The effects of CO2 enrichment on root proliferation of loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styracijlua L.) seedlings were studied under varied water and nitrogen (N) regimes and in competitive interaction. Seedlings of each species were grown from seed as monocultures or as 50:50 pine-sweetgum mixtures in 22-L pots filled with forest soil. Seedlings were exposed to either ambient (400 ppm) or CO2-enriched (ambient plus 400 ppm) air for 32 weeks in continuously stirred tank reactors. Detailed sampling of very fine roots (>0.5 mm diam.) showed a general increase (up to 2-fold) in root length density (RLD, cm m-3 with elevated CO2; however, the effects of CO2 on RLD differed according to species, culture type, water, and N availability. In monoculture, low water with low N conditions produced the largest RLD responses to elevated CO2: 75% increase for sweetgum and 31% increase for pine. In mixed culture, by contrast, the largest RLD responses to CO2 were observed under high water, high N regimes: pine showed a 110% increase and sweetgum a 96% increase. The total RLD of the standing crop in mixture under elevated CO2, high water, and high N was 2.6 cm m-3 compared with 1.6 cm m-3 in ambient CO2, with sweetgum accounting for >75% of the total RLD in both cases. These findings suggest that resource-rich rather than resource-poor soil environments could be th.e circumstances under which belowground interference from sweetgum would intensify in pine-sweetgum mixtures with rising atmospheric CO2.