Impacts of root herbivory on seedlings of three species of boreal forest trees

- Natural and simulated root herbivory similarly reduced aboveground plant growth.
- Seedlings of three tree species similarly responded to root herbivory.
- Larvae of Melolontha melolontha showed similar growth rates on three tree species.
- Larvae consumed more birch roots than conifer roots to gain similar weights.
- Roots were converted to insect biomass five-fold less efficiently than leaves.

Current meta-analysis has identified the shortage of information on responses of woody plants to root damage as the most critical gap in studies of belowground insect herbivory. We explored the impacts of natural and simulated root herbivory on seedlings of three boreal forest-forming trees, Pinus sylvestris, Picea abies and Betula pendula. We compared the effects of natural herbivory and mechanical damage by placing one larva of the common cockchafer, Melolontha melolontha, of different instars (to manipulate root loss) into a pot with a one-year-old seedling or we excised a portion of the roots by inserting a 13-mm diameter cork borer into the soil. During the experiment, which lasted from 2 to 8 weeks, the larvae consumed 53-73% of root biomass, whereas the mechanical damage treatment removed 30-40% of roots. Seedling mortality was low (5%) across all treatments, but the aboveground biomass decreased by 3-43% due to natural herbivory and by 6-30% due to mechanical damage. The M. melolontha larvae consumed a larger amount of roots from B. pendula than from either P. sylvestris or P. abies, but the larvae showed similar relative growth rates and mass gains, indicating a different efficiency of conversion of ingested food (ECI). The values of ECI calculated for larvae of M. melolontha (2.3-3.5%) appeared much lower than the 17% reported earlier for larvae of foliage-feeding beetles, presumably due to the high energetic cost of locomotion in the soil. Consequently, root-feeding insects consume much more plant biomass than leaf-feeding insects do to gain similar body mass, and therefore they inflict relatively more damage to plants per unit of mass gained. Based on the biomass of insect herbivores in European forests, we estimate that they consume 0.5% of the available fine root biomass. The effects of low levels of root damage on growth and reproduction of woody plants remain virtually unknown, emphasising the need for studies of plant- and ecosystem-level effects of minor but chronic root herbivory.