MULTIPLE INDIRECT EFFECTS OF PLANT LITTER AFFECT THE ESTABLISHMENT OF WOODY SEEDLINGS IN OLD FIELDS

I studied the effects of oak leaf litter, herb competition, and insect herbivory on the establishment of seedlings of Ailanthus altissima in New Jersey, USA. I performed two experiments in an early-successional site in two consecutive years. In the first experiment I tested the effects of litter and competition, and their interaction, on establishment and growth of tree seedlings. In the second experiment I studied the effect of insecticide application in combination with litter and herb competition on seedling establishment and growth, and on arthropod abundance. In the absence of competition, litter had no significant effect on the biomass of woody seedlings, though it delayed seedling emergence. Herb competition reduced the growth of the woody seedlings, but the addition of leaf litter reduced the density and biomass of the herbs, indirectly improving the growth of the woody seedlings. Litter increased both invertebrate damage to seedling cotyledons and seedling mortality. The application of insecticide increased seedling emergence and reduced seedling mortality, and more so in the presence of litter. The total number of arthropods and the number of plant-chewing arthropods were increased by the addition of litter and reduced by application of insecticide. Thus, litter had, simultaneously, a positive indirect effect on individual biomass through reduced competition, and a negative indirect effect on the number of individuals through increased seed and/or seedling predation, and a negative direct effect on seedling mortality.