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

The leaf carbon isotope ratio (δ¹³C) of C₃ 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 δ¹³C to aridity similar to within-species responses, and biotic homoeostasis, which predicts offsets in the δ¹³C of species occupying adjacent ranges. The Northeast China Transect spans 130-900mm 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 δ¹³C. The δ¹³C 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 C₃ plants, δ¹³C predicted N per unit leaf area (N_area) better than MI. The δ¹³C of C₄ plants was invariant with MI. SLA declined and N_area increased towards low MI in both C₃ and C₄ 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 C₃ plants.