One of the main goals in ecology is determining the mechanisms that control the abundance and distribution of organisms. Using data from 69 tropical forests worldwide I demonstrate that liana (woody vine) abundance is correlated negatively with mean annual precipitation and positively with seasonality a pattern precisely the opposite of most other plant types. 1 propose a general mechanistic hypothesis integrating both ecological and ecophysiological approaches to explain this pattern. Specifically the deep root and efficient vascular systems of lianas enable them to suffer less water stress during seasonal droughts while many competitors are dormant giving lianas a competitive advantage during the dry season. Testing this hypothesis in central Panama I found that lianas grew approximately seven times more in height than did trees during the dry season but only twice as much during the wet season. Over time this dry season advantage may allow lianas to increase in abundance in seasonal forests. In aseasonal wet forests however lianas gain no such advantage because competing plants are rarely limited by water I extend this theory to account for the local within-forest increase in liana abundance in response to disturbance as well as the conspicuous decrease in liana abundance at high latitudes.