Small birches are of large importance for beetle diversity in burned boreal forests

Sammanfattning

Forest fires create substrates and habitats many species are dependent on in boreal forests. Since forest fires have been mitigated since the 20th century these substrates and habitats have decreased and pyrophilous species are declining. The role of the fire favoured pine (Pinus sylvestris) in burned forests has been frequently covered when it comes to its post-fire connection to beetles, while the role of the similarly fire favoured birch (Betula pendula/pubescens) has not been equally covered. As birch has a generally high regenerative ability in Swedish forests, while it is not a particularly favoured tree species in the industry, the occurrence of birch is dominated by relatively small dimensions in Swedish forests. We sampled beetles after prescribed burnings in boreal forests of Sweden, to compare beetle abundance, diversity and species compositions on birches and pines after fire. We also registered trap tree diameters and local deadwood to compare beetle responses in relation to the amount of burned substrate surface area. The results show higher species richness and a higher number of associated species on burned birches compared to burned pines. Beetle species richness was slightly lower on larger diameters on pines, while no responses differed in relation to diameters on birch. This could possibly be caused by the narrow range of diameters in this study, reflecting commonly occurring birch diameter distributions in Swedish productive forests. Species compositions did not differ with any leverage between diameters regardless of tree species. A positive relationship between local deadwood volume and beetle diversity along with distinct species compositions was found, regardless of trap tree species. Local deadwood richness related to distinct species compositions only on pines. No relationships with beetle abundances were found. These results show that in the context of the common diameter distribution of the tree species in Swedish forests, birches can contribute to an overall higher beetle diversity in fire affected environments, side by side with pine, even when small. A high level of preference-driven attraction could be a possible explanation, as opposed to a larger surface area resulting in higher probabilities of more beetle individuals landing on a substrate. Since larger trees than sampled in this study could show other patterns, as earlier studies have indicated, beetle responses on burned birches of wider diameter ranges is necessary to keep investigating in future studies. Nevertheless, this study shows that smaller birches should not be overlooked when restoring values created by forest fire.