Elevated CO2 concentrations reduce C4 cover and decrease diversity of understorey plant community in a Eucalyptus woodland

Compared to tree responses to elevated (e)CO 2, little attention has been paid to understorey plant community responses in forest ecosystem studies, despite their critical role in nutrient cycling and the regeneration of overstorey species. Here, we present data on understorey responses from a 3-year Free-Air CO 2 Enrichment experiment in a native, phosphorus-limited Eucalyptus woodland in Australia (EucFACE). We conducted repeat surveys of the understorey plant community from 2012 to 2016, recording cover at the species level. Three years of eCO 2 significantly decreased the diversity (Shannon-Weaver; −30%) and species richness (−15%; c. −1 species per 4 m 2 plot) of graminoid species, and the cover of C 4 graminoids in both dominant (−38%) and subordinate (−48%) groups, relative to ambient conditions, leading to a significantly lower graminoid C 4:C 3 ratio (−59%) in the understorey plant community. The ratio of C 4:C 3 graminoids was negatively associated with soil nitrogen (N) availability suggesting that previously reported eCO 2-associated increases in N availability may contribute to (or be a consequence of) shifts in the composition of the graminoid community at the study site. There was, however, no effect of eCO 2 on the diversity of forb species, which represented the most species-rich functional group but only c. 1% of the understorey biomass. Synthesis. Our results suggest that eCO 2 influences competition between C 4 and C 3 graminoid species both directly and indirectly via increasing N availability. The shift towards lower C 4:C 3 ratios and enhanced dominance by C 3 species with their generally higher tissue N concentrations could further change soil nutrient availability and potentially accelerate community succession. Thus, eCO 2 has altered the diversity and composition of the understorey plant community in this woodland, with the potential for cascading consequences for trophic interactions and ecosystem function.