Abstract: The study showed that there was a difference in the species diversity of understory herbs between thinned and un-thinned stands. All of the diversity indices except evenness had higher values in thinned than in unthinned stands, suggesting that the diversity of herb layer was increased after thinning. According to the changes of herb density and the appearance or not of a species, all of the herb species were classified into three types, i . e . ,positive, neutral, and negative, which referred to a species newly appeared and having an obviously increased density after thinning, with no or no obvious changes in its appearance and density after thinning, and disappeared and having an obviously decreased density after thinning, respectively. Many new species were found in thinned stands, and most species found in both thinned and unthinned stands were negative species. In all 9 sub-plots, there were 11 mutual species found in spring and 10 mutual species in autumm, accounted for 17.74% and 15.15% of all the species in thinned stand, respectively . All of the diversity indices had the highest values in middle sub-plots, and most of them showed a consistent trend,i . e . , reduced from middle to side sub-plots. There was a definite correlation between most of the three types of species and light condition, which was similar in the two stands.

Key words: Pasture-crop rotation, Medicago sativa, Soil nitrogen, Soil phosphorus, Soil organic matter, Rotation pattern