Development of White Spruce Tree Islands in the Shrub Zone of the Forest-tundra
Development of White Spruce Tree Islands in the Shrub Zone of the Forest-tundra
复制标题
森林苔原灌丛区白云杉树岛的开发
作者：
P. Scott;R. Hansell
对加拿大曼尼托巴省丘吉尔东部森林苔原灌木带4株年龄在98年至400年以上的白云杉克隆岛的生长进行了研究。在1988年和1989年期间，研究人员监测了三个最年轻的岛屿上各20个相似年龄的茎的伸长情况，同时还监测了地面和空气的温度。在较年轻的岛屿上，茎对温度的日和年变化都表现出更灵活的反应。较年轻的岛屿在延长期间从霜冻事件中恢复得更快，在较冷的年份延长。早春变暖导致在生长季节出现之前发生融雪，导致延长期后期的水分胁迫。在云杉灌木的茎中，枝条集中在接近地面的地方，这是由于生长缓慢，而且由于雪的磨蚀，茎端反复丧失，茎上产生不定芽。在幼树中，遮阳和来自积雪的水分增加与羽毛苔藓的形成和深层活动层相吻合，导致地面温度更高，树木生长更快。正是在这一早期发育时期，树木最有可能发育出直立的茎。在后来的发展中，树的最低的树枝被贴压，长出根，成为二级茎，这个过程从中心茎向外继续。在老树岛上，泥炭堆积和针叶磨损会导致不利于针叶生长和维持的条件。因此，树冠可能变薄，这降低了它捕获雪的能力。当积雪减少时，地衣生长，永久冻土侵入土丘。继发茎在丘上的生长可能太慢，不能成功地发育成直立茎。因此，岛屿的发展在很大程度上取决于地面温度的变化，随着泥炭的积累和霜冻隆起使土丘升高，地面温度会变得更冷。温暖的春夏气候似乎对树岛不利。
The growth of four white spruce ( Picea glauca ) clonal islands ranging in age from ca. 98 years to more than 400 years was investigated in the shrub zone of the forest-tundra east of Churchill, Manitoba, Canada. The elongation of 20 similar-aged stems in each of the three youngest islands was monitored during 1988 and 1989, along with ground and air temperatures. Stems in the younger islands showed a more flexible response to both daily and annual variation in temperature. Younger islands showed faster recovery from frost events during elongation and longer periods of elongation in cooler years. Early spring warming that caused snowmelt to occur before the growing season appeared to result in moisture stress later in the period of elongation. In stems of spruce shrub, the branches are concentrated near ground level because growth is slow and adventitious buds develop on the stem after repeated loss of stem terminals through snow abrasion. In young trees, shading and increased moisture from trapped snow coincide with feather moss establishment and a deep active layer, resulting in higher ground temperatures and faster tree growth. It is during this early period of development that a tree may be best able to develop an erect stem. In later development, the lowest branches of trees become appressed, grow roots, and become second-order stems, and this process continues outward from the central stem. In older tree islands, peat accumulation and needle abrasion can lead to conditions less favourable for growth and maintenance of needles. Consequently, the canopy may thin, which reduces its ability to trap snow. When snow cover is reduced, lichen-heath establishes and permafrost intrudes into the mound. Subsequent growth of the secondary stems on the mound may be too slow to enable successful development of an erect stem. Thus, island development is largely dependent on changing ground temperatures, which become colder as peat accumulates and frost heaving elevates the mound. Warm spring and summer conditions appear to lead to unfavourable conditions for tree islands.