Effect of fire severity on understory vegetation in Da Xing’anling Mountains, NE China

As an essential disturbance factor in forest ecosystems, fire disturbance significantly impacts vegetation regeneration and succession. In this paper, the changes in species composition, aboveground biomass, species diversity, and coverage of the understory vegetation were analyzed using a field investigation method in the permafrost region of the Da Xing'anling Mountains, focusing on varying fire severities. The results showed that fourteen years after the fire, 1) the aboveground biomass (AGB) of understory vegetation was the highest in the lightly burned area; the AGB of shrub was the highest in the lightly burned area at 235.65 g·m⁻², followed by the severely burned area 206.32 g·m⁻², and was the least in the unburned area 139.33 g·m⁻². The herbaceous AGB was the highest in the lightly burned area, 42.77 g·m⁻², followed by the unburned area, 23.74 g·m⁻², and was the least in the severely burned area, 8.07 g·m⁻². 2) Forest fire changed the species diversity index of the understory vegetation: the species diversity index of the shrub layer was the highest in the lightly burned area and was significantly different from that of the severely burned area (P < 0.05); the species diversity index of the herb layer was the lowest in the severely burned area. 3) The change characteristics of shrub and herb coverage were opposite: coverage of shrubs was the highest in the severely burned area (84.20%), followed by the lightly burned area (69.20%), and the least was in the unburned area (67.67%); herbaceous coverages was the highest in the unburned area (43.50%), followed by the light burn (38.13%), and was the least in the severely burned area (5.73%). Light burns promoted plant growth somewhat; severe burn damage was serious, and serious fire should be avoided while maintaining species diversity.