AbstractWe developed an analytical method that quantifies the relative contributions of mortality and individual growth to ecosystem function and analysed the results from the first biodiversity experiment conducted in a tropical tree plantation. In Sardinilla, central Panama, over 5000 tree seedlings were planted in monoculture and mixed‐species plots. After 5 years of growth, mixed‐species plots yielded, on average, 30–58% higher summed tree basal area than did monocultures. Simulation models revealed that the increased yield of mixed‐species plots was due mostly to enhancement of individual tree growth. Although c. 1500 trees died during the experiment, mortality was highly species‐specific and did not differ consistently between biodiversity treatments. Our results show that the effects of biodiversity on growth and mortality are uncoupled and that biodiversity affects total biomass and potentially self‐thinning. The Sardinilla experiment suggests that mixed‐species plantings may be a viable strategy for increasing timber yields and preserving biodiversity in tropical tree plantations.