Species diversity of understory plants and its relationship with soil nutrients in three plantations in the hilly area of central Hainan

【Objective】To investigate the characteristics of understorey vegetation and soil nutrients in plantation forests in the hilly areas of central Hainan，and to reveal the main soil nutrient factors affecting the diversity of understorey vegetation in plantation forests in this area，with a view to providing theoretical basis for biodiversity conservation and scientific management of plantation forest ecosystems in the hilly areas of central Hainan and Hainan Tropical Rainforest National Park.【Method】 Three types of plantation forests（Acacia mangium forest，Eucalyptus robusta forest and Hevea brasiliensis forest）widely distributed in Fengmu Experimental Forestry Farm of Hainan were used as the research objects. Based on the understorey plant surveys and soil nutrient measurements，compared and analyzed the differences in the composition of the understorey vegetation species，the diversity of the plant species，and the content of the soil nutrients among the different forest stands，and revealed the main soil nutrient factors that affected the diversity of the understorey vegetation species through RDA ranking.【Result】The understorey vegetation of tE. robustahe three plantation forests was lush， with E. robusta forest having the highest number of understorey plant species（68），followed by A. mangium forest（60）， and H. brasiliensis forest had the lowest number of understorey plant species（58） . The Margalef index and ShannonWiener index of the shrub layer of each plantation forest were the highest in the A. mangium forest，and were significantly larger than those in the E. robusta forest and H. brasiliensis forest（P<0.05，the same below），Simpson index of A. mangium forest was the highest， and was significantly higher thean H. brasiliensis forest， that of E. robusta was not significantly different from the other two forests （P>0.05， the same below），whereas there was no significant difference in Pielou index among different forest stands， and Margalef index，Shannon-Wiener index，Simpson index and Pielou index were not significantly different between E. robusta forest and H. brasiliensis forest herb layer ，and were significantly higher than that of A. mangium forest. The soil pH of the three plantation forests ranged from 4.58 to 4.75，all of which were acidic. There were significant differences in soil nutrient contents among the three forests except for total nitrogen and available phosphorus，with E. robusta forest having the highest content of organic matter，total phosphorus，available potassium，and alkalihydrolyzed nitrogen，A. mangium forest having the highest content of total potassium，and H. brasiliensis forest having the highest content of total nitrogen and available phosphorus. The results of RDA analysis showed that organic matter and total nitrogen content explained 56.2% and 22.2% of the variation in plant species diversity respectively，and both of them reached the extremely significant level（P<0.01），which were the main soil factors affecting plant species diversity in the understorey of plantation forests in the area.【Conclusion】 The plantation forest understorey in the central hilly area of Hainan is rich in plant species，and there are significant differences in plant composition，plant species diversity and soil nutrients among the three plantation forests. Organic matter and total nitrogen are the main soil nutrient drivers affecting the plant diversity of the plantation forest understorey in this area，and organic and nitrogen fertilizers should be applied in the operation and management of plantation forest understorey in this area.