It has been hypothesized that elevated levels of atmospheric CO 2 (eCO 2 ) may facilitate the encroachment of woody plants into grasslands by reducing water stress. In eastcentral Minnesota, sandy soils frequently create drought conditions for plants, and water limitation inhibits the establishment of oaks into old fields situated on these soils. Some have argued that eCO 2 should slow secondary succession by favoring fast-growing early successional species. However, if oak encroachment into old fields is being inhibited by water stress, then eCO 2 could accelerate old-field succession in this region. The purpose of this study was to test the hypothesis that eCO 2 will increase the establishment success of oak seedlings in an old field environment. The study was conducted with CO 2 levels controlled by free air CO 2 enrichment (FACE). In May 1999, four oak (Quercus ellipsoidalis) acorns were planted in each of 24 plots in each of six experimental FACE rings (n 5 576), three of which received elevated levels (550 ppm) of CO 2 . 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, eCO 2 increased soil water levels in unweeded plots and enhanced oak establishment (survival and growth) in weeded plots. In 2006, after the eighth growing season following planting, survival was five times greater under elevated than ambient CO 2 . The results showed that under hot and dry conditions, eCO 2 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.

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