Litter of an alien tree, Casuarina equisetifolia, inhibits seed germination and initial growth of a native tree on the Ogasawara Islands (subtropical oceanic islands)

We investigated the effects of the accumulation of litter of an alien tree, Casuarina equisetifolia, on the initial establishment of Schima mertensiana, a tree native to the Ogasawara (Bonin) Islands, in the northwestern Pacific Ocean, in field and greenhouse experiments. The field experiment compared emergence of seedlings in forests dominated by C. equisetifolia with those in native forests, with and without litter removal. The greenhouse experiment compared seedling germination and biomass among 12 treatments that included two soil types (collected from the C. equisetifolia and native forests), two litter types (C. equisetifolia and native litter), and three amounts of litter (5, 15, or 45 g/pot). Significantly fewer S. mertensiana seedlings emerged at sites dominated by C. equisetifolia than at sites of native species in the field experiment. Litter removal tended to increase seedling emergence. Fewer and smaller S. mertensiana seedlings germinated with greater litter cover regardless of soil type or litter type in the greenhouse experiment. Our results demonstrate that C. equisetifolia litter suppresses germination and initial growth of S. mertensiana, and that the main cause of the suppression is the greater amount of litter accumulated under C. equisetifolia forest floor rather than factors such as the nature of the soil or litter between native and alien species.