Tree species identity drives soil organic carbon storage more than species mixing in major two-species mixtures (pine, oak, beech) in Europe

13 Pág. Instituto de Ciencias Forestales (ICIFOR)

Mixed forests are usually associated with higher aboveground carbon storage compared to the corresponding monocultures but information on the impact of tree species mixing on soil organic carbon (SOC) is still limited. Yet, maximizing SOC storage is crucial for ecosystem C sequestration and many other ecosystem services. This study used a triplet approach (ie. two-species mixed stand and respective pure stands at the same site) to assess the impact of tree species identity and mixing on SOC storage in eight pine-oak, eight pine-beech and five beech-oak triplets in Europe. We sampled the forest floor (FF) and 0–40 cm in the mineral soil per 10 cm interval. For each triplet type, we fitted basal area (BA) proportion of one component species (for species identity) and a BA-based plot-level True Shannon Diversity index (for species mixing) as explanatory variables for SOC stocks in linear mixed effects models, which included stone content and plot BA as covariates, and site as a random intercept. Considering the total soil depth (FF + 0–40 cm), species identity effect on SOC stocks was only significant for pine-beech and pine-oak triplets but explained more variability in SOC stocks than species mixing across triplet types. Species mixing effect was not significant for any triplet type in the total soil depth. While species identity consistently drove SOC storage in the topsoil layers across triplet types, species mixing explained more variability in SOC stocks in the deeper soil layers except for pine-oak triplets. The results showed that species identity is a stronger driver of SOC storage than species mixing. While tree species identity effect was strongly related to a conifers vs broadleaves signature, the drivers behind mixing effects remained elusive. The results suggest that targeted selection of tree species could better enhance SOC storage in European forests than a mere increase in species richness.

The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM – Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). Some triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). We are grateful to Karine Henin for her key role in the laboratory analyses of the samples. We appreciate the technical assistance by Benjamin Willems for soil sampling across Europe. We would like to thank Thomas Pérot for the dendrometric measurements in the Orléans sites. Finally, we are debtful to the two anonymous reviewers who contributed to improving a previous version of this manuscript.

Peer reviewed