Quality, decomposition and isopod consumption of tree litter produced under elevated CO2

Rising atmospheric CO2 is expected to alter plant tissue quality which in t urn could affect litter quality, decomposition, and carbon and nutrient tur nover. We tested this hypothesis using leaf litter of beech (Fagus sylvatic a) and branchlets (wood + bark) of spruce (Picea abies) produced under cont rasting CO2 concentrations in model ecosystems. Both types of litter produc ed under elevated CO2 had significantly lower N concentrations, but showed no CO2-related differences in carbon and lignin concentrations. Decompositi on rates (mass loss) assessed in a natural temperate forest were significan tly slower in litter produced at high CO2. However, this effect became stro nger in beech leaves but gradually disappeared in spruce branchlets over th e 331-d exposure period. Irrespective of CO2 treatment beech leaf litter lo st 16% of its initial N content. Spruce branchlets produced at low CO2 lost 50% of their initial N content, and those produced at high CO2 lost 26%. T wo isopod species representing native macro-decomposers consumed 36% more o f the high CO2-produced beech litter than they did of low CO2-produced beec h litter. Only small, and non-significant increases in consumption of high CO2-produced spruce branchlets were observed. Isopods feeding on high CO, l itter also produced more feces than those feeding litter from low CO2. Our results indicate that CO2-induced litter quality changes influence only certain stages of decomposition. and that these stages differ between diff erent litter types. Inhibitory effects of elevated CO2, however, may be com pensated by the positive feed-back of intensified "litter processing" of lo w quality litter by macro-decomposers. Consequently, the entire cycle of li tter production and decomposition must be included in the analysis of the p otential effects of rising CO2 on litter decomposition. This includes both micro- and macro-decomposer specific effects.