GoKawiilMeta-analysis
Search methodology
Suitable studies were identified through a systematic literature search. To create a comprehensive search string, a naive Boolean search string of (‘elevated CO 2 ’ or ‘high CO 2 ’) and ‘C 4 ’ and (‘grass*’ or ‘Poaceae’) was used initially in ISI Web of Science, resulting in 186 results (4 March 2021). The citation record and abstracts of all results were then exported for keyword co-occurrence network analysis using the R package Litsearchr52. This generated the final Boolean search string of (‘C 4 ’ or ‘C-4’) and (‘grass*’) and (‘elevat*’ or ‘increas*’ or ‘enrich*’) and (‘carbon dioxide’ or ‘CO 2 ’) and (‘biomass’ or ‘growth’ or ‘product*’ or ‘respons*’).
The final search terms were used in ISI Web of Science and SCOPUS, resulting in 768 unique results (March 2021). Google Scholar was not used because it does not recognize wildcard components (*) and a search without these components produced too many results (more than 36,000). Several e-theses repositories (White Rose repository, British Library EThOS and South African ETD Collections) were also queried, resulting in two further suitable studies.
Inclusion and exclusion criteria
The studies were screened by title and abstract and were selected if they investigated the responses of wild, C 4 grass species of grass-dominated ecosystems (grasslands and savannas) to elevated [CO 2 ]. Therefore, studies on non-grass, C 3 and domesticated species were excluded, as well as those from other ecosystems (for example, agricultural weeds and wetland species such as Spartina patens). We analysed data using a phylogenetically controlled method (see the ‘Modelling’ section below) so responses needed to be identified to the species level. Therefore, studies that did not isolate responses of particular species (for example, grassland community responses) were excluded.
For inclusion, studies had to include both ambient [CO 2 ] (aCO 2 ) and elevated [CO 2 ] (eCO 2 ) treatments. A range of [CO 2 ] has been used in both treatments, reflecting increasing atmospheric [CO 2 ] during the approximately 40-year timeframe of these studies and changing predictions of near-future [CO 2 ]. Ambient treatments used [CO 2 ] less than or equal to 450 ppm. [eCO 2 ] treatments ranged from 460 to 1,005 ppm. To investigate how C 4 grass responses to eCO 2 interacted with plant water status, studies that also included treatments relating to water availability (in addition to [CO 2 ] treatments) were included.
Studies had to report at least one of the following metrics: aboveground biomass, belowground biomass, total biomass, stomatal conductance measured at the growth [CO 2 ] (G s ) or light-saturated net CO 2 assimilation rates measured at the growth [CO 2 ] (A).
Data extraction
Grass responses at aCO 2 or eCO 2 were extracted either from the main body of the text, tables or Supplementary Information, or manually digitized from figures using WebPlotDigitizer53. When data were not available, we contacted authors directly to request data.
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