Abstract: [Objective]In order to provide a basis for enhancing the stability of plantations and maintaining land fertility sustainable management of plantations, this study explores the suitable thinning intensities for Chinese fir plantations from the perspective of developing understory vegetation and improving community structure.[Method]Three intensities were used for the thinning of Chinese fir plantations in Jiangle State owned Forest Farm of Fujian Province in 2013, namely low (LT, 10%-25%), moderate (MT, 25%-35%),high (HT, 40%-50%), and the control (CK, no thinning).In 2016, all trees in the sample plots were measured and species diversity, coverage, biomass of the understory vegetation and soil properties were in vestigated.A one-way analysis of variance (ANOVA) and Tukey multiple comparisons were used for significance test and Pearson correlation analysis between understory vegetation and soil properties was conducted.[Result]Three years after thinning, there were no community formed for the three thinning intensities CK, LT and MT, with a coverage of 25.52%,52.81%,and 58.98% respectively. While the shrub-fern community formed with a coverage of 100%after a high intensity thinning. In the shrub layer, the richness index, Simpson index, Shannon-Wiener index, coverage and biomass significantly increased with increasing thinning intensity while the evenness index displayed an opposite relation with the thinning intensity. In the herb layer, the richness index, Simpson index, Shannon-Wiener index, evenness index, coverage and biomass all significantly increased with increasing thinning intensity. Three years after thinning, compared with the CK, the organic matter content of the topsoil (0-10 cm) under the treatments of LT, MT, and HT increased by 71.41%, 39.31%,and 98.10%,respectively; the total nitrogen content increased by 82.76%, 51.27%, and 115.87% respectively; the content of available nitrogen increased by 49.47%, 25.59%, and 42.22% respectively; the available phosphorus content increased by 138.29%, 112.23%, and 174.29% respectively; the contents of available potassium increased by -16.25%, -24.25%, and 16.27%, respectively. Thinning intensity had no significant effect on pH, total phosphorus content and soil physical properties of top soil.The coverage and biomass of the shrub layer were positively correlated (P<0.05) with the organic matter, total nitrogen, total phosphorus and available phosphorus contents of topsoil. The coverage of the shrub layer was also positively correlated (P<0.05) with available potassium content. The coverage of herb layer was not strongly correlated with soil physical and chemical properties and the biomass of herb layer was positively correlated (P<0.05) with soil organic matter, total nitrogen, total phosphorus, available nitrogen, and available phosphorus contents of topsoil. There was no significant correlation between coverage and biomass of the understory vegetation with soil pH and physical properties.[Conclusion]The understory vegetation in the un-thinned Chinese fir plantations had low species diversity, coverage and biomass, unable to form a community structure and lack of stability. Thinning can promote the development of understory vegetation, leading to the increases of biodiversity, coverage, and biomass of the understory vegetation, formation of community structure, and improvement of the ability to maintain the soil fertility, and it is particularly evident for the high intensity thinning. For young Chinese fir plantation, a thinning intensity of 40%-50% was proposed to be suitable, and it adjusts the canopy coverage to 0.6-0.7.

Key words: Chinese fir, thinning intensity, understory, soilphysical and chemical properties