Quantitative study and modelling of the litter decomposition in a Europeanalluvial forest. Is there an influence of overstorey tree species on the decomposition of ivy litter (Hedera helix L.)?

The influence on the decomposition rate of ivy litter (Hedera helix L.) of three ligneous overstorey species (oak, Quercus robur L., white poplar, Pop ulus alba and ash, Fraxinus excelsior L.) that support ivy was studied in a n alluvial hardwood forest. The ivy provides an abundant litter at the end of spring. The decomposition of ivy litter and the nutrient release rate we re analyzed over four months during the growing season of the canopy trees, the hypothesis being that throughfall could slow down the mass loss I-ate of ivy depending on the support species. Mathematical models for mass loss and nutrient (P, N, K and Mg) release rates were developed. Mass loss rate and the release rate of magnesium, nitrogen and phosphorus show significant differences depending on species whereas no influence of species was obser ved on the release rate of potassium. The results illustrate the significan t effect of oak compared to ash and poplar in slowing down mass loss rate a nd nitrogen, phosphorus and magnesium release rates. The mass loss rate ove r time under the three species followed the proposed exponential model, whe reas the release rates of phosphorus and magnesium did not follow this mode l. This model is fitted to the data in two cases out of three for potassium . Magnesium and potassium are released more rapidly when nitrogen accumulat es and the phosphorus content does not change significantly. No species x d ate interaction was observed, except in the case of magnesium. The species effect is interpreted as an effect of composition of throughfalls and prese nce or absence of inhibitory substances such as phenolic compounds. (C) Els evier, Paris.