The relationship between biodiversity and ecosystem functioning (BEF) has emerged as a central issue in ecosystem ecology. Two broad classes of hypotheses, the mass ratio and niche complementarity effects, are proposed to explain the positive relation between biodiversity and ecosystem functioning in multiple experimental and natural systems. Environmental variation is noted as a key regulator of forest community biomass in natural systems and the under-story tree layer is often overlooked, as the canopy tree species are more likely to contribute to total community biomass. We determined community biomass of over-story, mid-story and under-story tree layers and employed environmental factors, functional diversity metrics of leaf traits to explain the variation in community biomass in an old-growth forest. We found that topographic and edaphic factors were of vital importance in regulating the standing biomass of each tree layer. Although functional diversity displayed positive effects on standing biomass across all tree strata, the standing biomass was more affected by functional identity than diversity in over-story and mid-story tree layers. Across all strata, the most significant correlation between functional diversity and standing biomass in under-story tree layer may suggest strengthened interspecific complementary interactions. Our study confirms that environmental variation is a strong driver of forest community biomass, and the relative importance of the mass ratio and the niche complementarity hypothesis vary across tree strata in explaining diversity effects on standing biomass. (C) 2020 The Authors. Published by Elsevier B.V.