During a recent fieldwork campaign with a logging operation in the southwestern Amazon, we weighed a single shihuahuaco tree (Dipteryx micrantha) at 76.1 tonnes dry mass, making it the most massive tropical tree ever recorded. Yet its diameter (158 centimetres) and its height (44 metres) were unexceptional. Marked variations in the biomass and species identity of large trees have significant implications for carbon accounting and the future of tropical forests.

Forest biomass is usually quantified using equations that include parameters such as trunk diameter, height and wood density. However, for this tree, commonly used indirect methods yielded a wide range of estimates, from 17% to 98% of its true mass. Although the biomass of large trees is notoriously difficult to predict, this sixfold range demonstrates the scale of the problem.

Carbon stocks can vary greatly among different species. Dipteryx trees have dense wood, slow growth and long lives (up to 1,000 years; J. Q. Chambers et al. Nature 391, 135–136; 1998). This is not true of other tree species in the same area. For example, a 115-cm diameter Cavanillesia umbellata weighed a mere 2.3 tonnes, which is 12 times lighter than a second D. micrantha of comparable size.

This enormous difference in the biomass of two trees of similar dimensions demonstrates the urgent need to improve our understanding of large-tree allometry.