Effects of aphid infestation on the biogeochemistry of the water routed through European beech (Fagus sylvatica L.) saplings

Mass outbreaks of herbivore insects perturb the functional properties of forests, by substantially altering water and nutrient cycling of infested trees. Less clear is how low to moderate herbivory affects the vertical flow of nutrients from trees to the soil. We report on the effects of low to moderate infestation levels of the woolly beech aphid (Phyllaphis fagi L.) on the nutrient dynamics and hydrology of European beech (Fagus sylvatica L.). We traced the vertical dynamics of macro and micronutrients via throughfall (TF), stemflow (SF) and Oa litter leachates from potted saplings underneath infested and uninfested control trees. The low infestation levels were insufficient to significantly alter TF and litter leachate composition, although, on average, the release of K and DOC and the immobilization of N (NH4-N) in TF solution were more pronounced under aphid infestation. SF chemistry was significantly altered by aphid activity exhibiting intensified fluxes of K (+159 %), Mg (+82 %), Mn (+93 %), S (+86 %), SO4-S (+62 %), DOS (+137 %), DOC (+51 %) and DON (+62 %), compared to control trees. SF is likely enriched by low and mid-level aphid activity due to its increased residence time in the canopy. It has interaction with large areas of aboveground vegetative surface area representing a cumulative flux of solutes and particulates from both foliar and woody surfaces from above-lying portions of the canopy. Accordingly, SF might be a useful indicator to evaluate the impact of sap- and leaf-feeding herbivores on forest ecosystems.