Figure 3.

Global trends in leaf gas-exchange capacity and water-use efficiency. Each data point is an individual leaf-level measurement from raw data compiled by Lin et al. (2015). A, Global nonlinear relationship between CO₂ assimilation rate (A) and stomatal conductance to water vapor (g_w) for 276 species with C3 photosynthesis from all major biomes. Note that measurements are for ambient atmospheric CO₂ concentration (±60 µL L⁻¹) and cover a broad range of leaf temperature (5°C–45°C). The fitted line is y = a − bc^x, where a = 40, b = 40, c = 0.19, n = 14,001, r² = 0.77, P = 0. B, Global decline in mean 1 – c_i/c_a, representing water-use efficiency, with increasing mean g_w, for species in A, using measurements above photosynthesis-saturating light (greater than 800 μmol m⁻² s⁻¹ photosynthetically active radiation). The fitted line is y = 0.144x^–0.38, n = 6,784, r² = 0.5, P = 0. C, Mean g₁ values obtained by fitting the BB model (Eq. 1, assuming g₀ = 0 and c_s ≈ c_a, as in Lin et al., 2015) to subsets of the data from Lin et al. (2015), representing C3 plants (as in A) and C4 plants from a temperate semiarid biome. Data sampled are for the relative humidity (H) range 0.1 to 0.9. Fitted lines are as follows: C3 plants, y = 12.7x, n = 12,521, r² = 0.87, P = 0, solid red line; C4 plants, y = 5.23x, n = 599, 27 species, r² = 0.86, P = 0, dashed red line. D, Mean g₁ values obtained as in C for subsets of the C3 plants, showing different biome-specific g₁ values. Fitted lines are as follows: C3 temperate biome, y = 13x, n = 11,010, 74 species, r² = 0.89, P = 0, green line; boreal biome, y = 7.8, n = 492, five species, r² = 0.95, P = 0, blue line.