Most tropical plants rely on animal seed dispersers to colonize new environments they will face as the climate changes. In a warming world, lowland tropical biota rely on elevational migrations to locate habitats that fit their ecological and physical requirements. The objectives of our study were to determine if upslope biotic or abiotic barriers currently exist to prevent plant range expansions along two tropical mountains: (1) Volcán San Martín Tuxtla (=Los Tuxtlas) in Mexico and (2) Volcán Barva in Costa Rica. We investigated if pre- and post-dispersal barriers exist to delimit the ranges of low-elevation plants. Using upslope transplant experiments of seedlings at multiple elevations, we assessed seedling survival in native (lowland) and novel (upslope) habitats on both mountains. In Los Tuxtlas, the three focal plant species showed different levels of success in colonizing upslope habitats. In Barva, all nine transplanted species suffered complete mortality at high elevations (2000 m a.s.l.) but had highest survival at middle elevations (1000 m a.s.l.). In Barva, we supplemented field experiments with laboratory work, using temperature-controlled incubators to assess seed resilience to novel temperature environments. We tracked seed germination success for 12 large-seeded lowland species and found that, overall, lowland species germinated well, even at an artificially high temperatures they do not currently experience (35° C). For both mountain ecosystems, some plants do not face post-dispersal barriers to upslope migration and may be able to colonize novel habitats to outpace global warming. However other species face strong pre-existing biotic barriers to survival at high elevations. This allows for potential upslope mobility, if animal seed dispersers deposit seeds as they move up mountains. The results of this study have implications for understanding and predicting plant ranges as mediated by seed dispersers in a changing world. If plants are unable to colonize novel highland habitats as biomes shift upward, these barriers will prevent successful elevational migration, resulting in lowland biotic attrition. Altered plant-animal interactions, especially seed dispersal and predation, will play major roles in redefining tropical plant and animal communities as the world warms.