Responses of photosynthetic rates to temperature in two conifers dominating at different elevations

The elevational changes in vegetation are related to the changes in environmental factors, such as temperature. Evergreen conifers Abies veitchii and A. mariesii dominate the subalpine zone between 1600 and 2000 m and between 2000 and 2500 m above sea level (a.s.l.), respectively, in central Japan. This study examined why their dominating elevations are separated by investigating the temperature–photosynthesis relationship at high and low elevations (1600 m and 2300 m a.s.l.). This study tested two hypotheses: (1) the optimal temperature (T) for the maximum photosynthetic rate (P) is lower in individuals grown at high elevation than at low elevation, and A. mariesii has lower T than A. veitchii at each elevation; and (2) P of individuals grown at low elevation is greater in A. veitchii than in A. mariesii, while that of individuals grown at high elevation is greater in A. mariesii than in A. veitchii. Saplings of the two conifers were sampled at high and low elevations. The photosynthetic rate was measured at the laboratory. Contrary to the hypotheses, both species had the same T at two elevations, and P was greater in A. veitchii than in A. mariesii, irrespective of the elevation where the individuals grew. The elevational distribution of the two species could not be explained by the temperature–photosynthesis relationship. Therefore, factors other than the temperature–photosynthesis relationship are important to understand the elevational separation of the two species.