Background
The authors introduce their most interesting study by stating that "woody vegetation is encroaching into grasslands throughout many regions of the world," and they note that explanations for this phenomenon have included, among other things, "elevated levels of atmospheric CO2 (Idso, 1992; Polley et al., 2003)." Hence, they decided to test the CO2 hypothesis at the Cedar Creek Natural History Area in east-central Minnesota, where they report that "water limitation represents a significant obstacle to the establishment of oaks in the site's old fields."

What was done
In May of 1999, four oak (Quercus ellipsoidalis) acorns were planted in each of 24 plots in each of six experimental FACE rings, three of which were maintained at an elevated atmospheric CO2 concentration of 550 ppm and three of which were maintained at the air's ambient CO2 concentration of 360 ppm. Half of the plots in each ring were weeded during the first three summers of the experiment; and in the summer of 2000, half of the plots received regular watering while the other half received no water during a 3.5-week period.

What was learned
Davis et al. report that "under hot and dry conditions, elevated CO2 increased soil water levels in unweeded plots and enhanced oak establishment (survival and growth) in weeded plots," so that "in 2006, after eight growing seasons, seedling survival was [an amazing] five times higher in the elevated than ambient CO2 plots."

What it means
In the words of the six Minnesota researchers, "the results showed that under hot and dry conditions, elevated CO2 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." In the specific situation investigated in their experiment, for example, they concluded that "increasing atmospheric levels of CO2 may be able to kick-start oak establishment into Cedar Creek's old fields."

References
Idso, S.B. 1992. Shrubland expansion in the American southwest. Climatic Change 22: 85-86.

Polley, H.W., Johnson, H.B. and Tischler, C.R. 2003. Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa. Plant Ecology 164: 85-94.