Fire severity and its induced permafrost degradation drive vegetation recovery and succession in the permafrost region in the northern Da Xing’anling Mountains, NE China

Abstract

Under a warming climate, forest fires are becoming more severe and prolonged, leading to permafrost degradation and alterations in soil properties, vegetation composition, and community dynamics. The Da Xing’anling Mountains in Northeast China is the only mid-high latitude permafrost region in China, and a critical region of boreal forests disturbed by forest fires. However, studies on the effects of forest fires on the vegetation of boreal permafrost regions are lacking. This study investigated post-fire soil and vegetation characteristics in larch forests through field surveys and chronosequence approaches. The results revealed that: forest fires decreased organic layer thickness, moss layer thickness (MLT), and soil nutrients, while increased pH, depth of seasonal thawing (DST), and soil temperature. After fires, the dominant shrub species shifted from Vaccinium vitis-idaea and Betula fruticosa to these two and Ledum palustre species; the importance values of Deyeuxia purpurea increased, with a shift in dominant herbs species from Carex bohemica to Deyeuxia purpurea, Carex cespitosa and Carex bohemica. Forest fires promoted the growth of tall shrubs and high growth herbs. As time progressed after fires, the height of herbs and the aboveground biomass of trees gradually recovered, the aboveground biomass and cover of shrubs initially decreased before subsequently increasing. Notably, the changes in aboveground biomass and cover of herbs were exactly the opposite of those observed in shrubs. Eight years after fire, species diversity increased, but decreased with increasing fire severity 14 years after fire. In the early post-fire period, NH₄⁺-N and total nitrogen (TN) were the most important factor affecting the characteristics of understory vegetation; as time progressed after fires, the DST, TN and MLT became the main influencing factor. Overall, vegetation recovered gradually with increasing time post-fire, with light burns facilitating faster recovery than severe burns. This study provides valuable insights for ecological restoration in boreal permafrost region.