In order to explore the characteristics of vegetation regeneration in the early stage of the burnt land of Yunnan pine forest.

This study focused on the aftermath of the “4·11” forest fire that occurred in Yuxi City, Yunnan Province in 2023. A total of 54 plots measuring 10 m×10 m were established to represent varying levels of fire intensity. Plant surveys and soil analyses were conducted for understory shrub and tree sapling regeneration, and redundancy analysis was employed to explore the relationship between understory plant recovery characteristics and environmental factors.

The results showed that 162 species of plants of 133 genera and 58 families were identified in the study area. Species diversity indices showed an overall trend of decreasing and then increasing with increasing fire intensity. The Margalef richness index (R) and diversity threshold (E) were positively correlated with fire intensity, and the difference was significant (P<0.05). Redundancy analysis showed that TN, fire intensity, forest stand, elevation and slope aspect are the main explanatory factors affecting species diversity. while the importance value of shrub layer was greatly affected by altitude and forest stand. The regeneration coverage of understory vegetation was 67% for unburned areas, 43% for lightly burned areas, and 32% for severely burned areas. The density of tree seedings was in the order of Low fire intensity > High fire intensity > unburned areas, The regeneration density of the uphill slope was higher than that of the downhill slope in the high fire intensity area. There was a significant negative correlation between tree seedings density and understory vegetation regeneration coverage (P<0.05).

Fire increased the diversity of understory vegetation to a certain extent, low fire intensity was most beneficial to the regeneration of P. yunnanensis seedlings. On severely burned sites, the density of seedings and the regeneration coverage are low, with the risk of soil erosion. The results were of great significance for predicting the structural and functional changes of forest ecosystem and scientific management after fire.

The forest soil organic carbon pool is a major component of forest ecosystems, in recent years, due to climate change, the frequency of forest fires has increased, which significantly affects forest ecosystems. This study helps to further understand the effects of different fire intensities on the soil properties of Yunnan pine pure forests in subtropical southwestern China, and provides a scientific basis for the restoration of the vegetation on the burnt sites in the area, and for the management of nutrients in the renewal of the sites.

Yunnan pine forest burnt site in the middle of Yunnan was taken as the research sample site half a year after the fire, and soil samples from different soil layers (0-20 cm and 20-40 cm) were collected in the non-burnt, light, medium and severe burning, and then determined the soil physicochemical indexes, SOC WSOC, ROC, MBC in different soil layers, and analysed the soil properties under different burning intensities. To study the significant differences and trends of soil properties under different fire intensities.

1) Different fire intensities significantly affected some soil physicochemical properties, in which soil bulk density, AK and AN increased with the increase of burning intensity, while SWC, STP and TN decreased with the increase of fire intensity; 2) SOC decreased with the increase of burning intensity, and the three kinds of soil reactive organic carbon (ROC) decreased with the increase of burning intensity, in which the decrease of the soil layer from 0 to 20 cm was more significant; 3) soil organic carbon and its reactive components were significantly positively correlated with AN and TN (P < 0.05), and significantly negatively correlated with AK (P < 0.05).

The burning resulted in a significant decrease in the content of soil active organic carbon components, changing some of the physical and chemical properties of the soil, and the performance was more significant under heavy burning, which can provide a scientific basis for the management of soil nutrients and vegetation restoration of Pinus yunnanensis Forest burning sites in the southwestern subtropical region.