•Mistletoe infestation was higher in pure Scots pine stands than in mixtures.•Associational resistance was partly driven by relative tree height.•Our study is the first to quantify associational ...resistance to a plant parasite.•In the study area Maritime pine was almost never infested with mistletoe.
The pattern that a given tree species suffers less damage when growing with heterospecific neighbors than amongst conspecific plants, i.e. associational resistance, is common for insect herbivores and many fungal pathogens. However, associational resistance to parasitic plants has never been tested in a replicated study. Using paired forest plots, we investigated whether tree diversity triggered associational resistance to a tree parasite, the European mistletoe Viscum album ssp. austriacum, by comparing pure stands of Scots pine (Pinus sylvestris) with mixtures of Scots pine and Maritime pine (Pinus pinaster) in northern Spain. Maritime pine, with 1.2% of trees being infested, was considered a non-host species in the study area. The infestation level of Scots pines was significantly higher in pure plots (45.1%) than in mixed plots of Scots pines and Maritime pines (25.4%). Our study is the first to quantify associational resistance to a plant parasite in mixed vs. pure forest stands and suggests that mechanisms proposed to explain associational resistance to insects and pathogens also apply to plant parasites. Scots pine trees that were taller than the surrounding trees had a higher infestation probability, in both pure and mixed stands. Scots pine trees growing in mixtures were slightly lower than Maritime pines, suggesting that associational resistance was partly driven by reduced relative tree height. However, the effect of plot type (pure vs. mixed) remained significant after the effect of tree height was accounted for, thus indicating that other factors also contributed to lower mistletoe infestation in mixed plots. In particular, the behavior of birds dispersing mistletoe seeds might differ in mixed vs. pure stands.
•This is a review of the potential use of community biotic effects to protect forests.•Candidate host community variables are described, discussed and evaluated.•Few methods permit scope to respond ...rapidly to new pests or pathogens.•The methods can form part of a long-term strategy against unknown threats.
In the light of the global increase in forest pests and diseases, accompanied by withdrawal of chemical pesticides and fungicides, we review recent advances in how forest tree resistance to pests and diseases might be influenced by manipulation of characteristics of the trees’ community of associated plants and other organisms. These include associational resistance (AR), effected by manipulation of tree species composition or richness, genotypic diversity, or by tree density/frequency of resistant species or genotypes, use of hybrids, and strategic use of bacterial and fungal symbionts to engender systemic induced resistance, or defense priming. Due to the rotation times in forest production systems, none of the community-based actions that attempt to disrupt a tree-pest or tree-pathogen interaction, can be used in a rapidly mobilised targeted response to currently known pests or pathogens or to unknown ones that are yet to emerge. The only exception is the possible use of non-pathogenic or mutualistic organisms to induce systemic resistance or prime tree defence systems against attack. Due to the range of permutations of participating species and assemblages and the idiosyncratic nature of their ecological interactions and processes, it is not currently possible to formulate predictive rules to protect forests using only these community-based methods. It is however possible to use them as part of a long-term prophylactic strategy by structuring future forests to increase their general resistance, and reduce the probability of impacts of pests and diseases, e.g. via diversification of tree species and genotypes.
Interactions between individual consumer and resource organisms can be modified by neighbors, e.g., when herbivory depends on the identity or diversity of neighboring plants. Effects of neighbors on ...consumer-resource interactions (“associational effects”) occur in many systems, including plant-herbivore interactions, predator-prey interactions (mimicry), and plant-pollinator interactions. Unfortunately, we know little about how ecologically or evolutionarily important these effects are because we lack appropriate models and data to determine how neighbor effects on individuals contribute to net interactions at population and community levels. Here we supply a general definition of associational effects, review relevant theory, and suggest strategies for future theoretical and empirical work. We find that mathematical models from a variety of fields suggest that individual-level associational effects will influence population and community dynamics when associational effects create local frequency dependence. However, there is little data on how local frequency dependence in associational effects is generated, or on the form or spatial scale of that frequency dependence. Similarly, existing theory lacks consideration of nonlinear and spatially explicit frequency dependence. We outline an experimental approach for producing data that can be related to models to advance our understanding of how associational effects contribute to population and community processes.
The natural enemies hypothesis predicts that the abundance and diversity of antagonists such as predators and parasitoids of herbivores increases with the diversity of plants, which can lead to more ...effective top-down control of insect herbivores. However, although the hypothesis has received large support in agricultural systems, fewer studies have been conducted in forest ecosystems and a comprehensive synthesis of previous research is still lacking.
We conducted a meta-analysis of 65 publications comparing the diversity, abundance or activity of various groups of natural enemies (including birds, bats, spiders and insect parasitoids) in pure vs. mixed forest stands. We tested the effects of forest biome, natural enemy taxon and type of study (managed vs experimental forest).
We found a significant positive effect of forest tree diversity on natural enemy abundance and diversity but not on their activity. The effect of tree diversity on natural enemies was stronger towards lower latitudes but was not contingent on the natural enemy taxon level.
Overall, our study contributes toward a better understanding of the “natural enemies hypothesis” in forest systems and provides new insights about the mechanisms involved. Furthermore, we outline potential avenues for strengthening forest resistance to the growing threat of herbivorous insects.
Summary
Associational resistance to herbivore and pathogen attack is a well documented ecological phenomenon and, if applied to agriculture, may reduce impact of pests and diseases on crop yields ...without recourse to pesticides.
The value of associational resistance through intercropping, planting multiple crops alongside each other, as a sustainable control method remains unclear, due to variable outcomes reported in the published literature. We performed a meta‐analysis to provide a quantitative assessment of benefits of intercropping for target plant resistance to plant‐parasitic nematodes and soil‐borne diseases.
We found that intercropping reduced damage to focal crops from nematodes by 40% and disease incidence by 55%. Intercropping efficacy varied with biological variables, such as field fertilisation status and intercrop family, and methodology, including whether study samples were potted or in fields.
Nematode control using intercropping was sufficient to offset reductions in focal crop yield from intercrop presence, making intercropping a viable agricultural tool. We identify key drivers for underpinning the success of intercropping and indicate areas for future research to improve efficacy. This study also highlights the potential benefits of harnessing ecological knowledge on plant–enemy interactions for improving agricultural and landscape sustainability.
Pest regulation is an important ecosystem service provided by biodiversity, as plants growing in species‐rich communities often experience associational resistance to herbivores. However, little is ...known about the respective influence of the quantity and identity of associated species on herbivory in focal plants. Using a meta‐analysis to compare insect herbivory in pure and mixed forests, we specifically tested the effects of the relative abundance of focal tree species and of phylogenetic distance between focal and associated tree species on the magnitude of associational resistance. Overall, insect herbivory was significantly lower in mixed forests, but the outcome varied greatly depending on the phylogenetic relatedness among tree species and the degree of herbivore feeding specialization. Specialist herbivore damage or abundance was positively related to relative abundance of their host trees, regardless of the phylogenetic distance between host and associated tree species. By contrast, tree diversity triggered associational resistance to generalist herbivores only when tree mixtures included tree species phylogenetically distant to the focal species. Synthesis and applications. Our study demonstrates that the establishment of mixed forests per se is not sufficient to convey associational resistance to herbivores if the identity of tree species associated in mixtures is not taken into account. As a general rule, mixing phylogenetically more distinct tree species, such as mixtures of conifers and broadleaved trees, results in more effective reduction in herbivore damage.
Forest pest damage is expected to increase with global change. Tree diversity could mitigate this impact, but unambiguous demonstration of the diversity–resistance relationship is lacking in ...semi-natural mature forests. We used a network of 208 forest plots sampled along two orthogonal gradients of increasing tree species richness and latitudes to assess total tree defoliation in Europe. We found a positive relationship between tree species richness and resistance to insect herbivores: overall damage to broadleaved species significantly decreased with the number of tree species in mature forests. This pattern of associational resistance was frequently observed across tree species and countries, irrespective of their climate. These findings confirm the greater potential of mixed forests to face future biotic disturbances in a changing world.
Although plants are sessile organisms, they can modulate their phenotype so as to cope with environmental stresses such as herbivore attack and competition with neighbouring plants. Plant‐produced ...volatile compounds mediate various aspects of plant defence. The emission of volatiles has costs and benefits. Research on the role of plant volatiles in defence has focused primarily on the responses of individual plants. However, in nature, plants rarely occur as isolated individuals but are members of plant communities where they compete for resources and exchange information with other plants. In this review, we address the effects of neighbouring plants on plant volatile‐mediated defences. We will outline the various roles of volatile compounds in the interactions between plants and other organisms, address the mechanisms of plant neighbour perception in plant communities, and discuss how neighbour detection and volatile signalling are interconnected. Finally, we will outline the most urgent questions to be addressed in the future.
Using nurse shrubs to improve tree seedling establishment in stressed environments is a common practice in forestry. Recent refinements of the stress‐gradient hypothesis suggest that positive nurse ...effects occur under intermediate stress and decline in the harshest conditions. Additionally, indirect facilitation (e.g. protection from herbivory) is expected in low‐stressed/productive systems according to the initial stress‐gradient hypothesis. However, there have been fewer investigations into the use of nurse shrubs to decrease herbivore impacts in stressed systems compared with the role of nurses in stress alleviation. This raises the need to clarify conditions in which the use of nurse shrubs would be most appropriate.
We conducted a field experiment in the sand dune forest of southwest France characterized by severe summer water shortages and the presence of different types of wild herbivores. In 2015 and 2016, Pinus pinaster seedlings were planted under shrubs Arbutus unedo or in shrub‐removed locations, in fenced/unfenced areas. Survival, cause of mortality and water stress were monitored for 1 year after plantation.
Summer drought was the main cause of mortality. Water stress increased in both years as summer progressed. We found direct facilitation only in July 2016 at intermediate water stress, due to a decrease in vapour pressure deficit under shrubs. These positive effects declined in late summer when stress was at a maximum. Rodent damage occurred under shrubs in spring whereas ungulate browsing was observed in open areas in autumn/winter. Overall seedling survival was equivalent under or outside shrubs during the first year, but was better under shrubs in the second one.
Synthesis and applications. Our results partially validate the refinement of the stress‐gradient hypothesis, with a decline of positive interactions in the most water‐stressed conditions. This draws attention to the difficulty of using shrubs as nurses to improve tree regeneration in the most xeric systems. In addition, we show that indirect associational effects, both positive and negative, could be as important as abiotic stress alleviation in a stressful ecosystem, and using shrubs to facilitate seedling survival could be efficient depending on the type of herbivore involved. This highlights the fact that considering plant–plant interactions can help in the design of management options, providing that up‐to‐date ecological theories are considered and that the different biotic and abiotic constraints are accurately evaluated.
Our results partially validate the refinement of the stress‐gradient hypothesis, with a decline of positive interactions in the most water‐stressed conditions. This draws attention to the difficulty of using shrubs as nurses to improve tree regeneration in the most xeric systems. In addition, we show that indirect associational effects, both positive and negative, could be as important as abiotic stress alleviation in a stressful ecosystem, and using shrubs to facilitate seedling survival could be efficient depending on the type of herbivore involved. This highlights the fact that considering plant–plant interactions can help in the design of management options, providing that up‐to‐date ecological theories are considered and that the different biotic and abiotic constraints are accurately evaluated.
Abstract
Insect herbivory plays a critical role in the regulation of plant populations and community structures. Although the strength of insect herbivory varies with neighbouring tree diversity and ...identity, it is not clear to what extent insect guilds and leaf traits of host plants mediate the relationships, particularly in diverse natural forests.
In this study, we measured leaf herbivory by chewing and mining insect herbivores in 442 saplings of nine species of deciduous trees in a warm‐temperate montane forest. We censused neighbouring trees and measured five leaf traits known to influence herbivory (i.e. specific leaf area‐SLA, thickness, carbon: nitrogen ratio‐C:N ratio, sodium‐Na and silicon‐Si content) on a subset of saplings.
Only herbivory by leaf‐mining insects was influenced by the identity and diversity of neighbouring trees. Specifically, the incidence of leaf‐miners decreased with the phylogenetic diversity of neighbouring trees and increased with conspecific adult density in the vicinity of focal trees. Furthermore, leaf mining herbivory was positively correlated with the amount of Si in the leaves of focal saplings, and leaf chewing herbivory decreased with increasing C:N ratio in the leaves. In contrast to our hypothesis, the leaf traits of focal saplings did not serve as a functional link between tree diversity and insect herbivory.
Synthesis
: Overall, this study highlights the importance of considering the specialisation of insect herbivores in understanding the impacts of the diversity and identity of neighbouring trees on the strength of insect herbivory.
PDF
