Impact of habitat associations on saproxylic beetle assemblages and their damage severity

Saproxylic beetles, as primary decomposers in forest ecosystems, play a crucial role in the decomposition of dead wood. However, there is a significant gap in understanding the extent of assemblages and damage caused by these insects, which is essential for managing the quality and utilization of dead wood resources in natural forests. This study employed the Bevan damage classification system to quantify the severity of saproxylic beetle damage to fallen trees, focusing on the boreal forest in the Green zone of Ulaanbaatar, the capital of Mongolia. A factorial design was used to assess the influence of forest landscape (north vs. south mixed forest), tree species (Siberian spruce Picea obovata and Siberian pine Pinus sibirica), and decay class (1-4) on beetle damage indices, abundance and feeding guilds (cambium consumers, wood borers, predators, parasitoids, and detritivores). Our findings reveal that decay class significantly affects beetle abundance and damage severity with early stages showing the highest values. Cambium consumers and wood borers were more abundant in decay class 1 (DC1) for downed spruce, with Ips typographus (24.7%) and Tetropium castaneum (15%) causing the most damage. For the Siberian pine, Monochamus galloprovincialis (9.8%) and Judolia sexmaculata (13.3%) were the most damaging in DC1 followed by Pityogenes conjunctus (10%). The results suggest that Siberian spruce may be more susceptible to saproxylic beetle damage than the Siberian pine, with structural features such as bark cover branch size and wood moisture playing a critical role, especially in early decay stages. Based on our findings, we recommend decay-stage-specific management approaches, particularly targeting early decay stages (DC1-DC2) where beetle damage is most severe. Practical strategies include early detection of freshly downed trees, bark removal to reduce suitable habitat for cambium consumers, and on-site processing techniques such as bark gouging or mechanical debarking. These methods allow deadwood biomass to be retained in the forest while reducing pest pressure, offering a viable alternative to salvage logging. Such approaches are especially relevant in protected areas, where they can support both pest control and biodiversity conservation objectives. However, given the geographic scope limited to boreal forests of Ulaanbaatar, caution should be exercised in extrapolating these recommendations to other regions without further study.