Evidence of a universal scaling relationship for leaf CO2 drawdown along an aridity gradient

P>The leaf carbon isotope ratio (delta 13C) of C-3 plants is inversely related to the drawdown of CO2 concentration during photosynthesis, which increases towards drier environments. We aimed to discriminate between the hypothesis of universal scaling, which predicts between-species responses of delta 13C to aridity similar to within-species responses, and biotic homoeostasis, which predicts offsets in the delta 13C of species occupying adjacent ranges. The Northeast China Transect spans 130-900 mm annual precipitation within a narrow latitude and temperature range. Leaves of 171 species were sampled at 33 sites along the transect (18 at >= 5 sites) for dry matter, carbon (C) and nitrogen (N) content, specific leaf area (SLA) and delta 13C. The delta 13C of species generally followed a common relationship with the climatic moisture index (MI). Offsets between adjacent species were not observed. Trees and forbs diverged slightly at high MI. In C-3 plants, delta 13C predicted N per unit leaf area (N-area) better than MI. The delta 13C of C-4 plants was invariant with MI. SLA declined and N-area increased towards low MI in both C-3 and C-4 plants. The data are consistent with optimal stomatal regulation with respect to atmospheric dryness. They provide evidence for universal scaling of CO2 drawdown with aridity in C-3 plants.