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| Mühlenberg, E; Stadler, B: Effects of altitude on aphid-mediated processes in the canopy of Norway spruce, Agricultural and Forest Entomology, 7, 133-143 (2005) | |
Abstract:
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.
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