Effects of wind and thermal conditions on timberline formation in central Japan: a lattice model

The upper distribution limit of tall tree species Abies mariesii is the timberline in central Japan, and dwarf pine Pinus pumila dominates above the timberline to near the summit. My previous studies suggested that the main cause of the timberline formation is the increase in mortality due to strong wind in winter rather than low growth due to low summer temperature. This study evaluated how wind velocity affects timberline formation and if the altitude of timberline moves upward due to high thermal conditions, by using a lattice model. Increase in wind velocity throughout the altitude lowered the altitudes of upper distribution limits of the two species. On the contrary, prolonged growth period due to high thermal conditions increased the upper distribution limit of P. pumila, and the upper distribution limit of A. mariesii was hardly affected by the change of growth period. However, the upward shift of the upper distribution limit of P. pumila due to the prolonged growth period in the model would not be realistic because P. pumila had already distributed up to near the summit. This study concludes that A. mariesii is a superior competitor to P. pumila at low altitudes with low wind velocity, but dwarf pine P. pumila can dominate at higher altitudes because A. mariesii suffers severe mechanical damage due to strong wind in winter, and that the altitude of the timberline does not move upward even under high thermal conditions due to global warming.