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

• Published in: The Journal of ecology Vol. 106; no. 4; pp. 1483 - 1494
• Main Authors: Hasegawa, Shun, Piñeiro, Juan, Ochoa-Hueso, Raúl, Haigh, Anthony M., Rymer, Paul D., Barnett, Kirk L., Power, Sally A.
• Format: Journal Article
• Language: English
• Published: Oxford John Wiley & Sons Ltd 01.07.2018; Blackwell Publishing Ltd
• Subjects: Availability; Biodiversity; Carbon dioxide; Cascading; Composition; Cynodon dactylon; Ecological succession; Ecosystem studies; Environmental changes; Eucalyptus; Eucalyptus woodland; Forest ecosystems; Free‐Air CO2 Enrichment; Functional groups; herbaceous vegetation; Herbivores; Interactions; Microlaena stipoides; Mineral nutrients; Nitrogen; Nutrient availability; Nutrient concentrations; Nutrient cycles; Nutrients; Phosphorus; Plant communities; Plant diversity; Plant-climate interactions; Ratios; Regeneration; Regeneration (biological); Soil; Species diversity; Species richness; Surveys; Terrestrial ecosystems; Tissue; Trophic relationships; understorey dynamics
• ISSN: 0022-0477; 1365-2745
• DOI: 10.1111/1365-2745.12943

1. Compared to tree responses to elevated (e)CO₂, 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 CO₂ Enrichment experiment in a native, phosphorus-limited Eucalyptus woodland in Australia (EucFACE). 2. We conducted repeat surveys of the understorey plant community from 2012 to 2016, recording cover at the species level. 3. Three years of eCO₂ significantly decreased the diversity (Shannon-Weaver; -30%) and species richness (-15%; c. -1 species per 4 m² plot) of graminoid speies, and the cover of C₄ graminoids in both dominant (-38%) and subordinate (-48%) groups, relative to ambient conditions, leading to a significantly lower graminoid C₄:C₃ ratio (-59%) in the understorey plant community. The ratio of C₄:C₃ graminoids was negatively associated with soil nitrogen (N) availability suggesting that previously reported eCO₂-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 eCO₂ on the diversity of forb species, which represented the most species-rich functional group but only c. 1% of the understorey biomass. 4. Synthesis. Our results suggest that eCO₂ influences competition between C₄ and C₃ graminoid species both directly and indirectly via increasing N availability. The shift towards lower C₄:C₃ ratios and enhanced dominance by C₃ species with their generally higher tissue N concentrations could further change soil nutrient availability and potentially accelerate community succession. Thus, eCO₂ 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.