Individual tree size inequality enhances aboveground biomass in homegarden agroforestry systems in the dry zone of Sri Lanka

Individual tree size variation, which is generally quantified by variances in tree diameter at breast height (DBH) and height in isolation or conjunction, plays a central role in ecosystemfunctioning in both controlled and natural environments, including forests. However, none of the studies have been conducted in homegarden agroforestry systems. In this study, aboveground biomass, stand quality, cation exchange capacity (CEC), DBH variation, and species diversity were determined across 45 homegardens in the dry zone of Sri Lanka. We employed structural equation modeling (SEM) to test for the direct and indirect effects of stand quality and CEC, via tree size inequality and species diversity, on aboveground biomass. The SEMaccounted for 26, 8, and 1% of the variation in aboveground biomass, species diversity andDBHvariation, respectively. DBHvariation had the strongest positive direct effect on aboveground biomass (beta= 0.49), followed by the non-significant direct effect of species diversity (beta= 0.17), stand quality (beta= 0.17) and CEC (beta=-0.05). There were non-significant direct effects of CEC and stand quality on DBH variation and species diversity. Stand quality and CEC had also non-significant indirect effects, via DBH variation and species diversity, on aboveground biomass. Our study revealed that aboveground biomass substantially increasedwith individual tree size variation only, which supports the niche complementaritymechanism. However, aboveground biomass was not considerably increased with species diversity, stand quality and soil fertility, which might be attributable to the adaptation of certain productive species to the local site conditions. Stand structure shaped by fewproductive species or independent of species diversity is amain determinant for the variation in aboveground biomass in the studied homegardens. Maintaining stand structure throughmanagement practices could be an effective approach for enhancing aboveground biomass in these dry zone homegarden agroforestry systems. (C) 2016 Elsevier B.V. All rights reserved.