In a field acid mist simulation experiment, Sitka spruce (Picea sitchensis) has been sprayed with different pollutant treatments:- N: NH4NO3,
S: Na2SO4, NS Acid: NH4NO3+H2SO4 and control: no spray. Treatment effects on the abundance of the green spruce aphidElatobium abietinumand honeydew production were assessed. In addition, needles were sampled for phyllosphere micro-organisms. In a manipulative experiment shoots were established and maintained as with or withoutE. abietinum infestation in order to determine the effects of infestation on needle loss and throughfall nutrient fluxes.
Aphid numbers were highest during the end of May and early June, with almost twice as many needles infested in the NS Acid treatment compared with the other treatments. Honeydew production was not affected by the treatments.
On infested shoots, increasing numbers of yellowing and dead needles were recorded above the throughfall collectors as the season progressed. The numbers of dead needles falling into the collectors were significantly higher beneath infested shoots. There were strong positive correlations between aphid numbers above the throughfall collectors, the number of yellowing and dead needles on the shoots and number of needles in the funnels of the throughfall collectors. Litter production was more affected by aphid number than by pollutant treatment.
Bacteria, yeasts and filamentous fungi were more prolific on infested needles and treatment effects on colony forming units (CFU"s) were most pronounced in the NS Acid treatment.
Fluxes of inorganic nitrogen beneath infested shoots were generally lower than beneath uninfested shoots. This effect was more pronounced in those treatments which supplied Nie., N, NS Acid. The combination of aphid infestation and N-addition exerted the strongest influence on nutrient fluxes. The fluxes of potassium and of organic carbon (DOC) were higher beneath infested shoots in all treatments, through most of the survey period.
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