Competition and disturbance affect elevational distribution of two congeneric conifers.

Climatic change will affect elevational vegetation distribution because vegetation distribution is related to thermal conditions. However, how elevational species distributions are determined by biotic and abiotic factors is not clear. The long-term plot census along an elevational gradient is indispensable to clarify mechanisms of elevational distribution of tree species. Two congeneric conifers, the less shade-tolerant and shade-tolerant . , dominate at low and high elevations, respectively, in the subalpine zone in Japan. This study investigated the population dynamics of the two species at three elevations (low, middle, high) for 13 years to examine why the two species dominated the different elevations from the viewpoints of competition and disturbance. This study showed that growth and survival rates were not highest at the most dominant elevations for each species. At the high elevation where . dominated and small disturbances frequently occurred, the recruitment rate of . was highest among the three elevations and that of . was largely decreased by tree competition. However, . was dominant earlier than . at the low elevation after large disturbances by the high growth rate of individual trees. Therefore, . was superior to . at the high elevation because of its high recruitment rate and large reduction of recruitment of . due to competition, while . was superior to . at the low elevation after large disturbances because of higher growth rate than . . It is suggested that the elevational distributions of the two species were determined by elevational changes in population dynamics in relation to competition and disturbance. Long-term observational studies of forest dynamics among various elevations are indispensable to predict the effects of climatic change on vegetation distribution.