Nature2014-06-24 5:58 PM

老树的重要性 Rate of tree carbon accumulation increases continuously with tree size

论文摘要 

老龄林和它们的叶子比新森林固氮量小,但这个规律是否适用于单个树木?这项研究利用对超过400个树种所做的一项全球性分析显示,它并不适用。相反,较大和较老的树要比较年轻和较小的树更快地积累碳。这一点可以通过将随着年龄增长叶子数量增加和森林密度降低考虑进去之后而在其他层面上所产生的效应得到解释。大树的快速生长意味着,相对于它们的数量来说,它们在森林对陆地碳循环和全球气候系统的反馈中可能会发挥一个与其数量不相称的重要作用。

Abstract 

Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest carbon cycle—particularly net primary productivity and carbon storage—increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree’s total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.

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