Conspecific negative distance dependent seedling mortality and negative density-dependent seedling survival (CNDD) are among the most important mechanisms of species diversity maintenance. Recent research has demonstrated that CNDD is greater for species with abundant seeds, due mainly to attack by density-dependent pathogens. If seed number were correlated negatively with seed size, species with smaller seeds would exhibit much greater negative CNDD. We explored whether CNDD is affected by seed size. We evaluated the strength of CNDD as a log response ratio of the probability of mortality among seedlings at conspecific and heterospecific sites and at high and low densities as functions of distance and density, respectively, in a seed-sowing experiment using eight hardwood species in a temperate forest in northern Japan. As a function of distance, CNDD attributable to pathogens was correlated positively with seed mass at low density, but not at high density, whereas seed mass was not correlated with CNDD attributable to invertebrate or vertebrate herbivores. As a function of density, seed mass was not correlated with CNDD attributable to pathogens or invertebrate or vertebrate herbivores. The ability of large-seeded species to better tolerate CNDD suggests a novel mechanism through which seed size may contribute to CNDD variation and promote species coexistence due to distance-dependent attack by pathogens, particularly those bearing leaf diseases.
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