Below‐ground responses to insect herbivory in ecosystems with woody plant canopies: A meta‐analysis

Abstract Insect herbivory can have important consequences for the functioning of terrestrial ecosystems. Despite a growing recognition of the role of herbivores in above‐ground–below‐ground interactions, our current understanding is mainly restricted to studies of vertebrates in grassland and tundra ecosystems, while ecosystems with tree‐like canopies (termed forests below) and invertebrates remain understudied. Here, we assess the current state of knowledge of one key aspect of plant–herbivore interactions by conducting a meta‐analysis of the peer‐reviewed literature on the below‐ground consequences of above‐ground insect herbivory in forest ecosystems. Main results are reported as aggregated relative effect sizes (Cohen's d ). We find that above‐ground insect herbivory reduced below‐ground carbon (C) allocation by plants to roots (−0.56) and root exudation (−0.85), causing shifts in root–symbiont communities, for example, a decrease (−0.67) in the abundance of ectomycorrhizal fungi. Microbial decomposer abundances showed no significant responses, while soil faunal abundances increased (0.50). C and nitrogen (N) mineralization rates (C: 0.48, N: 0.48) along with nutrient leaching (C: 0.30, N: 0.77) increased, with a stronger response to outbreak relative to background insect densities. The negative responses increased in strength in colder and dryer biomes while positive responses were reinforced in warmer and wetter biomes, thus extending previously shown effects for vertebrate herbivores to also include insect herbivory. The positive response by soil fauna to insect herbivory was the notable exception. This may be associated with the limited physical soil disturbance caused by insects compared to ungulates. Furthermore, we identified an under‐representation in the literature of large areas of boreal and tropical biomes calling for research priorities to fill these knowledge gaps. We present three recommendations for future research: addressing (a) biological drivers of biogeochemistry and response pathways, (b) knowledge gap from boreal and tropical forests, and (c) heterogeneity of herbivore disturbances. Synthesis. Insect herbivores significantly accelerate soil C and N cycling during outbreaks in forest ecosystems, but we lack knowledge on the underlying biological drivers. Overall, below‐ground responses to insect herbivory are similar to vertebrate herbivory responses, which may simplify implementing herbivory effects into ecosystem models. Nonetheless, we identify a few important differences and general knowledge gaps on which we base recommendations for future research.