Effects of altitude on aphid-mediated processes in the canopy of Norway spruce
1 The abundance of aphids and their honeydew are important in shaping the ecology of food web interactions and nutrient cycling in forests of Norway spruce. Here, the effects of the different environmental conditions at two study sites located at different altitudes (500 m, 765 m a.s.l.), in the Fichtelgebirge, north-eastern Bavaria, Germany, on the abundance of Cinara pilicornis and their influence on epiphytic microorganisms on shoots of Picea abies were compared. Subsequent changes in throughfall fluxes were measured over a period of 12 weeks beneath infested and reference trees. In a laboratory experiment, the effects of ultraviolet (UV) radiation on microbial mortality and C and N concentrations in leachates were determined.
2 The warmer and drier conditions at the low altitude site favoured an early onset to aphid multiplication in spring compared with the high altitude site, where aphid numbers peaked 3 weeks later.
3 The presence of honeydew was associated with a significant increase in the total number of cultured epiphytic filamentous fungi, yeasts and bacteria in 12 of the 18 sample units, indicating better culturability or growth, whereas altitude had no significant effect on cultured cell numbers. By contrast to the reference trees, the high dissolved organic carbon (DOC) and hexose-C fluxes beneath infested trees at the peak in aphid abundance, in June and July, resulted in a concomitant decrease in the fluxes of total inorganic nitrogen beneath infested trees (low altitude: −19.7%; high altitude: −52.3%). Fluxes of organic nitrogen were significantly higher beneath infested trees at the time of infestation. Similarly, potassium fluxes in throughfall increased 1.6–2.0-fold in response to aphid infestation.
4 The exposure of infested and uninfested shoots of Norway spruce to UV-A and UV-B radiation only weakly affected epiphytic microbial mortality and did not affect the concentrations of the different nitrogen compound in leachates. However, bacteria, tended to be more active in the leachates collected from infested shoots, which resulted in the higher concentrations of aminosugar-N. The aphids had a more pronounced effect on the concentrations of DOC in leachates, with average DOC concentrations being 4.2-fold higher than in leachates from uninfested shoots.
5 It is suggested that, even at very low densities, aphids exert a strong influence via honeydew on the performance of microorganisms, and nutrient and energy flow, in spruce forests.