What was done
In the words of the authors, they describe "how elevated CO2 affects aboveground biomass within the understory community of a temperate deciduous forest at the Oak Ridge National Laboratory sweetgum (Liquidambar styraciflua) free-air carbon dioxide enrichment (FACE) facility in eastern Tennessee, USA," where growing-season (April to November) CO2 treatments -- ambient (aCO2) and enriched (eCO2) -- were initiated in 1998. For the years 2001-2003 and 2008, when total understory aboveground biomass was assessed, the dominant sweetgum trees were approximately 16 meters tall during the first period and 18 meters tall in the second; and for these four years the daytime aCO2 and eCO2 concentrations in the forest understory averaged 474 and 577 ppm, respectively, for an eCO2-aCO2 difference (103 ppm) that was considerably less than that experienced in the sweetgum canopy above (161 ppm).

What was learned
Souza et al. report that "across years, aboveground biomass of the understory community was on average 25% greater in eCO2 than in aCO2 plots." In addition, they found that "in 2001-03, little of the understory biomass was in woody species." In fact, they report that "herbaceous species made up 94% of the total understory biomass across CO2 treatments." In 2008, however, they discovered that the contribution of herbaceous species to total understory biomass had dropped to 61% in the aCO2 plots, and that it had declined all the way to 33% in the eCO2 plots.

What it means
The growth stimulation provided to the understory plants in the sweetgum plantation was truly outstanding: a 25% increase for a mere 103-ppm increase in the CO2 concentration of the air to which they were exposed. Likewise, the rapid shift from a predominately herbaceous understory to one that was fully two-thirds woody in the eCO2 plots a mere five years later was also rather amazing, indicating, in the words of the five researchers, a "more rapid development of the tree regeneration layer and faster successional development of the stand," which they state could "influence long-term forest dynamics." In addition, we note that their results provide support for the theory that atmospheric CO2 enrichment is enabling woody plants to expand their ranges into areas that had previously supported primarily herbaceous plants. For more on this fascinating and important phenomenon, see Range Expansion (Woody Plants) in our Subject Index.