Growing forest trees in species mixtures reduces damage by insect herbivores and fungal pathogens on average, yet the sign of the effect for a particular host species reverses with the host's pest-preference status, neighbour species identity, drought stress, spatial scale and stand age. The open problem is to determine the conditions — stated in measurable community and context parameters — under which tree species diversity suppresses versus amplifies pest and disease damage to a given host species, and to establish the mechanisms responsible, jointly for insect herbivores and pathogens. Recent syntheses, including the largest synthesis to date (2026, 2315 observations), show that average effects are firmly established while sign prediction and mechanistic attribution remain explicitly unresolved. The problem is open.

Background

Forest managers worldwide increasingly plant and retain mixed-species stands in the hope that tree diversity buffers against insect pests and fungal diseases, but whether a mixture protects or exposes a particular host species has resisted generalization. Two contrasted outcomes have names: associational resistance (AR), in which a focal tree suffers less herbivore or pathogen damage when surrounded by heterospecific neighbours, and associational susceptibility (AS), its opposite. The classical candidate mechanisms pull in opposite directions. The resource concentration hypothesis predicts that specialised herbivores and pathogens more easily find, and remain on, hosts concentrated in pure stands, so mixing dilutes attack; the natural enemies hypothesis predicts more predators and parasitoids, hence stronger top-down control of herbivores, in diverse stands. Conversely, generalist consumers may profit from dietary mixing in diverse stands (facilitation), and a heterospecific neighbour can worsen a host's exposure, for example by altering the stand microclimate or by changing how conspicuous the focal tree is (apparency, often measured as the height contrast between a focal tree and its neighbours). At the biogeographic scale, the same tension appears as dilution (low host dominance impedes pest establishment) versus facilitation (more niches support more pest species).

The empirical record shows both signs. A meta-analysis of 119 studies found herbivory on average lower in mixed forests, with oligophagous insects (feeding on few related host species) almost always reduced but polyphagous insects (feeding broadly) responding variably, and mixture composition mattering more than species richness (Jactel & Brockerhoff 2007). At the other extreme, subcontinental data for the conterminous United States show a hump-shaped relationship in which pest diversity rises with tree diversity at low diversity (facilitation) and falls at high diversity (dilution) (Guo et al. 2019). Within single experiments the sign is host-specific: in an early-stage diversity experiment, neighbourhood diversity simultaneously increased leaf removal for some tree species, decreased it for others and had no effect on yet others, with AR for oaks against leaf miners and maples against anthracnose but AS for birches against leaf miners and red cedars against cedar-apple rust (Grossman et al. 2019). After a severe drought in a young European diversity experiment, infestation risk by the six-toothed spruce bark beetle Pityogenes chalcographus decreased with tree diversity for the main hosts Norway spruce and European larch but increased for the less-preferred pines (Berthelot et al. 2021). In a French oak diversity experiment, species richness lowered leaf-miner, chewer and powdery-mildew damage, but the strength of the effect was modulated by irrigation (drought treatment), and the presence of birch increased chewer and mildew damage while decreasing leaf-miner damage (Field et al. 2020).

Later syntheses confirm the average but not the sign. A meta-analysis of about 600 cases shows average resistance contingent on herbivore diet breadth and stand composition, with specialist damage mainly reduced in phylogenetically distant mixtures (Jactel et al. 2021). The first pathogen-specific systematic review and meta-analysis finds overall reduced pathogen damage that is not universal and hinges on neighbour identity rather than richness (Field et al. 2025). The largest synthesis to date (2315 observations from 316 studies) finds average reductions of roughly 21–32% in mixtures but signs that depend on context (moderator) variables such as consumer diet breadth and stand age, with herbivore effects shifting from negative to positive as stands age (Huang et al. 2026). Species-specific invasion models conclude that biodiversity effects are variable and preclude broad generalizations (Ward et al. 2022), and a 2023 paper states plainly that a successful framework explaining the wide variation in the sign and magnitude of plant–herbivore associational effects is still lacking (Leonard et al. 2023). Proposed partial predictors — phylogenetic distance of neighbours to the pest's preferred hosts (Gougherty & Davies 2024), leaf-trait mediation (Wang et al. 2024), volatile-organic-compound diversity (Zhu et al. 2026) — each explain part of the variance, and an integrative five-factor mechanistic framework has been proposed but not validated (Halliday et al. 2025).

Problem Statement

Determine the conditions under which forest tree species diversity suppresses versus amplifies damage by insect pests and fungal pathogens to a given host species, and the mechanisms responsible, jointly for both consumer types. "Conditions" are to be stated in terms of measurable community and context parameters: the focal host's species identity and its host-preference status for the local pest complex; herbivore diet breadth and pathogen host specificity; neighbour species identity and its phylogenetic and functional distance from the focal host; the proportion of host versus non-host trees in the stand; abiotic stress, notably drought; spatial scale and neighbourhood extent; and stand age or developmental stage. These parameters delimit the space of conditions to be considered; an answer identifies which of them drive the sign for the host in question and need not resolve every one. Determine how these parameters map onto the sign — and ideally the magnitude — of the associational effect for particular host species, and attribute the sign to specific mechanisms (for example resource concentration, natural-enemy recruitment, apparency, microclimate modification, volatile-mediated host finding, or trait- and defence-mediated changes). The components — condition identification and mechanism isolation — jointly serve this single objective; an average effect size across all hosts and consumers does not answer the question, because the average conceals the documented sign reversals, and coverage of only insects or only pathogens does not either, because the question spans both.

References

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