Department of Biology, Faculty of Science, Shinshu University, Asahi 3-1-1, Matsumoto 390-8621, Japan.
Ann Bot. 2012 May;109(6):1165-74. doi: 10.1093/aob/mcs043. Epub 2012 Mar 25.
Altitudinal timberlines are thought to move upward by global warming, a crucial topic in ecology. Tall tree species (the conifer Abies mariesii and the deciduous broad-leaved Betula ermanii) dominate the sub-alpine zone between 1600 and 2500 m a.s.l., the timberline, on Mount Norikura in central Japan. Dwarf pine Pinus pumila dominates above the timberline to near the summit (3026 m a.s.l.). This study evaluated how the timberline formed on Mount Norikura by examining altitudinal changes in stand structure and dynamics around the timberline.
One hundred and twenty-five plots of 10 m × 10 m were established around the timberline (2350-2600 m a.s.l.). Trunk diameter growth rate during 6 years was examined for A. mariesii, B. ermanii and P. pumila. Mortality during this period and mechanical damage scars on the trunks and branches due to strong wind and snow were examined for A. mariesii only.
The density, maximum trunk height and diameter of A. mariesii in plots decreased with altitude. The maximum trunk height of B. ermanii decreased with altitude, but density and maximum trunk diameter did not decrease. In contrast, the density of P. pumila abruptly increased from around the timberline. A strong negative correlation was found between the densities of P. pumila and tall tree species, indicating their interspecific competition. Trunk diameter growth rates of A. mariesii and B. ermanii did not decrease with altitude, suggesting that these two tall tree species can grow at the timberline. The ratio of trees with mechanical damage scars increased with altitude for A. mariesii, a tendency more conspicuous for larger trees. The mortality of larger A. mariesii was also greater at higher altitude. Tall tree species may not increase their trunk height and survive around the timberline because of mechanical damage.
This study suggests that the altitudinal location of the timberline is mainly affected by mechanical damage due to strong wind and snow rather than by growth limitation due to low temperature. Therefore, the timberline would not move upward even under global warming if these growth and mortality characteristics do not change for a long time.
海拔树线被认为随着全球变暖而上移，这是生态学中的一个关键议题。在日本中部的枪岳，海拔 1600 至 2500 米的亚高山带（即树线），以高大的树种（针叶树冷杉和阔叶树山毛榉）为主。在树线以上到近山顶（海拔 3026 米），主要生长着矮小的松树赤松。本研究通过考察树线周围林分结构和动态的海拔变化，评估了枪岳树线的形成方式。
在树线（海拔 2350-2600 米）周围设置了 125 个 10 米×10 米的样地。对冷杉、山毛榉和赤松的树干直径生长速率进行了 6 年的监测。仅对冷杉进行了同期死亡率的监测，并对树干和树枝上因强风和积雪造成的机械损伤痕迹进行了检查。
样地中冷杉、山毛榉的密度、最大树干高度和直径随海拔升高而降低。山毛榉的最大树干高度随海拔升高而降低，但密度和最大树干直径没有降低。相比之下，赤松的密度则在树线附近急剧增加。赤松和高大树种之间的密度呈强烈的负相关，表明它们存在种间竞争。冷杉和山毛榉的树干直径生长速率随海拔升高而没有降低，表明这两种高大树种可以在树线处生长。冷杉树干有机械损伤痕迹的树木比例随海拔升高而增加，对于较大的树木，这种趋势更为明显。较大的冷杉死亡率也随海拔升高而增加。高大树种可能不会因为机械损伤而增加树干高度并在树线周围存活。
本研究表明，树线的海拔位置主要受强风和积雪造成的机械损伤的影响，而不是低温导致的生长限制。因此，如果这些生长和死亡率特征在很长一段时间内没有变化，即使在全球变暖的情况下，树线也不会上移。
