Changes in needle quality and larch bud moth performance in response to CO2 enrichment and defoliation of treeline larches

Abstract. 1. It is hypothesised that the larch bud moth cycle is controlled by host‐tree foliage quality. In a Free Air CO₂ Enrichment (FACE) experiment at the Swiss alpine treeline (2180 m a.s.l.), the effects of elevated CO₂ and previous year defoliation on needle quality of larch and the performance of the larch bud moth were investigated. 2. Starch and lignin concentrations increased and water content decreased in elevated CO₂‐grown needles compared with ambient CO₂ concentration. Defoliation resulted in reduced N, water, starch, and sugar concentrations in needles of the next year generation. No interactions between elevated CO₂ and defoliation on needle quality were observed. 3. Needle quality changes due to needle maturation over the course of the experiment, however, were much larger than the effects of elevated CO₂ and defoliation. For example, N concentration was on average 38% lower and lignin concentration 55% higher at the end of the experiment (early July 2003) than at the beginning (mid June 2003). 4. On non‐defoliated trees, larch bud moth larvae grew somewhat more slowly under elevated CO₂ compared with ambient CO₂ concentration. If, however, trees had been defoliated, this response was reversed, with a faster growth of larch bud moth on high CO₂‐exposed trees than on control trees. Pupal weight was not affected by either CO₂ treatment or defoliation. 5. These results suggest that the larch bud moth has to cope with large changes in food quality due to needle maturation during its development, and that additional CO₂‐ and defoliation‐induced alterations in needle chemistry have comparatively minor influences on larch bud moth performance at the treeline.