Abstract: The size structure and spatial pattern of seedlings of Castanopsis carlesii Hayata, were studied with the contiguous grid quadrat method. The causes and dynamics of the mortality of different grade of seedlings were analysed. The spatial pattern of each grade of seedlings was tested with"v/m ratio"," Morisita’s dispersion index", and "Lloyd’s patchiness index" etc. The variation of the spatial pattern sampled with different quadrats is discussed.It showed that the seedling bank of C.carlesii(Hemsl.)Hayata population in this area was abundant, with 3.3 seedlings per m^-2, and the size structure was a typical pyramid shape, which indicated a developing population. Each grade of seedlings had respective mortality. From gradeⅠto grade Ⅴ, the curve of mortality had two peaks. Analysis of the seedlings sampled with different quadrats showed an aggregated distribution pattern. The C.carlesii(Hemsl)Hayata seedling population’s aggregated distribution pattern was primarily due to the dispersion character of the seeds.The differences among all size grade of seedlings were due to the interspecific competition.

Key words: Weedy rice, Luolijing (Oryza sativa), Biology, Stress-resistance