Variations in the foliar δ13C and C3/C4 species richness in the Japanese flora of Poaceae among climates and habitat types under human activity

Abstract

For 383 Poaceae species harvested over the Japanese islands and stored as herbarium specimens along several decades, we determined C3 and C4 types of photosynthesis from leaf stable carbon isotope ratio (δ13C). Then, we sought the relationships between C4 species richness and climatic factors or habitat types. Except for the two Panicum species (P. lanuginosum and P. scoparium) having the possibility of C3-C4 intermediate, 227 and 154 species were classified into C3 and C4. The C4 species richness increased from northern to southern islands in Japan, positively correlated with mean annual air temperature. Greater C4 species richness in the seashore habitats, and smaller C4 species richness in the shaded, wet and highland habitats would be related to the photosynthetic responses to local environmental factors such as irradiance level and temperature regime. No difference of leaf δ-value of C3 Poaceae was obtained between the habitats with different soil water availability, suggesting the less importance of soil water availability on leaf water-use efficiency in C3 Poaceae species in Japan having humid climate. Additionally, possible effects of human activity around the harvested time or site on leaf δ-value were estimated, because the habitat includes the sites with high human activity. Leaf δ-value was decreased with sampling year, and it was higher in the densely inhabited district for both C3 and C4. They are probably due to a historical decrease in the atmospheric δ-value via increasing human activity, and high gas emission at the districts of high human density. © 2009 The Ecological Society of Japan.