Distribution of C_3 and C_4 Plants and Changes in Plant and Soil Carbon Isotope Ratios with Altitude in the Kirigamine Grassland, Japan

Mo Wenhong, Nishimura Noboru, Soga Yukiko, Yamada Kyoko, Yoneyama Tadakatsu

doi:10.14941/grass.50.243

The aim of this study was to clarify how temperature underlines the proportion of C_3 and C_4 grasses along an alitudinal gradient in the Kirigamine grassland of central Japan. We investigated the altitudinal patterns of C_3 and C_4 plants in the Kirigamine grassland (1,500 to 1,920m a. s. l.), and analyzed its relationship to temperature, topography and soil carbon and nitrogen contents based on field survey data of 1992 to 1995. The C_4-grass percentage (measured as % of total grass coverage or biomass) decreased and the C_3-grass percentage increased with altitude, indicating the existence of a transition between C_3 and C_4 grasses in the grassland. Seasonal changes in the proportion of C_4 and C_3 grasses of the C_3-C_4 transition indicated that there was a shift of productive activities from C_3 to C_4 grasses with increasing temperature. The significant correlation between percentage of C_4 and C_3 grasses and temperature-related factors suggests that temperature is of primary importance in determining the proportions of C_4 and C_3 grasses along the altitudinal gradient. Precipitation, soil C and N contents, and topographical factors (slope inclination and aspect) showed less influence on the transition in the grassland. On the other hand, the C_3-C_4 balance point (C_3 and C_4 grasses each constituting 50% coverage) in the Kirigamine grassland had a mean August (the warmest month) minimum temperature of 13℃, which was in rough agreement with the balance point in other temperate regions but higher than that of tropical regions. This suggests that C_4 plants require warmer summer for distribution to higher latitudes. We measured carbon isotope ratios (δ^<13>C) of vegetation and found that the values decreased with increasing altitude, and indicating the transition from C_4 to C_3 grasses with increasing altitude as well. Moreover, we clarify the changes in altitudinal pattern of C_3 and C_4 plants in past vegetation of the Kirgamine grassland, based on the changes in carbon isotope ratios of present vegetation and soil organic matter with increasing altitude. The δ^<13>C value of soil organic carbon at the 1660-m site was -18.2‰, while the present vegetation δ^<13>C value was -18.6‰, suggesting a similar C_3/C_4 ratio in the past (recorded in soil δ^<13>C value) as at the present at this site. However, at the 1715-m and 1830-m sites, the soil δ^<13>C values were -19.5‰ and -21.2‰, whereas the present vegetation δ^<13>C values were -22.3‰ and -26.1‰, respectively. These results strongly suggest that a greater proportion of C_4 plants occupied the communities at the higher altitudes in the past.

Distribution of C_3 and C_4 Plants and Changes in Plant and Soil Carbon Isotope Ratios with Altitude in the Kirigamine Grassland, Japan

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