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Abstract

Aim: Within C3 plants, photosynthesis is a balance between CO2 supply from the
atmosphere via stomata and demand by enzymes within chloroplasts. This process is
dynamic and a complex but crucial aspect of photosynthesis. We sought to understand
the spatial pattern in CO2 supply–demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (Δ13C), which provide an integrative
record of CO2 drawdown during photosynthesis.
Location: Global.
Time period: 1951–2011.
Major taxa studied: Vascular plants.
Methods: We assembled a database of leaf carbon isotope ratios containing 3,979
species–site combinations from across the globe, including 3,645 for C3 species. We
examined a wide array of potential climate and soil drivers of variation in Δ13C.
Results: The strongest drivers of carbon isotope discrimination at the global scale
included atmospheric pressure, potential evapotranspiration and soil pH, which explained
44% of the variation in Δ13C. Addition of eight more climate and soil variables
(each explaining small but highly significant amounts of variation) increased the explained
variation to 60%. On top of this, the largest plant trait effect was leaf nitrogen
per area, which explained 11% of Δ13C variation.
Main conclusions: By considering variation in Δ13C at a considerably larger scale than
previously, we were able to identify and quantify key drivers in CO2 supply–demand
balance previously unacknowledged. Of special note is the key role of soil properties,
with greater discrimination on low‐pH and high‐silt soils. Unlike other plant traits,
which show typically wide variation within sets of coexisting species, the global pattern
in carbon stable isotope ratios is much more conservative; there is relatively
narrow variation in time‐integrated CO2 concentrations at the site of carboxylation
among plants in a given soil and climate.