Influences of forest fires on the permafrost environment: A review

In boreal and arctic regions, forest fires exert great influences on biogeochemical processes, hydrothermal dynamics of the active layer and near-surface permafrost, and subsequent nutrient cycles. In this article, the studies on impacts of forest fires on the permafrost environment are reviewed. These studies indicate that forest fires could result in an irreversible degradation of permafrost, successions of boreal forests, rapid losses of soil carbon stock, and increased hazardous periglacial landforms. After forest fires, soil temperatures rise; active layer thickens; the release of soil carbon and nitrogen enhances, and; vegetation changes from coniferous forests to broad-leaved forests, shrublands or grasslands. It may take decades or even centuries for the fire-disturbed ecosystems and permafrost environment to return to pre-fire conditions, if ever possible. In boreal forest, the thickness of organic layer has a key influence on changes in permafrost and vegetation. In addition, climate warming, change of vegetation, shortening of fire return intervals, and extent of fire range and increasing of fire severity may all modify the change trajectory of the fire-impacted permafrost environment. However, the observations and research on the relationships and interactive mechanisms among the forest fires, vegetation, carbon cycle and permafrost under a changing climate are still inadequate for a systematic impact evaluation. Using the chronosequence approach of evaluating the temporal changes by measuring changes in the permafrost environment at different stages at various sites (possibly representing varied stages of permafrost degradation and modes), multi-source data assimilation and model predictions and simulations should be integrated with the results from long- and short-term field investigations, geophysical investigations and airborne surveys, laboratory testing and remote sensing. Future studies may enable quantitatively assess and predict the feed-back relationship and influence mechanism among organic layer, permafrost and active layer processes, vegetation and soil carbon under a warming climate at desired spatial and temporal scales. The irreversible changes in the boreal and artic forest ecosystem and their ecological and hydrothermal thresholds, such as those induced by forest fires, should be better and systematically studied.