School of Geography and the Environment, Environmental Change Institute, University of Oxford, South Parks Road, Oxford OX1 3QY, UK.
Philos Trans R Soc Lond B Biol Sci. 2011 Nov 27;366(1582):3225-45. doi: 10.1098/rstb.2011.0062.
The allocation of the net primary productivity (NPP) of an ecosystem between canopy, woody tissue and fine roots is an important descriptor of the functioning of that ecosystem, and an important feature to correctly represent in terrestrial ecosystem models. Here, we collate and analyse a global dataset of NPP allocation in tropical forests, and compare this with the representation of NPP allocation in 13 terrestrial ecosystem models. On average, the data suggest an equal partitioning of allocation between all three main components (mean 34 ± 6% canopy, 39 ± 10% wood, 27 ± 11% fine roots), but there is substantial site-to-site variation in allocation to woody tissue versus allocation to fine roots. Allocation to canopy (leaves, flowers and fruit) shows much less variance. The mean allocation of the ecosystem models is close to the mean of the data, but the spread is much greater, with several models reporting allocation partitioning outside of the spread of the data. Where all main components of NPP cannot be measured, litterfall is a good predictor of overall NPP (r(2) = 0.83 for linear fit forced through origin), stem growth is a moderate predictor and fine root production a poor predictor. Across sites the major component of variation of allocation is a shifting allocation between wood and fine roots, with allocation to the canopy being a relatively invariant component of total NPP. This suggests the dominant allocation trade-off is a 'fine root versus wood' trade-off, as opposed to the expected 'root-shoot' trade-off; such a trade-off has recently been posited on theoretical grounds for old-growth forest stands. We conclude by discussing the systematic biases in estimates of allocation introduced by missing NPP components, including herbivory, large leaf litter and root exudates production. These biases have a moderate effect on overall carbon allocation estimates, but are smaller than the observed range in allocation values across sites.
生态系统净初级生产力（NPP）在冠层、木质组织和细根之间的分配是描述该生态系统功能的一个重要指标，也是正确表示陆地生态系统模型的一个重要特征。在这里，我们整理和分析了一个关于热带森林 NPP 分配的全球数据集，并将其与 13 个陆地生态系统模型中 NPP 分配的表示进行了比较。平均而言，数据表明所有三个主要组成部分的分配相等（平均 34±6%为冠层，39±10%为木质组织，27±11%为细根），但木质组织与细根之间的分配存在很大的站点间差异。对冠层（叶、花和果实）的分配则表现出较小的变化。生态系统模型的平均分配接近数据的平均值，但分布范围更广，有几个模型报告的分配分区超出了数据的分布范围。在无法测量生态系统 NPP 的所有主要组成部分的情况下，凋落物是总 NPP 的一个很好的预测指标（线性拟合通过原点的 r²为 0.83），茎生长是一个中等预测指标，细根生产则是一个较差的预测指标。在各站点之间，分配变化的主要组成部分是木质组织和细根之间的分配转移，而冠层的分配则是总 NPP 的一个相对不变的组成部分。这表明，分配的主要权衡是“细根与木质组织”之间的权衡，而不是预期的“根与茎”之间的权衡；这种权衡最近基于理论基础被提出，用于解释老龄林分的情况。最后，我们讨论了由于缺少 NPP 组成部分（包括食草动物、大的凋落物和根分泌物的产生）而导致的分配估计中的系统偏差。这些偏差对整体碳分配估计有一定影响，但小于各站点之间分配值的观察范围。
