settings for the detection of the PPM nests. The recommended methods can be extended to other pest systems, especially when specific symptoms can be associated with an insect pest species.

With increasing deforestation, questions are being raised about the risk of zoonotic disease to humans. To better assess the role of forest in the emergence of tick-borne diseases, we conducted a meta-analysis of the scientific literature to compare the abundance or diversity of ticks between forest and open habitats (natural or anthropogenic) and a meta-regression to test how tick abundance is influenced by the abundance of their vertebrate hosts in forest habitats. We found that Ixodes ticks were on average more abundant and diverse in forests than in any other non-forested habitats, the difference being more pronounced with mixed deciduous- coniferous than with deciduous forests. At the forest scale, exophilic Ixodes tick abundance was positively influenced by the abundance of their ungulate hosts. Our results suggest that mixed forests represent the habitats with the highest level of tick hazard. However, more studies are needed to assess the risk of transmission of tick-borne diseases in forests, which also depends on the prevalence of pathogens and the exposure of people.

Key message In mixed stands of Pinus pinaster and Pinus pinea , fewer insect vectors of the pinewood nematode (PWN) were captured than in pure P. pinaster stands. This finding has practical implications for PWN disease management, including the recommendation to improve the diversity of maritime pine plantations and to conserve stone pines in infected areas. Context The PWN is an invasive species in European pine forests, being vectored by the longhorn beetle Monochamus galloprovincialis . The presence of less preferred host trees may disrupt the insect vector dispersal and slow the spread of the disease. Aims The aim of the study was to compare the abundance of M. galloprovincialis in pure stands of Pinus pinaster , a preferred host tree, pure P. pinea stands, a less preferred host, and mixtures of these two species. Methods We selected 20 mature pine stands varying in % P. pinaster and % P. pinea in Spain. In each stand, we installed 3 pheromone traps to catch M. galloprovincialis . We related trap catches to stand and landscape composition. Results The level of capture of M. galloprovincialis was highest in pure P. pinaster stands and decreased with increasing proportion of P. pinea . Conclusions The presence of stone pine mixed with maritime pine significantly reduces the local abundance of the PWN insect vector. The most plausible mechanism is that P. pinea emits odors that have a repulsive effect on dispersing beetles.

Habitat degradation and climate change are main drivers of insect species loss worldwide, raising concern about natural forest replacement by tree monocultures in a context of more frequent disturbances like drought. Carabid beetles are emblematic species in ecology because they are often used as indicators of biodiversity and they have important functional roles, particularly predation. We used a tree diversity experiment with half of the plots irrigated and the other half under summer water stress to test the combined effects of tree species mixing and drought on carabid species and functional diversity. We observed a qualitative effect of drought on carabid communities, with species turnover mainly due to predator species loss in the drier (non‐irrigated) plots. We also found that species richness and activity density were lowest in pure pine plots and highest in mixture of pine and birch at low tree density. The likely underlying mechanism is the better provision of food and shelter resources in mixed forests. The association of pine with birch species could compensate for the loss of carabid beetles observed between pine monocultures benefiting from the water regime of the last century and those subject to current droughts. This suggests that diversifying plantation forests is a promising way to increase their resilience to the adverse effects of climate change.

Urbanization is recognized as an important driver of the diversity and abundance of tree associated insect herbivores, but its consequences for insect herbivory are controversial. 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 effect of urbanization and local canopy cover on insect herbivory in the pedunculate oak (Quercus robur) throughout most of its geographic range in Europe. The damage caused by chewing insect herbivores as well as the incidence of leaf-mining and gall-inducing herbivores consistently decreased with increasing urbanization around focal oaks. Herbivory by chewing herbivores increased with increasing forest cover, regardless of urbanization. In contrast, an increase in local canopy cover buffered the negative effect of urbanization on leaf-miners and strengthened its effect on gall-inducers. These results show the complexity of plant-herbivore interactions in urbanized areas, highlighting that the presence of local canopy cover within cities has the potential to attenuate or modify the effect of urbanization on biotic interactions.

Aim:Leaves support a large diversity of fungi, which are known to cause plant diseases, induce plant defences or influence leaf senescence and decomposition. To advance our understanding of how foliar fungal communities are structured and assembled, we assessed to what extent leaf flush and latitude can explain the within- and among-tree variation in foliar fungal communities. Location:A latitudinal gradient spanning c. 20 degrees in latitude in Europe. Taxa:The foliar fungal community associated with a foundation tree species, the pedunculate oak Quercus robur. Methods:We examined the main and interactive effects of leaf flush and latitude on the foliar fungal community by sampling 20 populations of the pedunculate oak Quercus robur across the tree's range. We used the ITS region as a target for characterization of fungal communities using DNA metabarcoding. Results:Species composition, but not species richness, differed between leaf flushes. Across the latitudinal gradient, species richness was highest in the central part of the oak's distributional range, and foliar fungal community composition shifted along the latitudinal gradient. Among fungal guilds, the relative abundance of plant pathogens and mycoparasites was lower on the first leaf flush, and the relative abundance of plant pathogens and saprotrophs decreased with latitude. Conclusions:Changes in community composition between leaf flushes and along the latitudinal gradient were mostly a result of species turnover. Overall, our findings demonstrate that leaf flush and latitude explain 5%-22% of the small- and large-scale spatial variation in the foliar fungal community on a foundation tree within the temperate region. Using space-for-time substitution, we expect that foliar fungal community structure will change with climate warming, with an increase in the abundance of plant pathogens and mycoparasites at higher latitudes, with major consequences for plant health, species interactions and ecosystem dynamics.

Summary Many plant species produce multiple leaf flushes during the growing season, which might have major consequences for within‐plant variation in chemistry and species interactions. Yet, we lack a theoretical or empirical framework for how differences among leaf flushes might shape variation in damage by insects and diseases. We assessed the impact of leaf flush identity on leaf chemistry, insect attack and pathogen infection on the pedunculate oak Quercus robur by sampling leaves from each leaf flush in 20 populations across seven European countries during an entire growing season. The first leaf flush had higher levels of primary compounds, and lower levels of secondary compounds, than the second flush, whereas plant chemistry was highly variable in the third flush. Insect attack decreased from the first to the third flush, whereas infection by oak powdery mildew was lowest on leaves from the first flush. The relationship between plant chemistry, insect attack and pathogen infection varied strongly among leaf flushes and seasons. Our findings demonstrate the importance of considering differences among leaf flushes for our understanding of within‐tree variation in chemistry, insect attack and disease levels, something particularly relevant given the expected increase in the number of leaf flushes with climate change.

Conspecific insect herbivores co-occurring on the same host plant interact both directly through interference competition and indirectly through exploitative competition, plant-mediated interactions and enemy-mediated interactions. However, the situation is less clear when the interactions between conspecific insect herbivores are separated in time within the same growing season, as it is the case for multivoltine species. We hypothesized that early season herbivory would result in reduced egg laying and reduced performance of the next generation of herbivores on previously attacked plants. We tested this hypothesis in a choice experiment with box tree moth females (Cydalima perspectalis Walker, Lepidoptera: Crambidae). These females were exposed to box trees (Buxus sempervirens L., Buxaceae) that were either undamaged or attacked by conspecific larvae earlier in the season. We then compared the performance of the next generation larvae on previously damaged vs undamaged plants. Previous herbivory had no effect on oviposition behaviour, but the weight of next generation larvae was significantly lower in previously damaged plants. There was a negative correlation between the number of egg clutches laid on plants by the first generation and the performance of the next generation larvae. Overall, our findings reveal that early season herbivory reduces the performance of conspecific individuals on the same host plant later in the growing season, and that this time-lagged intraspecific competition results from a mismatch between the oviposition preference of females and the performance of its offspring.