Tree species identity mediates mechanisms of top soil carbon sequestration in a Norway spruce and European beech mixed forest

Abstract

Key message: Combined effects of litterfall and root turnover significantly increase topsoil carbon stocks in Norway spruce and European beech mixed forests, indicating local complementarity effects mediated by tree species mixtures. Context: The establishment of mixed stands by intermingling individuals of European beech and Norway spruce is an ongoing trend in adaptive forest management strategies. However, our understanding of the potential of these strategies to promote C sequestration remains limited. Aims: This study aims to assess the effect of species composition on SOC stock in a mixed forest of Norway spruce and European beech. Methods: We studied C stocks in the uppermost soil layers in two stands dominated either by Norway spruce or European beech and in a mixture of both species. We evaluated the effect of litterfall and root turnover on soil organic carbon (SOC) stocks and its spatial distribution by combining structural equation models and geostatistical techniques. Results: C stocks in the forest floor were highest in Norway spruce, whereas in the mineral soil, the highest values were in the mixed stand. The proportion of Norway spruce litterfall was positively related to C stock in the forest floor across stands. Root turnover was positively related to C stock in the mineral soil of the mixed stand. Conclusion: Our results confirm a contrasting role of root turnover and litterfall between soil layers in the studied stands, suggesting that tree species composition can mediate the spatial distribution of SOC stocks in mixed forests.