Treefall gaps are hypothesized to maintain diversity by creating resource-rich heterogeneous habitats necessary for species coexistence. This hypothesis however is not supported empirically for shade-tolerant trees the dominant plant group in tropical forests. The failure of gaps to maintain shade-tolerant trees remains puzzling and the hypothesis implicated to date is dispersal limitation. In central Panama we tested an alternative O^biotic interferenceO. hypothesis: that competition between growth forms (lianas vs. trees) constrains shade-tolerant tree recruitment survival and diversity in gaps. We experimentally removed lianas from eight gaps and monitored them for 8 years while also monitoring nine un-manipulated control gaps. Removing lianas increased tree growth recruitment and richness by 55 46 and 65% respectively. Lianas were particularly harmful to shade-tolerant species but not pioneers. Our findings demonstrate that competition between plant growth forms constrains diversity in a species-rich tropical forest. Because lianas are abundant in many tropical systems our findings may apply broadly.