Dynamics Study on Soil and the Characteristics of Understory Vegetation Under Different Restoration Modes in Subtropical Forests

The standing forests with different ages (5, 8, 21, 27, and 40 years) under two restoration modes of subtropical natural restoration (secondary forest) and artificial plantation of Cunninghamia lanceolata were selected as the research objects. By measuring the soil properties and the characteristics of understory vegetation in the soil layer of 0-10 cm, the restoration trend and interaction process of the understory vegetation and the soil properties in different restoration modes were discussed. The results showed that with the development of the stand, the carbon and nitrogen storage in soil of Cunninghamia lanceolata forest and secondary forest did not change significantly, and the soil carbon-nitrogen ratio remained unchanged. Forest type had a significant effect on the soil carbon and nitrogen storage, and the soil carbon and nitrogen storage in the secondary forest was significantly higher than that in Cunninghamia lanceolata forest at 5 and 8 years of stand age. With the development of the stand, the soil phosphorus storage of Cunninghamia lanceolata forest showed a trend of increasing-decreasing-increasing, and reached the lowest at 27 years of Cunninghamia lanceolata mature period, while the carbon-phosphorus ratio and nitrogen-phosphorus ratio reached the highest. However, during the development of the secondary forest, the soil phosphorus storage decreased first and then increased, and reached the lowest at 21 years of stand age. The carbon and nitrogen storage in the surface soil of the 100-year secondary forest was similar to that of the 40-year secondary forest, but the phosphorus storage was at a lower level. The correlation analysis showed that the coverage of shrub layer was significantly positively correlated with the soil carbon and nitrogen storage in Cunninghamia lanceolata forest. At 5 and 8 years of stand development, the coverage and abundance of shrub layer in Cunninghamia lanceolata forest were significantly lower than those in the secondary forest, but as time went on they tended to be the same at 27 years of stand age. The results showed that Cunninghamia lanceolata forest was more conducive to the rapid recovery of above-ground biomass in the short-term recovery, and its management measures increased the abundance of herbaceous layer. However, the coverage and abundance of shrub layer were sacrificed at the early stage of stand development. The natural restoration could form the complex forest communities more quickly. In the culture and management process of Cunninghamia lanceolata forest, the felling of vegetation in the understory shrub layer could be reduced, and maintaining a certain level of understory vegetation was beneficial to the improvement and restoration of the soil fertility conditions.