Tree species diversity drives above-ground carbon sequestration through light-related trait shifts

Abstract

Functional traits can vary in response to tree species mixing, which in turn might influence biomass production and, consequently, carbon (C) sequestration in diverse forests. However, evidence for consistent broad-scale patterns in tree trait responses, particularly regarding trait identity and their contribution to above-ground biomass outcomes, remains limited. Using data from even-aged forest stands in 11 tree diversity experiments in Europe and Brazil, encompassing 40 tree species, we estimated the influence of species mixing on above-ground biomass components (woody, litterfall and understory biomass), as well as effects of mixing on plasticity-driven changes in species- and community-level functional traits. At the community level, specific leaf area (SLA) and leaf area index (LAI) were higher in mixtures than expected values based on monocultures, while leaf nitrogen per area decreased, and leaf nitrogen per mass remained stable. SLA increases were primarily due to the response of less dominant tree species. Woody and litterfall biomass increased in mixtures, whereas understory biomass remained unchanged. At the species level, diversity-driven plastic changes were observed in multiple traits, but only SLA showed a consistent shift across species. Tree diversity effects on above-ground biomass were influenced by both functional diversity and diversity-driven trait shifts, where increased SLA and LAI enhanced woody biomass accumulation, while higher LAI in diverse stands reduced understory biomass. Together, these results show that tree species mixing alters canopy structure and light-related traits, with shifts in SLA and LAI constituting key pathways through which mixed forests accumulate more woody biomass.