Elevational changes in productivity of saplings relate to distribution of two congeneric tree species

Elevational changes in vegetation are associated with changes in environmental factors, an example of which is provided by the shade-tolerant Abies mariesii and less shade-tolerant Abies veitchii, which dominate forests at high and low elevations, respectively, in subalpine zones of central Japan. In this study, we sought to establish the factors underlying the differential elevational dominance of these two species from the perspective of sapling growth and survival. It is assumed that the growth and survival of saplings is greater at higher rates of surplus production (the value obtained by subtracting the minimum net production to maintain the current sapling leaf mass from the total net production), as sapling leaf mass gradually declines with time if saplings cannot maintain the current sapling leaf mass, thereby increasing the risk of premature mortality. In this regard, we aimed to verify the following two hypotheses: (1) at low elevations, the surplus production rate of A. veitchii is greater than that of A. mariesii in canopy gaps, and vice versa in the forest understory; and (2) at high elevation, the surplus production rate of A. mariesii is greater than that of A. veitchii in both forest understory and canopy gaps. The results obtained in this study were consistent with our two stated hypotheses. In addition, at the low elevation site, the rate of the growth in height of A. veitchii in canopy gaps was greater than that of A. mariesii, indicating that A. veitchii can dominate after disturbance at low elevations. The findings of this study indicate that the differential elevational distribution of the two Abies species can be attributed to interspecific differences in surplus production rates. We believe that these findings will be useful for predicting changes in the distribution of vegetation in response to climate change.