Forest gaps play an important role in the forest structure change, succession and species diversity. Moreover, the gap size directly affects the forest regeneration. In this study, the gap size and regeneration of Pinus tabulaeformis plantation in the Yanshan Mountain were investigated by the line transects method to analyze the effects of gap size on the regeneration dynamics, and on the density of regeneration seedling and grass diversity. The results showed that there were many medium and small itaps in the near-mature P. tabulaeformis plantation, but rare large gaps. The gaps were basically filled with the regenerated P. tubulaeformit seedlings. In the gaps of 20 - 100 m2 size there was the earliest forest renewal, and the 10 - 15-year-old seedlings accounted for more than 80% of the total regenerated seedlings. The gaps of 20 - 40 m size had obviously higher renewal density than the others, but the height of regenerated saplings was generally shorter than that in the smaller gaps. Species diversity and richness indices of herbaceous showed hump-shaped curves with increasing gap size, and the niaxes of both indices appeared in 60 -80 m gaps.
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