Effects of altitude and competition on growth and mortality of the conifer Abies sachalinensis

Altitudinal gradients are convenient subjects to investigate plant responses to air temperature. Plant growth and mortality are also affected by competition at any altitude. This study investigated the effects of altitude and competition on absolute diameter growth rate (ADGR) and mortality of the conifer Abies sachalinensis by using 13-year data. This study was done at two altitudes (200 and 1,000 m a.s.l.) in northern Japan. Local crowding by conifers and broad-leaved trees reduced ADGR of target trees. ADGR was lower in high altitude than low altitude at any DBH and any degree of local crowding because of the short growing season. Observed size-dependent mortality was a U-shaped pattern against DBH at the two altitudes. Smaller and larger trees tended to die of suppression (standing-dead) and disturbances (stem-broken and uprooting), respectively. Mortality of standing-dead trees was negatively correlated with ADGR, irrespective of altitude, i.e., ADGR was a good predictor of mortality. Thus, mortality of standing-dead trees was estimated to be greater at high altitude than low altitude at any degree of local crowding because ADGR was lower at high altitude than low altitude. By contrast, mortality due to disturbances was slightly greater at low altitude than high altitude. Thus, this study showed that a short growth period decreases growth and increases mortality due to suppression at high altitude. Although global warming may increase growth and survival of individual trees at high altitude by prolonging the growth period, prediction on mortality due to disturbances needs caution because the mortality is largely affected by frequency and intensity of disturbances.