A positive diversity–productivity relationship has been demonstrated in planned experiments, but the evidence from field studies of natural forests is contradictory. We studied aboveground forest productivity in 28 subtropical montane forests on Mt. Fanjingshan and assessed the relationship between productivity and tree diversity, as well as climatic, edaphic, topography, and stand structural factors likely to influence productivity. The main determinants of aboveground net primary productivity (NPP) were climate factors and stand properties. They explained 35% of the variance in NPP. The percentage of the variance in NPP explained by stand properties, climate, soil properties, topography, and species richness were 55%, 30%, 7%, 5%, and 3%, respectively. Topography had no direct effect on production, but they had a significant indirect effect. Elevation had an indirect effect on production via tree height, and slope steepness had a significant indirect effect through climate factors. However, tree diversity had nonsignificant direct and indirect effects on productivity. We concluded that the productivity of highly diverse montane forests is primarily controlled by climate factors and stand structural properties rather than species diversity.