Distribution of C3 and C4 plants and changes in plant and soil carbon isotope ratios with altitude in the Kirigamine grassland, Japan

The aim of this study was to clarify how temperature underlines the proportion of C3 and C4 grasses along an alitudinal gradient in the Kirigamine grassland of central Japan. We investigated the altitudinal patterns of C3 and C4 plants in the Kirigamine grassland (1,500 to 1,920 m 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 C4-grass percentage (measured as % of total grass coverage or biomass) decreased and the C3-grass percentage increased with altitude, indicating the existence of a transition between C3 and C4 grasses in the grassland. Seasonal changes in the proportion of C4 and C3 grasses of the C3-C4 transition indicated that there was a shift of productive activities from C3 to C4 grasses with increasing temperature. The significant correlation between percentage of C4 and C3 grasses and temperature-related factors suggests that temperature Is of primary importance in determining the proportions of C4 and C3 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 C3-C4 balance point (C3 and C4 grasses each constituting 50% coverage) in the Kirigamine grassland had a mean August (the warmest month) minimum temperature of 13 deg C, which was in rough agreement with the balance point in other temperate regions but higher than that of tropical regions. This suggests that C4 plants require warmer summer for distribution to higher latitudes. We measured carbon isotope ratios (delta 13C) of vegetation and found that the Values decreased with increasing altitude, and indicating the transition from C4 to C3 grasses with increasing altitude as well. Moreover, we clarify the changes in altitudinal pattern of C3 and C4 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 delta13C value of soil organic carbon at the 1660-m site was -18.2%o, while the present vegetation delta13C value was -18.6%o. suggesting a similar C3/C4 ratio in the past (recorded in soil delta13C value) as at the present at this site. However, at the 1715-m and 1830-m sites, the soil delta13C values were -19.5%o and - 21.2%o. whereas the present vegetation delta13C values were - 22.3%o and - 26.1%o, respectively. These results strongly suggest that a greater proportion of C4 plants occupied the communities at the higher altitudes in the past.