Dendrochronological studies are limited in tropical regions because not many tree species form annual growth rings. This work reports an evaluation of the dendrochronological potential of tropical ash (Fraxinus uhdei) and its use as a bioindicator of fossil CO2 concentration in urban areas by means of radiocarbon analysis on growth rings. We analyzed a cross-section of a tree that grew during the period 1932–2007 in San Luis Potosí, one of the most industrialized cities in Mexico. The Δ14C values obtained follow the same variation pattern as the calibration curve of the Northern Hemisphere (NH) zone 2 (Hua and Barbetti 2004), with the peak centered in 1964, but they are lower by up to 124′. The high correlation coefficient (r = 0.990, p < 0.001) between the variation patterns indicates that this species does form annual growth rings, and the lower values can be attributed to the 14C dilution caused by fossil CO2 emissions. The magnitude of the Suess effect varied between −6.9% and −0.5%, equivalent to fossil CO2 concentrations ranging between 21.9 and 1.5 ppmv. The Suess effect and fossil CO2 values have significant variations with no apparent monotone increasing trend, suggesting that the CO2 emissions during the studied period have diverse sources. It is concluded that F. uhdei has potential for dendrochronological studies in tropical areas because its growth rings are formed annually and, furthermore, it can be used as a bioindicator of atmospheric 14C variations and fossil CO2 concentration in urban areas.