Abstract. We explore the use of active volcanoes to determine the short- and long-term effects of elevated CO2 on tropical trees. Active volcanoes continuously but variably emit CO2 through diffuse emissions on their flanks, exposing the overlying ecosystems to elevated levels of atmospheric CO2. We found tight correlations (r2=0.86 and r2=0.74) between wood stable carbon isotopic composition and co-located volcanogenic CO2 emissions for two of three investigated species (Oreopanax xalapensis and Buddleja nitida), which documents the long-term photosynthetic incorporation of isotopically heavy volcanogenic carbon into wood biomass. Measurements of leaf fluorescence and chlorophyll concentration suggest that volcanic CO2 also has measurable short-term functional impacts on select species of tropical trees. Our findings indicate significant potential for future studies to utilize ecosystems located on active volcanoes as natural experiments to examine the ecological impacts of elevated atmospheric CO2 in the tropics and elsewhere. Results also point the way toward a possible future utilization of ecosystems exposed to volcanically elevated CO2 to detect changes in deep volcanic degassing by using selected species of trees as sensors.