Research on Species Diversity of Pinus radiata Plantation under Different Afforestation Models in Dry-hot Valley of the Minjiang River

According to the field experiment, the species diversity of Pinus radiata pure forest, Pinus radiata and Cupressus chengiana mixed forest, Pinus radiata, Cupressus chengiana and Robinia pseudoacacia mixed forest was studied in Wenchuan, Lixian and Maoxian dry-hot valleys of the Minjiang River. The results showed that: (1) Ailanthus altissima was the common accompanying tree species in the tree layer of P. radiata plantation, Lespedeza bicolor was the dominant species in the shrub layer, and Artemisia had great regeneration potential in the herb layer. The species richness was in the order of herb layer > shrub layer > tree layer, and Asteraceae, Gramineae and Leguminosae were the dominant species under the forest. (2) During the time of 5—20 years, with the extension of time, the growth of average tree height and diameter at breast height (DBH) of P. radiata pure forest increased, but the number of shrub and herb species and vegetation coverage decreased gradually. The growth of average tree height and DBH of P. radiata and C. chengiana mixed forest was low, and the number of shrub and herb species under the forest had little change. The growth of average tree height and DBH of P. radiata, C. chengiana and R. pseudoacacia mixed forest increased slowly, but the species diversity and biomass of shrub and herb under the forest showed an upward trend. (3) In the practice of afforestation in the dry-hot valley of the Minjiang River, P. radiata, C. chengiana and R. pseudoacacia mixed forests with suitable density could be used as the main afforestation model. For the P. radiata pure forests to be tending, native tree species such as R. pseudoacacia, A. altissima and L.bicolor were preferred to be replanted after thinning to increase species diversity.