Phylogenetic diversity and community wide-trait means offer different insights into mechanisms regulating aboveground carbon storage

Both attributes of functional traits and phylogenetic diversity influence ecosystem functions, but which of these factors is most important is still poorly understood in natural systems. Using data from West African forests and tree savannas, we analyse how (i) phylogenetic diversity complements attributes of functional traits in explaining aboveground carbon (AGC); (ii) phylogenetic diversity relates with attributes of functional traits along gradients of phylogenetic signal; and (iii) pathways between phylogenetic diversity and attributes of functional traits relate AGC to soil and climate. Phylogenetic diversity was measured as standardised effect size of Mean Pairwise Distance (sesMPD) and Mean Nearest Taxon Distance (sesMNTD). Functional dispersion (FDis) and community weighted mean (CWM) were calculated for four traits related to leaf economics spectrum and plant life-history. Functional traits-based models explained 11 % of AGC variability. With two out of the four traits being phylogenetically conserved, incorporating phylogenetic diversity in the models increased the explained variance in AGC by 15 %. The slope of phylogenetic diversity-trait relationship was more responsive to trait conservatism for FDis than CWM. AGC was positively influenced by sesMPD and CWM of plant maximum height. In turn, CWM of plant maximum height increased with higher soil nitrogen and climate moisture, whereas sesMPD was negatively related with climate moisture. Although FDis was positively associated with sesMPD, it was not as important as sesMPD and CWM of plant maximum height in influencing and relating AGC to soil nitrogen and climate moisture. Our results suggest that phylogenetic diversity is important for AGC but does not fully reflect the functional mechanisms pertaining to community-wide trait means. The study also demonstrates the role of environment in regulating AGC, which operates through differences in community fitness driven by tall plant stature, and evolutionary processes whereby closely related species are maintained in less arid environments.