Wildfires provide more diverse habitats than prescribed burns for saproxylic beetles and wood decay fungi in Swedish boreal landscapes

Boreal forests provide important ecosystem services such as climate regulation, wood production and biodiversity. Fire determines forest dynamics and generates habitat for many species but fire frequency and severity are expected to increase as a result of global warming. In Fennoscandian boreal forests forest management and fire suppression have reduced the burnt areas and amount of deadwood since early 1900s, causing habitat loss and subsequent declines of many saproxylic fungi and beetles. Here, prescribed burns are used as a tool to restore habitats of crucial importance for biodiversity and are well-documented to promote fire associated insects in the short term and fungi in the long run. As the first study to compare wildfires and prescribed burns in landscapes with a long history of fire suppression we examine how large wildfires and prescribed burning contribute to biodiversity of saproxylic fungi and beetles after a decade by addressing the question: How does deadwood volume and composition, and species richness, abundance and assemblage composition of wood fungi and beetles differ between a) wildfires and prescribed burns and b) between burned areas and unburned areas. We use a study design with three large wildfires paired with three unburnt controls, and five prescribed burns. We sampled beetles with flight intercept traps and conveyed detailed deadwood and polypore field inventories. We found that the prescribed burns incompletely mimicked wildfire, with prescribed burns only containing half the volumes of deadwood with significantly lower volumes of decidouous wood but significantly more highly decayed deadwood.and only hosting a subset of the wood fungi assemblages. Beetle assemblages recovered largely after 12 years with similar species richness, abundance and assemblage composition, but species associated with old-growth stands were still rare in burned areas. For wood fungi, assemblage composition differed between burned and unburned stands and species associated with decayed wood were not detected in burned areas after 12 years. We show that to meet biodiversity goals with prescribed burning it is important to aim for higher tree mortality and higher deadwood production than was achieved in this study. This can be done by burning larger areas to include more variation, to burn at dryer conditions, and/or to combine burning with the creation of deadwood. We show that wildfires will provide high volumes of deadwood that provide valuable substrates for saproxylic species for many years to come.To facilitate conservation it is thus also important to protect wildfire areas and avoid salvage logging, especially when old forests burn.