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

The leaf carbon isotope ratio (δ(13) C) 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 δ(13) C to aridity similar to within-species responses, and biotic homoeostasis, which predicts offsets in the δ(13) C 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 δ(13) C. The δ(13) C 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, δ(13) C predicted N per unit leaf area (Narea) better than MI. The δ(13) C of C4 plants was invariant with MI. SLA declined and Narea 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.