Effects of forest fire on successional trajectory of understory vegetation in boreal permafrost region in Northeast China

Abstract

In boreal permafrost regions, forests are among the primary ecotypes, where fire severity, extent and frequency have been rising due to a warming climate. Following a fire, the composition (species types and diversity), structure (arrangement and layering of trees and undergrowth), and successional trajectory (changes in the ecosystem over time) of forest vegetation are modified. These changes have significant impacts on ecosystem carbon balance, ecosystem services, and forest management, influencing carbon release and storage, biodiversity, hydrological cycles, as well as fire prevention measures and risk assessment. However, at the southern edge of the boreal forest, where permafrost ecosystems are particularly vulnerable to climate change and wildfire disturbances, few studies have focused on post-fire understory vegetation renewal and succession. In this study, we selected six burned areas in Northeast China, including four larch forests and two shrub wetlands, to examine vegetation succession pathways under varying fire severities from 4 to 32 years after wildfires. The results showed that in larch forests, herbaceous vegetation, particularly graminoids, benefited from fire disturbances in the early post-fire period, showing significant increases in coverage, biomass, height, and species diversity. In the mid- post-fire period (four to ten years after the fire), shrubs, especially tall shrubs, began to benefit from forest fires, showing significant increases in cover, biomass and height. Tall shrub species increased during the early post-fire period (first four years), then declined, gradually recovering in the middle and late post-fire period (10 to 32 years). In larch forests, fire severity and post-fire increase in soil temperature and moisture contents constrained shrub growth while promoting herbaceous vegetation. Similarly, in shrub wetlands, shrub coverage and biomass remained higher at burned sites than at unburned ones, while the opposite trend was observed for herbaceous vegetation. In contrast to larch forest, shrub coverage significantly increased in shrub wetlands following a fire. During the period of 17 to 32 years after the fire, the height and species diversity of herbs and shrubs were lower at the burned sites compared to unburned sites, leading to the transformation of herb-dominated wetlands into shrub-dominated ones. Fire severity and permafrost degradation significantly inhibited herb growth while promoting shrub growth in shrub wetlands. These fire-induced effects intensified with rising fire severity, and the legacy effects of forest fires on herbs and shrubs persisted for up to 32 years after the event. Additionally, forest fires increased the species diversity of flowering plants. Therefore, this study provides valuable data to support the restoration and management of vegetation following forest fires in boreal forest regions.