What was done
Five-year-old plants of heather (Calluna vulgaris) were collected from a Scottish moor and grown in open-top chambers receiving atmospheric CO2 concentrations of 350 and 600 ppm for 20 months and additions or no additions of nitrogen fertilization to determine the effects of elevated CO2 and soil nitrogen on plant growth and tissue chemistry.  In addition, at two different times during the experiment, Operophtera brumata larvae, whose outbreaks have caused extensive damage to heather moorland in recent years, were allowed to feed upon current-year shoots for up to one month to determine if elevated CO2 or nitrogen affects the feeding pattern or development of this voracious winter moth.

What was learned
Elevated CO2 did not significantly affect the growth of heather as measured by current-year shoot lengths.  In addition atmospheric CO2 enrichment did not alter the C:N ratio or total phenolic concentration of foliage.  However, elevated CO2 tended to increase photosynthetic rates (30% stimulation), regardless of nitrogen treatment, and significantly reduced rates of stomatal conductance.  Thus, growth in elevated CO2 significantly improved the water-use efficiency of heather.  In contrast, high nitrogen applications significantly increased shoot growth, decreased the C:N ratio in foliage, but had no effect on photosynthesis or stomatal conductance.

The survivorship of larvae placed upon CO2-enriched foliage was not significantly different from that of larvae placed upon ambiently-grown foliage, regardless of nitrogen treatment.  In addition, feeding upon CO2-enriched foliage did not affect larval growth rates, development, or final pupal weights, all of which were significantly increased by nitrogen fertilization.

What it means
As the air's CO2 content continues to rise, heather plants will likely exhibit increased water-use efficiency, resulting from CO2-induced enhancements in photosynthesis and reductions in stomatal conductance.  In addition, because photosynthetic increases are unlikely to change foliar C:N ratios, increasing atmospheric CO2 concentrations will not make heather a more appealing host for the winter moth Operophtera brumata.  Consequently, the authors concluded that their study "provides no evidence that increasing atmospheric CO2 concentrations will affect the potential for outbreak of Operophtera brumata on this host."  Thus, the Operophtera brumata-Calluna vulgaris system is unlikely to respond to future changes in atmospheric CO2 concentration.