Understorey plant community assemblage of Australian Eucalyptus woodlands under elevated CO2 is modulated by water and phosphorus availability

Aims: Given the key functional role of understorey plant communities and the substantial extent of forest cover at the global scale, investigating understorey community responses to elevated CO2 (eCO2) concentrations, and the role of soil resources in these responses, is important for understanding the ecosystem-level consequences of rising CO2 concentrations for forest ecosystems. Here, we evaluated how experimentally manipulating the availabilities of the two most limiting resources in an extremely phosphorus-limited eucalypt woodland in eastern Australia (i.e. water and phosphorus) can modulate the response of the understorey community to eCO2 in terms of germination, phenology, cover, community composition and leaf traits. Methods: We collected soil containing native soil seed bank to grow experimental understorey plant communities under glasshouse conditions. Important Findings: Phosphorus addition increased total plant cover, particularly during the first 4 weeks of growth and under high water conditions, a response driven by the graminoid component of the plant community. However, the treatment differences diminished as the experiment progressed, with all treatments converging at ∼80% plant cover after ∼11 weeks. In contrast, plant cover was not affected by eCO2. Multivariate analyses reflected temporal changes in the composition of plant communities, from pots where bare soil was dominant to high-cover pots dominated by a diverse community. However, both phosphorus addition and the interaction between water availability and CO2 affected the temporal trajectory of the plant community during the experiment. eCO2 also increased community-level specific leaf area, suggesting that functional adaptation of plant communities to eCO2 may precede the onset of compositional responses. Given that the response of our seed bank-derived understorey community to eCO2 developed over time and was mediated by interactions with phosphorus and water availability, our results suggest a limited role of eCO2 in shaping plant communities in water-limited systems, particularly where low soil nutrient availability constrains productivity responses.