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The purpose of this study was to explore the characteristic of structure, species composition and species diversity in secondary mixed deciduous forest at Thung Salaeng Luang National Park, Lower North of Thailand. This vertical stratification of the tree canopy can affect the growth of young trees on the ground surface, especially that of saplings and seedlings. Ground species can establish themselves very quickly when the light intensity is high enough and especially when the light can penetrate directly to the ground during gap formation. The data of tree individuals have served to give insight into the stand density, basal area, and frequency number of DBH class ranges. There were a large number of DBH class ranges but there were low
DBH values. The species composition of secondary mixed deciduous forest is low rich and diversity but that is high density. The comparison between secondary mixed deciduous forest at Thung Salaeng Luang National Park and other forest is tree density higher than other forest but number of species is similarity or lower. The dominant species of trees were Haldina cordifolia, Albizia odoratissima and Lagerstroemia duperreana. The IVI values of trees were 132.91, 17.78 and 14.22, respectively. Species diversity compared to other forests,
The Fisher's, Shannon-Wiener's index and Simpson's index in these studies are found to be lower than other forests. The size class distribution of trees in secondary mixed deciduous forest is shown to be on L-shape curve, which is high abundance of small trees.
