INVESTIGATING POST-FIRE TREE GROWTH AND VEGETATION RECOVERY. A comparison of fire resilience in mixed and coniferous forests

Abstract

Climate change entails risks of more frequent and intense weather events such as droughts, thunderstorms and a prolonged fire season. Forest fires are essential as a natural disturbance that creates variation in distribution of species and ages, as well as reducing competition of nutrients, water and light. Despite the high importance of forest fires, it is in human interest to prevent forest fires in environmental, social and financial aspects. The interests could include maintaining forests as a carbon sink, secure residents and properties in fire areas, as well as protecting forest capital from destruction of forest fires. The risks of uncontrollable destructive forest fires can be reduced by creating more fire resilient forests with the ability to recover from a forest fire, without significant loss. Simultaneously, the desired environmental, social and economic aspects can be fulfilled. This thesis investigates post-fire growth and vegetation recovery for mixed and coniferous forests, aiming to compare their fire resilience. The study is conducted in the nature reserves Nötberget Södra and Ängrabrännan, in Ljusdal municipality, which experienced a significant forest fire in 2018. Tree-ring width (TRW) of pine and the normalized difference vegetation index (NDVI) were analyzed before and after the fire, to assess regrowth and vegetation recovery patterns. Results indicate that the mixed forest of pine, spruce and birch exhibit a higher fire resilience compared to pure coniferous forests. Specifically, mixed forests showed an earlier and more substantial post-fire TRW regrowth and an ability in vegetation recovery while preventing significant loss. The reason for high fire resilience for mixed forests could be explained by the higher diversity that a mixed forests possess, where different species possess varied tolerances for different climate and weather conditions. Some tree species might withstand drought, while another is less ignitable. In that way, the entire forest population is not at risk of collapse in an extreme weather event or another attack. As discussed in this thesis, it is important to enlighten that all mixed forests do not have a high fire resilience. If tree species with different fire sensitive traits are combined, the risk of ignition in a forest could increase. This thesis has contributed with more knowledge about fire resilience in mixed and coniferous forests. There are currently knowledge gaps about specific tree species combinations to optimize the forests’ fire resilience and its productivity. Further research is required to meet future challenges regarding climate change and human interests with environmental, social and financial aspects.