Effects of forest structure and climate on decomposition processes and decomposer communities

Temperate production forests have experienced a homogenization of forest structure under established management regimes, leading to a loss of biodiversity and changes in ecosystem functions, as well as a decreased resilience to disturbances. Management approaches such as the Enhancement of Structural Beta Complexity (ESBC) aim at reintroducing heterogeneity in production forests by emulating natural disturbances and succession through silvicultural manipulations. By breaking down organic matter and making it available for other organisms, decomposition processes and their associated invertebrate communities are an integral part of nutrient and carbon cycling. Animal derived necromass, such as carcasses and dung, is especially nutrient rich and provides due to its ephemeral nature resources for specialized decomposer communities. Through their tunnelling behaviour common dung beetle and burying beetle species, such as Anoplotrupes stercorosus and Nicrophorus vespilloides, play an important role in forest soil functioning by improving aeration and increasing nutrient input into the soil. Controlled by factors such as microclimate, which are directly influenced by forest stand structure, these processes and communities are likely strongly affected by forest stand homogenisation.

As part of the BETA-FOR research unit this study aims at disentangling the relationship between homogenization and decomposition in temperate production forests with a focus on decomposition rates and decomposer diversity. We have introduced different ESBC treatments by creating deadwood and canopy openings at eleven forests sites in production forests in Germany, each site comprised of nine ESBC-plots and nine control-plots (closed forests). On each plot, decomposition rates of different necromass, such as animal dung and rat carcasses, were determined by exposing the materials to plot conditions for specific lengths of time in spring and in summer 2023. Pitfall traps baited with dung and carcasses were installed directly afterwards to investigate decomposer diversity.

We found no difference in gamma- and beta-diversity of dung beetles and necrophagous beetles between ESBC-forests and control forests. However, Dung removal rates and dung beetle biomass decreased with increasing temperature. Dung beetle abundance and biomass, as well as dung removal rates were lower in summer and in warmer regions, this effect was especially strong on those plots of the ESBC-forests that had open forest canopies. Additionally, alpha diversity of both dung beetles and necrophagous beetles was lower on plots with open forest canopies in all regions.

This demonstrates that some important forest communities might not benefit from increased structural heterogeneity in forest stands and even react negatively to some aspects, such as more openings in the forest canopy. Canopy openings, especially in combination with higher temperatures, negatively impacted dung beetle communities, showing that under future climate warming and changes in forest structure these communities might face increased pressures.