Common species play a selection effect on the biomass at the species level but not at the community

Abstract

The influence of the selection effect and complementarity on the plant diversity-biomass relationship was investigated. We included 662 woody species across 101 savanna sites. At the species level, a normalized rarity continuum was used as the predictor variable, with a normalized above- (NAGB) and below-ground biomass (NBGB) as response variables. At the community level, the predictors were the richness of rare and common species, while the response variables were NAGB and NBGB per site. Generalized Additive Models (GAM) were used to test the relationship between plant diversity and biomass across total species, species categorized by growth forms (shrubs, trees, and shrubs/trees), and savannas vegetation types (open, woody, and woodland). Rarity continuum showed a negative correlation with NABG and BGB, indicating that common species play a selection effect at species level. Therefore, asymmetric competition mediated by common species is a major mechanism for biomass buildup at the species level. However, the richness of rare species showed a positive relationship with NAGB and NBGB. This suggests a positive complementarity effect supported by niche overlap increasing biomass at the community level, favoring rare over common species. The shift of these mechanisms seems to depend on the interaction between growth forms and vegetation types across different scales. These results emphasize the importance of incorporating both species level and community level processes into plant species conservation and management strategies.