Nemoral saproxylic insects in a changing environment. The effects of forestry on broadleaf saproxylic insects in Scania, southern Sweden

Abstract

The negative effects of intense forestry on insect biodiversity has long been studied for the Scandinavian coniferous insects. Likewise, southern Sweden is undergoing a decline in old broadleaf forest when it is being logged or replaced with Norway spruce (Picea abies). About 70% of the Swedish red listed insects are found in the nemoral (broadleaf) zone, many of them being dead wood and broadleaf dependent, but still, recent studies are lacking on the effects of forestry from this area. As global temperatures rise, broadleaf trees such as beech (Fagus sylvatica) are expected to expand northward, which could create a biodiversity wasteland where intense forestry replaces beech with spruce plantation. Between April and August 2025, 40 pieces of dead wood gathered from pairs of beech and spruce stands were used to hatch saproxylic insects, comparing abundance in relation to stand, decay class and wood volume as well as species richness. In total, 167 individuals from 26 species emerged, 4 of these emerging from spruce, making the difference in counts large. Most abundant species in beech were deciduous, the most common being a red listed species of cranefly. The almost complete lack of individuals in spruce logs is a sign that many nemoral insects struggle to find suitable habitats in spruce plantations, regardless of decay class or volume. Although, species richness was positively affected by higher decay class in both stands. The properties of the stands are arguably different – one being a young plantation and the other a natural stand native to the area, but investigating the effects of local saproxylic biodiversity is urgent to understand the effects of intensified forestry, especially as many species connected to F. sylvatica are already threatened. I find sharp differences in the species abundance, where spruce plantations in the nemoral zone seem to lack suitable substrate for saproxylic insects.