The effects of mammal and insect herbivory on above and belowground C allocation by tree seedlings in a temperate forest

Herbivory is a key ecosystem process in terrestrial systems that influences belowground processes. In forest environments, insect herbivory alone can lead to the loss of 2-15% of foliar biomass annually. When plants experience aboveground herbivory, they may change the amount of carbon allocated to the soil via their roots, either by increasing or decreasing root exudation and turnover. These changes in carbon allocation can influence the structure and activity of the root-associated microbial communities. Similarly, herbivory by mammals can affect soil communities by changing the input of easily accessible C through contributions like plant litter and excrement. However, how plants manage C allocation above and belowground in response to mammalian versus insect herbivory remain poorly understood. Therefore, we assessed if plants differentially respond to mammalian vs. insect herbivory via changes in the quantity of C entering the soil in root exudates, resulting in shifts in soil biotic communities with consequences for C cycling and storage. To do this, we carried out a field-based ¹³C pulse chase experiment in a temperate forest, in which we subjected 3-year-old oak seedlings (Q. robur) to simulated herbivory by insects and mammals followed by ¹³C enrichment. Following plant assimilation of the labelled carbon (¹³C) we tracked carbon allocation to root exudates, leaves, roots, rhizosphere soil, soil fauna and continuously monitored soil ¹³CO₂ efflux for five days. During the two-month experimental period, conducted on 36 seedlings, we observed that soil carbon efflux increased over time across all treatments. This is likely because plants were at a later stage phenologically, better able to assimilate C and therefore more C available to allocate belowground. Further results are being analysed and will be presented at the conference. Quantifying these cascading and interactive above-ground, below-ground effects is a research priority given global change-induced changes in invertebrate communities, current forest management strategies and rates of change in mammal populations.