Despite their diversity, the structure of most tropical rainforests is highly predictable. Scientists have described the various sizes of the trees by a simple mathematical relationship called a power law.In a new study using data from a rainforest in Panama, researchers determined that competition for sunlight is the underlying cause of this common structure, which is observed in rainforests around the globe despite differences in plant species and geography. The new finding can be used in climate simulations to predict how rainforests absorb excess carbon dioxide from the atmosphere.