Non-additive effects of biotic interactions on litter decomposition

Recent studies examining the effects of litter mixtures on rates of decay and nutrient release have focused at the level of plant species, however the role of mixtures of plant genotype or herbivory, have received relatively little attention.  Focusing on a series of field and common garden studies with Populus, we examined the relative influence of plant genotype or previous herbivory on rates of litter decay in litter mixtures.  Overall we found non-additive effects of decomposing litters in mixture such that plant genotype or previous herbviory can explain 30-64% of the variation in litter decay and nitrogen or phosphorus release.  However, when decomposed in genotype mixtures with and without previous herbviory, the effects of herbivory reduce the variation due to genotype and homogenize the overall genotype effect.  These data suggest that biotic interactions at fine scales may have important but little appreciated roles on litter dynamics in forest ecosystems.