In natural forests, it is increasingly suggested that stand factors are far more important for community biomass and productivity than biodiversity, but the relative importance of stand factors vs. diversity on ecosystem stability, and how their relative roles change with grain size, still remain unclear. Using inventory data from tropical forest plots in southwestern China from 2004 to 2010, we found that stand factors were clearly more stronger drivers than diversity for forest biomass and productivity (at each grain size from 400 m2 to 0.25 ha), while diversity was predominate for temporal stability of biomass and productivity. The effect of diversity on biomass and productivity increased with increasing grain size, but did not change clearly for ecosystem stability. Functional diversity was more important for ecosystem functions and stability than taxonomic and phylogenetic diversity, and richness was more important than the other two diversity components (evenness and divergence). Our results reconcile the recent debate on the relative importance of diversity vs. stand factors on ecosystem properties, and suggest that forest management to adjust stand structure is an effective way to increase forest carbon storage rapidly, but biodiversity conservation may be crucial for long-term ecosystem stability under climate change.