What was done
The authors grew seedlings of five woody leguminous species (Acacia farnesiana, Gleditsia triacanthos, Leucaena leucocephala, Parkinsonia aculeata and Prosopis glandulosa) for close to one month in greenhouses maintained at atmospheric CO2 concentrations of 390 and 700 ppm under well-watered and water-stressed conditions to determine the effects of elevated CO2 and soil moisture on drought survivorship.

What was learned
Seedlings grown under conditions of elevated atmospheric CO2 exhibited greater rates of photosynthesis and more favorable leaf water potentials than plants grown in ambient air.  In addition, elevated CO2 increased seedling biomass from 11 to 43%, except in Leucaena, whose biomass was indifferent to atmospheric CO2 concentration.  Also, the CO2-enriched seedlings reached the 50% drought-survivorship point four days later than seedlings grown in ambient air.  Moreover, the CO2-enriched seedlings survived 11 days longer than control seedlings when subjected to maximum drought conditions.

What it means
As the air's CO2 content increases, the woody species studied will likely respond by exhibiting greater rates of photosynthesis and biomass production.  In addition, they will likely display a greater ability to deal with conditions of water stress and should thus exhibit increased survivorship under drought conditions.  These phenomena could well lead to woody-plant encroachment upon grasslands, fostering their transformation into woodlands.