Abstract: Drought can greatly impact the biodiversity of an ecosystem and play a crucial role in regulating its functioning. However, the specific mechanisms by which drought mediate the biodiversity effect (BE) on community biomass in above- and belowground through functional traits remain poorly understood. Here, we conducted a common garden experiment in a greenhouse, which included two plant species richness levels and two water addition levels, to analyze the effects of biodiversity on aboveground biomass (AGB), belowground biomass (BGB) and total biomass (TB), and to quantify the relationship between BEs and functional traits under drought conditions. Our analysis focused on partitioning BEs into above- and belowground complementarity effect (CE) and selection effect (SE) at the species level, which allowed us to better understand the impacts of biodiversity on community biomass and the underlying mechanisms. Our results showed that plant species richness stimulated AGB, BGB and TB through CEs. Drought decreased AGB, BGB and TB, simultaneously. In addition, the aboveground CE was positively associated with the variation in plant height. SEs in above- and belowground were negatively correlated with the community mean plant height and root length, respectively. Furthermore, drought weakened the aboveground CE by decreasing variation in plant height, resulting in a reduction in AGB and TB. Our findings demonstrate that the complementarity of species is an important regulator of community biomass in above- and belowground, the dynamics of biomass under environmental stress are associated with the response of sensitive compartments.

Key words: biodiversity effect, drought, plant height, root length, aboveground biomass, belowground biomass