Understanding the relationship between biodiversity and ecosystem productivity is a primary goal of contemporary ecology. However, the richest ecosystems, that is, tropical forests, remain underrepresented in global biodiversity-productivity research. We investigate this relationship in the Atlantic Forest, one of the world's most biodiverse and threatened tropical ecosystems. We analysed tree species richness and aboveground biomass data as a proxy for productivity in conjunction with environmental variables, including past and current climatic conditions, changes in forest cover, soil attributes, and topography. We investigated the shape of the richness–productivity relationship and employed spatial path analysis to assess how diversity influences productivity and how environmental factors modulate this relationship. Species richness sets a lower bound on biomass but does not determine it. Path analysis indicated that plant species richness is primarily driven by climatic stability, soil attributes, and forest cover. It also reinforced the limited effect of species richness on biomass, which was primarily driven by energy and water availability. These findings contribute to understanding the mechanisms that sustain productivity in tropical forests and shed light on assembly processes in the Atlantic Forest. These findings have important implications for conservation and management strategies aimed at preserving both biodiversity and ecosystem services in the Atlantic Forest. • The Brazilian Atlantic Forest has the highest level of diversity and productivity. • Examining their association clarifies the nature and limits of this relationship. • Species richness sets a lower bound on biomass but does not determine it. • Abundance-based explanations, independent of diversity, prevail for productivity. • Preserving top-diversity areas sustains high productivity levels, not the reverse.