Aphid infested Norway spruce are "hot spots" in throughfall carbon chemistry in coniferous forests

Abstract

Throughfall chemistry in deciduous and coniferous forests is characterized by a high spatial and temporal variability, the sources of which are largely unknown. Aphids on Norway spruce (Picea abies (L.) Karst.) produce large quantities of honeydew, which is rich in sugars. We investigated the effect of changes in aphid abundance on throughfall carbon chemistry of Norway spruce using a rainfall simulation experiment. Aphid numbers in the experiment paralleled field infestations. Infested trees showed significantly higher concentrations of dissolved organic carbon (DOC) and hexose-C in throughfall solutions compared with uninfested trees. There was also a pronounced seasonal trend in the concentrations of these carbon compounds with highest concentrations recorded at the time of peak aphid abundance at the end of June - beginning of July. The throughfall from a spruce tree showed no spatial (center vs. periphery) differences in DOC and hexose-C concentrations. The number of aphids on a tree was the best predictor of the carbohydrate concentration of throughfall, explaining more than 60% of the variability. During the period of aphid infestation (May-September) average estimated field fluxes of DOC in throughfall collected under heavily infested trees ("hot spots") were 11.6 and 240 times higher than those recorded under moderately and uninfested trees, respectively. The results implicate herbivores as a major potential source of the spatiotemporal heterogeneity in flows of nutrients in forests.