Fig. 2: Predicted gas-exchange rates and water relations closely match observations for 18 species.

a–c, Pooled data from all 18 species comparing assimilation rate A (a), stomatal conductance g_s (b) and leaf-internal-to-external CO₂ ratio χ (c) for different values of soil (predawn leaf) water potential ψ_s. d, Predicted values of soil-to-leaf water-potential difference Δψ compared to observations for (1) six species for which midday leaf water potentials were reported in the corresponding experiments, and thus measured under the same environmental conditions as the gas-exchange rates (circles), and (2) two species (Pseudotzuga menziesii and Olea europea var. Meski) for which values were obtained from literature ⁶⁷ (triangles). Colours indicate soil water potential relative to the stomatal closure point (ψ_g88) of the species; thus, yellow points represent soil water potentials at or beyond stomatal closure. Black lines show linear regressions, while grey lines are the 1:1 lines that represent perfect predictions. In c, we ignore points with ψ_s < ψ_g88 (yellow points) while calculating the regression line, since there is a known bias in predictions of χ beyond stomatal closure (see Discussion).