Abstract: Clarifying the temporal effects of different thinning intensities on understory plant diversity can provide theoretical basis for the sustainable management of P. massoniana plantations. We investigated the understory plant diversity of 20 years old P. massoniana plantations after 1, 5 and 15 years of thinning under four different thinning intensities (control, no thinning; light thinning, 25%; moderate thinning, 45%; heavy thinning, 65%) in Wuxi-angshan National Forest Park, Nanjing, Jiangsu Province. The results showed that: (1) the number of understory plant species after 1, 5, and 15 years of thinning was 36, 42, and 60, respectively. Compared with one year after thinning, the number of plant species was increased by 20 new species after five years of thinning, including five regenerated tree species. Compared with five years after thinning, the number of plant species was increased by 26 new species in the stands after 15 years of thinning, including seven regenerated tree species. (2) Species richness was the highest in heavy thinning treatment after one year of thinning, with Smilax china, Symplocos paniculata, and Rubus corchorifolius as the dominant tree species. After five and 15 years of thinning, species richness of understory plants increased under different thinning intensities, with the highest value in moderate thinning. Meanwhile, Zelkova serrata, Melia azedarach, Ulmus parvifolia and other native tree species gradually appeared. (3) With the increases of duration after thinning, richness and diversity index of shrubs and herbs increased significantly (P<0.05), while the dominance index decreased with years, and the evenness index exhibited no change. (4) After thinning of one year, richness and diversity index of shrubs and herbs were the highest in heavy thinning, while the dominance index and evenness index were the highest in light thinning. After five years of thinning, richness, diversity index, and evenness index of shrubs and herbs were the highest in moderate or heavy thinning, and the dominance index was the highest in the unthinning plot. After 15 years of thinning, most plant diversity indices were the highest under moderate thinning. In conclusion, species richness of understory plants increased after thinning. Thinning had positive effects on understory plant diversity. The richness index and diversity index of shrub and herbs increased significantly in moderate and heavy thinning, which was beneficial to the settlement of native tree species and to the transformation from coniferous plantation to coniferous and broad-leaved mixed forest.

Key words: Pinus massoniana plantation, thinning intensity, understory plant diversity, dynamic effect.