Comparison of Spatial Distribution Patterns of Seed Rain between Larch Plantations and Adjacent Secondary Forests in Northeast China

The conversion of pure larch plantations into mixed larch-broadleaf forests by promoting the natural regeneration of native broadleaved tree species of secondary forests in larch plantations is a basic strategy in Northeast China. This strategy can solve the problems existing in the larch plantations, such as mono-species composition, decreased soil fertility, and unsustainable productivity. Seed production and dispersal, the beginning of a natural regeneration process that links the end of the reproductive cycle of adult plants and their subsequent life cycle stages, are crucial to successful regeneration. The first objective of this study was to determine the spatial patterns of seed rain for broadleaved tree species produced by the common spatial distribution types of larch plantations at an independent hillside: the contour type (secondary forests and larch plantations are located at the same slope position and aspect) and the downslope type (larch plantations are located at the downslope of the adjacent secondary forests at the same aspect). The second objective was to determine the contributions of seed rain to the soil seed bank for the two common types of seed dispersal (wind- and gravity-dispersed species). We found that the seed densities of these two dispersal types tended to decrease along the gradient from secondary forests to boundary and to larch plantations of the contour type of stands. The seed density of the wind-dispersed species in the downslope type of stands was significantly higher than that in the contour type of stands, particularly in the larch plantation portion of a stand. The similarity in species composition between the seed rain and the seed bank in the downslope type of stands (0.36 +/- 0.06) was significantly higher than that in the contour type of stands (0.14 +/- 0.04). The contribution of the seed rain to the seed bank of the wind dispersed species (13.0-18.6%) was considerably greater than that of the gravity-dispersed species (1.4-1.8%). Our results indicated that the relationships between the seed rain and the seed bank were significantly influenced by both the spatial distribution types of larch plantations and the dispersal types of species. The natural regeneration potential of wind -dispersed species may be more critical in converting larch plantations into mixed forests in the downslope type of stands.