Effects of dwarf-bamboo understory on tree seedling emergence and survival in a mixed-oak forest in northern Japan: a multi-site experimental study

A dense bamboo undergowth and thick layer of its slowely decaying litter belong to the main factors inhibiting the establishment of woody species in forest stands of northern Japan. The successful regeneration often hinges on bamboo dieback after mass flowering or small-scale disturbances that remove the understory cover and create open sites suitable for seed germination and growth. We conducted a field experiment with repeated observations of emergence and survival of tree seedlings in undisturbed Sasa senanensis understory and experimental gaps simulating different disturbance events to determine mechanisms of bamboo interference with tree seedling regeneration. The experiments were repeated in four sites to test if seedling emergence was related to differences in site overstory composition and thus to differential seed input. Very few seedlings germinated under the Sasa layer and also few seedlings emerged in microsites where the Sasa above-ground biomass was removed but litter left intact. Only severe disturbance that exposed soil led to a significant increase in the number of seedlings. Smaller-seeded species with longer-distance seed dispersal such as Betula spp. and Abies sachalinensis were generally more abundant than larger-seeded species such as Quercus mongolica and Acer mono. The mechanism by which Sasa cover reduced seedling emergence was not uniform among tree species: emergence of smaller-seeded species - Betula spp, Abies sachalinensis and Picea glehnii - is reduced primarily by low levels of soil exposure, Acer mono emergence appears to be related to the reduced light level under Sasa cover, emergence of Kalopanax pictus and Phellodendron amurense is reduced by high litter production of Sasa, and Quercus mongolica suffered a high level of seed predation. The fact that seedlings of different species responded in different ways to particular treatments shows the importance of the regeneration niche for the maintenance of species diversity. The importance of bamboo understory as an ecological filter that shapes the future canopy composition and structure through differentially influencing tree species establishment is discussed.