Aphid infestation affecting the biogeochemistry of European beech saplings

Abstract:

Mass outbreaks of herbivore insects are known to perturb the functional properties of forests. However, it is less clear how endemic to moderate aboveground herbivory affects the vertical flow of nutrients from tree canopies to the soil. Here, 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.). In a potted sapling experiment, we followed the vertical dynamics of nutrients via throughfall (TF), stemflow (SF) and litter leachates (LL) collected over ten weeks underneath infested and uninfested control trees.

Aphid infestation amplifies the fluxes of K⁺, Mn²⁺ and particulate nitrogen (0.45µm < PN < 500 µm) in TF solution  by 42% for K⁺, 59% for Mn²⁺ and 13% for PN relative to the control. In contrast, fluxes of NH₄-N and SO₄-S diminished during peaking aphid abundance by 26 and 16%, respectively. Differences in canopy-derived dissolved nitrogen and carbon compounds, sulfur (S), Ca²⁺, Mg²⁺, Na⁺ were < 10%. The effect of aphid abundance on nutrient dynamics was most notable in TF and SF and diminished in LL.

Aphid infestation greatly altered the SF fluxes of DOC, K⁺, Mn²⁺, DON and sulfur-species, which were significantly concentrated at the tree base by “funneling” the rainfall through the canopy biomass to the trunk. Normalized to one square meter, water and nutrient fluxes were amplified by a factor of up to 200 compared to TF.

Imaging of leaf surfaces by scanning electron microscopy exhibited notable differences of the surface morphology and microbiology of control, lightly infested, and heavily infested leaves. This observation might point to an aphid-mediated alteration of the phyllosphere ecology triggering the microbial uptake of NH₄-N and SO₄-S and its transformation to particulate N by magnified biomass growth of the phyllosphere microflora, consequently changing the chemical partitioning and temporal availability of nitrogen.