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 8th 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.
