Technical Abstract: Belowground responses to CO2 enrichment remain understudied relative to aboveground parameters. Further, there is a paucity of information on the long-term effects of CO2 on tree species. Sour orange trees (Citrus aurantium L.), grown in an Avondale loam in Phoenix, AZ, were exposed to ambient and elevated (ambient + 300 ppm) levels of atmospheric CO2 for 17 years. At study termination, soil cores were collected to determine how long-term CO2 enrichment affected horizontal (0.5, 1.0, 1.5, and 2.0 m from each tree) and vertical (0 - 105 cm in 15 cm increments) fine root dry weight and length distribution patterns. Overall, elevated CO2 increased both root length (35.6%) and root dry weight (39.1%) densities. There was no effect of CO2 concentration on horizontal root distribution patterns. However, significant CO2 by depth interactions were noted for both root length and root dry weight densities with differences occurring in the top two depths only. Elevated CO2 increased fine root length density by 64.5 and 57.2% at the 0 - 15 and 15 - 30 cm depth increments; respective increases for root dry weight density were 80.3 and 82.8%. These large responses occurred under water and nutrient supply reflective of orchard conditions and suggest that long-term citrus productivity can be enhanced under future levels of atmospheric CO2.