What was done
Using free-air CO2 enrichment (FACE) technology, the authors studied the effect of a modest 130-ppm increase in atmospheric CO2 concentration on surface soil moisture content (to a depth of 7 cm) beneath a mixture of initially-two-year-old deciduous tree seedlings native to northern Japan (Betula platyphylla, B. maximowicziana, Quercus mongolica, Ulmus davidiana, Fagus crenata and Acer mono), where they grew for a period of two years on either brown forest soil or volcanic ash soil, both of which soils are typical of that region.

What was learned
As best we can determine from the graphical presentation of Eguchi et al.'s data, during the months of June, July, August and September of 2003 and 2004, the average surface soil moisture content beneath the CO2-enriched trees was approximately 28% greater than it was beneath the ambient-treatment trees.

What it means
What is particularly interesting about this result is what caused it. The Japanese researchers report that "transpiration per leaf area decreased under elevated CO2 in both soil conditions," as one might have expected would be the case; but they note that "total [our italics] transpiration per seedling increased [our italics] under elevated CO2," which they "attributed to the increase in total leaf area per seedling." Concomitantly, however, they found that evaporative water loss from the forest floor "decreased significantly under elevated CO2," which they attributed to "the reduction in soil surface temperature owing to an increase in leaf area index that blocked direct sunlight at the stand floor." This phenomenon apparently predominated over the former one, with the end result being the "same as that found for grassland (Obrist et al., 2003)," i.e., an increase in surface soil moisture content beneath the stand that produced the greater leaf area.

Reference
Obrist, D., Verburg, P.S.J., Young, M.H., Coleman, J.S., Schorran, D.E. and Arnone, J.A. 2003. Quantifying the effects of phenology on ecosystem evapotranspiration in planted grassland mesocosms using EcoCell technology. Agricultural and Forest Meteorology 118: 173-183.