Elevated atmospheric CO2: a nurse plant substitute for

It has been hypothesized that elevated levels of atmospheric CO2 (eCO2) may facilitate the encroachment of woody plants into grasslands by reducing water stress. In east-central 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 eCO2 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 eCO2 could accelerate old-field succession in this region. 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 ellipsoi-dalis) acorns were planted in each of 24 plots in each of six experimental FACE rings (n5 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 plot