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The effects of the surrounding stand type and forest site productivity on the composition and size of understory trees were examined in middle-aged Cryptomeria japonica plantations. A negative correlation was found between the stem density of hardwood colonized and the site index as an indicator of forest site productivity. The effect of the site index on the colonization of hardwood is greater than that of the measures of current light intensity. The stem density of tall hardwood species with seed dispersal type by birds was positively correlated with the number of surrounding hardwood forests. In addition, the mean heights of all life-forms were positively correlated with the number of surrounding hardwood forests. In conclusion, as forest site productivity decreases and the number of surrounding hardwood forests increases, many hardwoods appear to colonize Cryptomeria japonica plantations.
