Leaf litter age regulates the effect of native and exotic tree species on understory herbaceous vegetation of riparian forests

Decomposing litter is regarded as the most important source of allelochemicals released into soil. In this study, a greenhouseexperiment was designed to assess the net effect of differently aged leaf litter from exotic (Ailanthus altissima, Robinia pseu-doacaciaandUlmus pumila) and native riverine trees (Populus alba, Populus nigraandUlmus minor) on the germination andgrowth of three herb species (Trifolium repens, Dactylis glomerataandChenopodium album). We also characterized the chem-ical composition of litter samples at different litter ages (0, 1, 2 and 3 months) based on phenolic compounds,fibers and ergos-terol (as a measure of fungal biomass) contents. Overall, litter from both native and exotic species had a negative effect onshoot and root growth of target species, indicating that phytotoxic effects of litter predominate over positive effects. The inhibi-tion effect of the exotic species was similar or even lower than that of the natives, which does not support the Novel WeaponsHypothesis. Among exotic trees,U. pumilashowed the highest inhibition effect on the growth of the target species.T. repenswas the most sensitive target species. The importance of litter age varied with both target and donor species. In general,D.glomeratawas more inhibited by fresh litter,C. albumby half-decomposed litter ofU. pumilaandR. pseudoacaciaand byfresh litter ofA. altissimaandT. repenswas more inhibited by fresh litter ofA. altissimaandP. albaand by highly decomposedlitter ofU. minor. The concentration of total phenolics andflavonoids decreased while acid detergentfiber, lignin and ergosterolincreased with increasing litter age. Hydroxybenzoic and protocatechuic acids and theflavonoid quercetin were detected in alllitter species and at most of the litter ages, while gallic, chlorogenic, vanillic, coumaric and rosmarinic acids were species-spe-cific and they were only detected in fresh litter. Ergosterol concentration appeared as the strongest constrictor of inhibitoryeffect