Department of Biological Sciences, University of Alaska Anchorage, Anchorage, AK 99508, USA.
Oecologia. 2010 Aug;163(4):833-43. doi: 10.1007/s00442-010-1597-1. Epub 2010 Mar 14.
Two opposing hypotheses have been presented to explain reduced tree growth at the treeline, compared with growth in lower elevation or lower latitude forests: the carbon source and sink limitation hypotheses. The former states that treeline trees have an unfavorable carbon balance and cannot support growth of the magnitude observed at lower elevations or latitudes, while the latter argues that treeline trees have an adequate carbon supply, but that cold temperatures directly limit growth. In this study, we examined the relative importance of source and sink limitation in forest and treeline white spruce (Picea glauca) in three mountain ranges from southern to northern Alaska. We related seasonal changes in needle nonstructural carbohydrate (NSC) content with branch extension growth, an approach we argue is more powerful than using needle NSC concentration. Branch extension growth in the southernmost Chugach Mountains was much greater than in the White Mountains and the Brooks Range. Trees in the Chugach Mountains showed a greater seasonal decline in needle NSC content than trees in the other mountain ranges, and the seasonal change in NSC was correlated with site-level branch growth across mountain ranges. There was no evidence of a consistent difference in branch growth between the forest and treeline sites, which differ in elevation by approximately 100 m. Our results point to a continuum between source and sink limitation of growth, with high-elevation trees in northern and interior Alaska showing greater evidence of sink limitation, and those in southern Alaska showing greater potential for source limitation.
有两个相反的假说被提出，来解释与低海拔或低纬度森林相比，林线树木的生长减缓：碳源和碳汇限制假说。前者表明，林线树木的碳平衡不利，无法支持在低海拔或低纬度观察到的那种规模的生长，而后者则认为，林线树木有足够的碳供应，但低温直接限制了生长。在这项研究中，我们研究了南阿拉斯加三个山脉的森林和林线白云杉（Picea glauca）中源和汇限制的相对重要性。我们将针叶非结构性碳水化合物（NSC）含量的季节性变化与枝条延伸生长联系起来，我们认为这种方法比使用针叶 NSC 浓度更有力。最南端的楚加奇山脉的枝条延伸生长比怀特山脉和布鲁克斯山脉大得多。楚加奇山脉的树木比其他山脉的树木表现出更大的季节性针叶 NSC 含量下降，而 NSC 的季节性变化与整个山脉的枝条生长有关。在森林和林线地点之间，没有证据表明枝条生长存在一致的差异，这些地点的海拔相差约 100 米。我们的结果表明，生长的源和汇限制之间存在一个连续体，阿拉斯加北部和内陆的高海拔树木表现出更大的汇限制证据，而阿拉斯加南部的树木则表现出更大的源限制潜力。
