Effects of understory vegetation and litter on the establishment of Abies sachalinensis and Picea jezoensis seedlings in a conifer forest in Hokkaido, northern Japan.

摘要

The effects of understorey vegetation on the establishment of Abies sachalinensis and Picea jezoensis seedlings on the ground were studied at a coniferous forest in Hokkaido, northern Japan. We measured the density of seedlings in four types of understorey vegetation (dwarf bamboo, sedge, fern and sparse vegetation) and on fallen logs. We examined the effects of the understorey vegetation on seedlings by removing above-ground vegetation and litter and by observing the progress of seedling cohorts in each of the four vegetation types for 12 years. The experiment revealed that both the above-ground vegetation and litter acted as a barrier for seedling establishment. The emergence of P. jezoensis seedlings was impaired much more seriously than that of A. sachalinensis seedlings especially by litter on the forest floor, but their emergence was as efficient as that of A. sachalinensis seedlings when we simultaneously removed both the above-ground vegetation and litter. P. jezoensis can therefore regenerate from the ground only after soil disturbance that produces bare ground. The twelve-year observation of seedling cohorts showed that the final outcome of understorey vegetation inhibition on seedling establishment was similar between the dwarf bamboo vegetation type and other vegetation types (sedge and fern). The inhibitory effect was also found to be associated with above-ground biomass and litter thickness, but not with vegetation type. A. sachalinensis seedlings can establish themselves to some degree on the forest floor with <50% vegetation cover, because the seedlings survived for 12 years after emergence in plots with sparse vegetation. Although seedlings of both species were found more frequently on logs than on the ground, "safe sites" for seedling establishment may vary with the abundance of the understorey vegetation, because areas other than logs occupied 85% of the forest floor. The difference in the sensitivity between the two species to the inhibitory effect of vegetation on seedling establishment on the ground thus affects the species composition of seedlings, and consequently may also affect the species composition of canopy trees.