Competitive interactions between established grasses and woody plant seedlings under elevated CO₂ levels are mediated by soil water availability

The expansion of woody plants into grasslands has been observed worldwide and is likely to have widespread ecological consequences. One proposal is that woody plant expansion into grasslands is driven in part by the rise in atmospheric CO₂ concentrations. We have examined the effect of CO₂ concentration on the competitive interactions between established C₄ grasses and woody plant seedlings in a model grassland system. Woody plant seedlings were grown in mesocosms together with established C₄ grasses in three competition treatments (root competition, shoot competition and root + shoot competition) under ambient and elevated CO₂ levels. We found that the growth of the woody plant seedlings was suppressed by competition from grasses, with root and shoot competition having similar competitive effects on growth. In contrast to expectations, woody plant seedling growth was reduced at elevated CO₂ levels compared to that at the ambient CO₂ level across all competition treatments, with the most plausible explanation being reduced light and soil water availability in the elevated CO₂ mesocosms. Reduced light and soil water availability in the elevated CO₂ mesocosms was associated with an increased leaf area index of the grasses which offset the reductions in stomatal conductance and increased rainfall interception. The woody plant seedlings also had reduced ‘escapability’ (stem biomass and stem height) under elevated compared to ambient CO₂ levels. Our results suggest that the expansion of woody plants into grasslands in the future will likely be context-dependent, with the establishment success of woody plant seedlings being strongly coupled to the CO₂ response of competing grasses and to soil water availability.