Background/Question/Methods Rising concentrations of atmospheric carbon dioxide ([CO2]) are likely to have effects on terrestrial ecosystems via both direct and indirect effects on understory plant communities. Elevated [CO2] can directly affect understory plant communities through stimulation of photosynthesic responses and overall performance of plant species. Elevated [CO2] can indirectly affect understory plant communities via direct effects on forest structure. Here, we 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. From 2001 to 2003, we estimated species-specific, woody vs. herbaceous, and total aboveground biomass by harvesting subplots within the established understory plant community using four 1 m × 0.5 m quadrats in each FACE plot. In 2008, we estimated herbaceous biomass as previously, but used allometric relationships to estimate woody biomass across two 5 m × 5 m quadrats in each FACE plot. We asked if: (1) CO2 enrichment affected total understory biomass; and (2) whether such effects could be explained by effects of CO2 enrichment on understory community composition.

Results/Conclusions After 11 years of CO2 enrichment, understory community aboveground biomass was 25% greater in elevated [CO2] compared to that in ambient [CO2] plots. In 2001-2003, very little of the understory biomass was in woody species; herbaceous species made up 94% of the total understory aboveground biomass in both ambient and elevated [CO2] treatments. Woody species increased in importance, especially in elevated [CO2], and in 2008 the contribution of herbaceous species to total understory aboveground biomass was 61% in ambient [CO2] and 33 % in elevated [CO2]. Plant community composition did not differ between ambient [CO2] and elevated [CO2] treatments respectively. This shift in relative abundance of plant functional types reflects responses of the community to changes in the structure of the forest through time. Specifically, increases in tree sapling abundance caused a decline in biomass of herbaceous species especially under elevated [CO2]. However, species composition did not differ between ambient and elevated [CO2] suggesting that changes in total aboveground biomass were not related to [CO2] effects on performance of particular plant species. Our results suggest that woody sapling establishment and subsequent growth may be faster as atmospheric [CO2] increases, and this could have longer-term repercussions on forest community dynamics.