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Effects of Elevated CO2 and Nitrogen Deposition on Spruce Needle Quality and Consumption by Nun Moths
Hattenschwiler, S. and Schafellner, C. 1999. Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees. Oecologia 118: 210-217.

What was done
Seven-year-old spruce trees (Picea abies) grown at atmospheric CO2 concentrations of 280, 420, and 560 ppm and various nitrogen deposition treatments for three years had needle quality assessments completed before allowing nun moth larvae to feed upon current-year needles for 12 days to determine the effects of elevated CO2 and nitrogen on needle quality and larval feeding patterns. The nun moth is a voracious defoliator that resides in most parts of Europe and East Asia between 40 and 60 N latitude, and is commonly regarded as the "coniferous counterpart" to its close relative the gypsy moth, which feeds primarily upon deciduous trees.

What was learned
Elevated CO2 significantly enhanced needle starch, tannin, and phenolic concentrations, while significantly decreasing needle water and nitrogen contents. Thus, atmospheric CO2 enrichment reduced overall needle quality in the eyes of this foliage-consuming insect, as nitrogen content is the primary factor associated with leaf quality. In contrast, increasing nitrogen deposition tended to have the opposite effect on needle quality; for it lowered starch, tannin, and phenolic concentrations, while increasing needle nitrogen content. However, the positive influence of nitrogen deposition on needle quality was not large enough to completely offset the quality reductions caused by atmospheric CO2 enrichment.

Larvae placed upon CO2-enriched foliage consumed less total needle biomass than larvae placed upon ambiently-grown foliage, regardless of nitrogen treatment. Consequently, the larvae exhibited reduced relative growth rates and attained an average biomass that was only two-thirds of that attained by control larvae consuming foliage produced at 280 ppm.

What it means
As the CO2 content of the air rises, spruce needles will likely exhibit structural and chemical changes that will provide them with enhanced protection from herbivory by nun moth larvae. In addition, the authors conclude that, "altered needle quality in response to elevated CO2 will impair the growth and development of Lymantria monacha larvae." Hence, the larvae will likely consume smaller quantities of spruce needles in the future, which should lead to reductions in spruce tree destruction caused by this voracious defoliator.

Reviewed 15 January 2000