Identifying the stand properties that support both high biodiversity and carbon storage in German forests

Forest ecosystems face threats related to human-driven degradation, climate change, and biodiversity loss. Addressing these challenges requires management strategies that combine biodiversity conservation with climate change mitigation. Here, we aimed to identify manageable local-scale forest properties that promote biodiversity at multiple trophic levels while also promoting carbon storage and sequestration. We combined data on the diversity of nine taxonomic groups (plants, birds, moths, molluscs, soil fungi, active soil bacteria, cercozoan and endomyxan soil protists, oomycotan soil protists, and nematodes), with above- and belowground carbon storage in 150 temperate forest plots in three regions of Germany. These were dominated by European beech, Scots pine, Norway spruce, and sessile and pedunculate oak. We then investigated the relationships between multiple forest structure and management variables, and multiple biodiversity and carbon storage and sequestration measures. Soil carbon did not respond to deadwood input or any other variable, except in spruce-dominated forests where a higher proportion of other tree species had positive effects on soil carbon storage. Carbon storage in trees was lower in pine- and spruce-dominated stands than in beech stands where it increased with mean tree diameter. Carbon sequestration (i.e. stand uptake) in trees decreased with mean tree diameter. Mean tree diameter was positively related to the biodiversity of multiple taxa, especially taxonomic richness of forest specialist birds; as well as red-listed birds in pine stands. Beech-dominated stands harboured a higher taxonomic richness of many investigated taxa compared to stands dominated by conifers (especially pine). One exception to this was the richness of plant species and forest specialist plants, which were highest in spruce plantations. Deadwood input had limited effects on biodiversity with few exceptions such as bacteria diversity, probably because many deadwood dwelling organisms were not measured in this study. By showing that forests of larger trees with a high proportion of broadleaved trees can promote both biodiversity and carbon storage, our results could help inform sustainable local-scale forest management in Central Europe. These findings can form the basis of further larger-scale studies investigating such relations at larger spatial scales to inform landscape-level recommendations for sustainable multifunctional forest management.