Aphids on Norway spruce and their effects on forest-floor solution chemistry

Many species of aphids on spruce excrete large quantities of honeydew. As p roviders of carbohydrates, it is likely that they affect several ecosystem functions by fueling biological processes which are energy-limited. In a ra infall simulation experiment, we manipulated the level of aphid infestation on spruce and studied the effects of honeydew on forest-floor solution che mistry collected underneath infested and uninfested trees. A non-destructiv e sampling method was used to allow repeated measurements using the same fo rest floor during the experimental period. Although the input of dissolved organic carbon (DOC) underneath infested tr ees to the forest floor was considerably higher compared to uninfested tree s, carbon concentrations of forest-floor leachates did not differ between t reatments. Concentrations of dissolved organic nitrogen (DON), NO3-N and N- total of forest-floor leachates, underneath infested and uninfested trees f ollowed a marked seasonal trend with low concentrations recorded in June an d after-frost treatment of the forest floor, while the highest concentratio ns were recorded in July/August. Statistically significant differences in s oil solution properties underneath infested and uninfested trees were recor ded in July when honeydew-affected forest-floor leachates had lower concent rations of NH4-N, N-total, a lower conductivity and a higher pH. Despite a large input of honeydew, no pronounced seasonal trend was found in the carb on leachate concentrations of the forest floor (e.g. DOG, hexose-C). Discri minant function analysis showed that forest-floor leachates can be classifi ed according to the experience of frost and nitrogen concentrations, which were affected by the honeydew of aphids. At the end of the experiment, the number of micro-organisms present in the forest floor was not significantly different between treatments but was higher in the O-h compared to the O1f horizon. Microbial communities did not appear to be severely influenced b y deep temperatures. It seems likely that, in coniferous forests, aphids ca n considerably reduce local fluxes of nitrogen from the forest floor. (C) 1 999 Elsevier Science B.V. All rights reserved.