We at TERN acknowledge the Traditional Owners and Custodians throughout Australia, New Zealand and all nations. We honour their profound connections to land, water, biodiversity and culture and pay our respects to their Elders past, present and emerging.
This work was funded by the Terrestrial Ecosystem Research Network (TERN), an Australian Government National Collaborative Research Infrastructure Strategy (NCRIS) project.
The purpose of this work was to create maps of C3 and C4 abundance in Australia, and a vegetation δ13C isoscape for the continent. Maps of C3 and C4 plant abundance and stable carbon isotope values (δ13C) across terrestrial landscapes are valuable tools in ecology to investigate species distribution and carbon exchange. Australia has a predominance of C4-plants, thus monitoring change in C3:C4 cover and δ13C is essential to national management priorities.
Data were analysed in the R statistical environment (R Core Team 2019). TERN plot data were imported using the ‘ausplotsR’ package, a package which enables the import and analysis TERN plot survey data
The Australian δ13C vegetation isoscape was constructed using data primarily sourced for the year 2015. Climate conditions in 2015 for Australia were considered average and fire occurrence and intensity were relatively low. Thus, a 2015 isoscape should be a good representation of modern average conditions in Australia. The % woody cover layer was designated 100% C3 vegetation. This introduces a potential source of error because some groups of shrubs may use either C3 or C4 photosynthesis. We were unable to identify an accurate way to distinguish and model C4 shrub cover. Consequently, we made the simplifying assumption that all woody cover is C3. In some grid cells the total % C3 cover values exceed 100%. This is because % C3 cover includes both % woody cover and the % C3 herbaceous cover that may be growing beneath the % woody cover, as described in Munroe et al. (2022). Because our approach assumed all woody cover was C3, % C4 cover never exceeded 100%.
C3, C4 and δ13C maps can be used to quantify and compare C3 and C4 distribution at a landscape scale. Isoscapes are useful in the study of food web dynamics and animal migration. These data could also be used to calculate fractional productivity of different photosynthetic pathways.
We used vegetation and land-use rasters to categorize grid-cells (100 m2) into woody (C3), native herbaceous, and herbaceous cropland (C3 and C4) cover. Field surveys and environmental factors were regressed to predict native C4 herbaceous cover. These layers were combined and a δ13C mixing model was used to calculate site-averaged δ13C values.