The impact of spruce aphids on nutrient flows in the canopy of Norway spruce

Abstract

Summary1 This study investigated the effects of honeydew from aphids in the canopy of Norway spruce (Picea abies (L.) Karst.) on the nitrogen chemistry of throughfall using a rainfall simulation experiment. Throughfall collected beneath infested trees was compared with that from beneath uninfested trees, while standardizing the quality and quantity of the precipitation and plant age.2 Honeydew excreted by Cinara pilicornis (Hartig) and C. costata (Zett.) significantly increased the concentrations of dissolved organic carbon (DOC) and hexose‐C in throughfall. The average concentrations of nitrogenous compounds (NH4‐N, NO3‐N) in throughfall collected beneath infested trees were significantly lower than beneath uninfested trees.3 Multiple regression analysis indicated that the amount of rain and NH4‐N concentrations were the best predictors of the concentrations of dissolved organic nitrogen (DON) in throughfall. Parameters that were closely associated with the level of infestation (DOC, hexose‐C concentrations) did not have a direct relationship with DON. About 40% of the reduction in the concentration of DON in the throughfall was attributed to aphid–micro‐organism interactions.4 Particle amino nitrogen (PAN)‐concentrations were highest under infested trees in July after aphid numbers had declined, indicating a concomitant decline in microbial biomass after honeydew becomes a limiting resource.5 The comparison of the concentrations of different nitrogen compounds in throughfall of infested and uninfested trees indicated that aphids affect the carbon and nitrogen cycles in the phyllosphere by providing energy that fuels the metabolism of the micro‐organisms. These processes seem to occur very rapidly.6 We discuss the significance of the results and the prospects of linking the ecology of micro‐organisms and herbivores with flows of nutrients through the canopy of trees.