Changes in leaf Δ13C of herbarium plant species during the last 3 centuries of CO2 increase

Δ¹³C were determined for herbarium specimens of 12 C₃ plants (trees, shrubs and herbs) collected during the last 240 years in Catalonia, an area with a Mediterranean climate. Values were 19.91 (S.E. = 0.32, n= 21) for 1750–1760, 19–86 (S.E. = 0.21, n= 49) for 1850–1890 and 19.95 (S.E. = 0.29, n= 25) for 1925–1950, and decreased significantly to 18.87 (S.E. = 0.31, n= 29) for 1982–1988. More irregular temporal changes were found in Δ¹³C of two C₄ species, but they also suggest a decrease in discrimination in recent decades. These results suggest that either carbon assimilation rates have increased or stomatal conductance has decreased, and therefore, that there has been an increase in water use efficiency over the last few decades.