It has been hypothesized that elevated levels of atmospheric CO₂ (eCO₂)_, may facilitate the encroachment of woody plants into grasslands by reducing water stress.  The purpose of this study was to test the hypothesis that eCO₂ will increase the establishment success of oak seedlings in an old field environment.  The study was conducted with CO₂ levels controlled by free air CO₂ enrichment (FACE).  In May 1999, four oak (Quercus ellipsoidalis) acorns were planted in each of twenty-four plots in each of six experimental FACE rings (n=576), three of which received elevated levels (550 ppm) of CO₂.  Half the plots in each ring were weeded during the first three summers of the experiment.  In summer 2000, water input was manipulated during a 3.5 week period, during which half the plots received regular watering while the other half received no water.  Summer 2001 was dry, receiving 35% less rainfall than the mean level.  Under hot and dry conditions, eCO₂ increased soil water levels in unweeded plots and enhanced oak establishment (survival and growth) in weeded plots.  In 2006, after the 8^th growing season following planting, survival was five times greater under elevated than ambient CO₂ .  The results showed that under hot and dry conditions, eCO₂ can act like a nurse plant for tree seedlings growing in bare and unshaded areas, increasing seedling survival and growth, and thereby expanding the establishment window for trees encroaching into a grassland environment.