Stand structural diversity rather than species diversity enhancesaboveground carbon storage in secondary subtropical forests in Eastern China

Abstract. Stand structural diversity, typically characterized by variances in tree diameterat breast height(DBH) and total height, plays a critical role in influencing aboveground carbon (C) storage. However, few studies have considered the multivariate relationships of aboveground C storage with stand age, stand structural diversity, and species diversityin natural forests. In this study, aboveground C storage, stand age, tree species, DBH and height diversity indices, were determined across 80 subtropical forest plotsin Eastern China. We employed structural equation modelling (SEM) to test for the direct and indirect effectsof stand structural diversity, species diversity, and stand age on aboveground C storage. The three final SEMs with different directions for the path between species diversityand stand structural diversity had a similar goodness of fit to the data. They accounted for 82 % of the variation in aboveground C storage, 55–59 % of the variation in stand structural diversity, and 0.1 to 9 % of the variation in species diversity. Stand age demonstrated strong positive total effects, including a positive direct effect ( β  =  0.41), and a positive indirect effectvia stand structural diversity ( β  =  0.41) on aboveground C storage. Stand structural diversity had a positive direct effect on aboveground C storage ( β  =  0.56), whereas there was little total effect of species diversityas it had a negative direct association with, but had a positive indirect effect, via stand structural diversity, on aboveground C storage. The negligible total effect of species diversityon aboveground C storage in the forests under study may have been attributable to competitive exclusion with high aboveground biomass, or a historical logging preference for productive species. Our analyses suggested that stand structural diversity was a major determinant for variations in aboveground C storage in the secondary subtropical forests in Eastern China. Hence, maintaining tree DBH and height diversity through silviculturaloperations might constitute an effective approach for enhancing aboveground C storage in these forests.