Ecol Appl. 2014 Jun;24(4):680-98. doi: 10.1890/13-0070.1.
Tropical forests play a vital role in the global carbon cycle, but the amount of carbon they contain and its spatial distribution remain uncertain. Recent studies suggest that once tree height is accounted for in biomass calculations, in addition to diameter and wood density, carbon stock estimates are reduced in many areas. However, it is possible that larger crown sizes might offset the reduction in biomass estimates in some forests where tree heights are lower because even comparatively short trees develop large, well-lit crowns in or above the forest canopy. While current allometric models and theory focus on diameter, wood density, and height, the influence of crown size and structure has not been well studied. To test the extent to which accounting for crown parameters can improve biomass estimates, we harvested and weighed 51 trees (11-169 cm diameter) in southwestern Amazonia where no direct biomass measurements have been made. The trees in our study had nearly half of total aboveground biomass in the branches (44% +/- 2% [mean +/- SE]), demonstrating the importance of accounting for tree crowns. Consistent with our predictions, key pantropical equations that include height, but do not account for crown dimensions, underestimated the sum total biomass of all 51 trees by 11% to 14%, primarily due to substantial underestimates of many of the largest trees. In our models, including crown radius greatly improves performance and reduces error, especially for the largest trees. In addition, over the full data set, crown radius explained more variation in aboveground biomass (10.5%) than height (6.0%). Crown form is also important: Trees with a monopodial architectural type are estimated to have 21-44% less mass than trees with other growth patterns. Our analysis suggests that accounting for crown allometry would substantially improve the accuracy of tropical estimates of tree biomass and its distribution in primary and degraded forests.
热带森林在全球碳循环中起着至关重要的作用，但它们所包含的碳量及其空间分布仍不确定。最近的研究表明，一旦在生物量计算中考虑到树高，除了直径和木材密度之外，在许多地区，碳储量估计都会减少。然而，在一些树木高度较低的森林中，较大的树冠大小可能会抵消生物量估计的减少，因为即使是相对较短的树木，在森林冠层内或上方也会形成大而明亮的树冠。虽然当前的异速生长模型和理论侧重于直径、木材密度和高度，但树冠大小和结构的影响尚未得到很好的研究。为了检验考虑树冠参数在多大程度上可以提高生物量估计的准确性，我们在亚马孙西南部的一个没有直接生物量测量的地区，对 51 棵树（直径 11-169 厘米）进行了收割和称重。我们研究中的树木的树冠占地上总生物量的近一半（44% +/- 2%[平均值 +/- SE]），这表明考虑树冠的重要性。与我们的预测一致，包括高度但不考虑树冠尺寸的关键泛热带方程，低估了所有 51 棵树的总生物量 11%到 14%，主要是因为对许多最大的树木的严重低估。在我们的模型中，包括树冠半径极大地提高了性能并减少了误差，特别是对于最大的树木。此外，在整个数据集上，树冠半径比高度（6.0%）更能解释地上生物量的更多变化（10.5%）。树冠形态也很重要：具有单轴结构类型的树木的质量估计比具有其他生长模式的树木少 21-44%。我们的分析表明，考虑树冠的异速生长将大大提高热带地区对树木生物量及其在原始和退化森林中的分布的估计的准确性。
