The ratio of litter production to wood production is higher in tropical than in temperate forests. This difference led to the recent hypothesis that there is less competition for light among tropical forest trees than among temperate forest trees. An alternate explanation is proposed here: Lianas account for the greater ratio of litter production to wood production in tropical forests. This follows from the fact that the major structural difference between typical temperate and tropical forests is that the latter have ten times as many liana individuals as do the former. Because of their growth form lianas have intrinsically higher ratios of leaf production to wood production and a high percentage of the leaf biomass of tropical forests is known to come from lianas.