Data from: The importance of crown dimensions to improve tropical tree biomass estimates

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 %), 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 severe 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 dataset, crown radius marginally 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发挥着至关重要的作用，但其固碳总量与空间分布格局仍未明确。已有研究表明，在生物量测算中同时纳入树高、胸径与木材密度这三项参数后，多数区域的碳储量估算结果会有所下调。但在部分树高较低的森林中，更大的冠幅或许能抵消生物量估算值的下调幅度——因为即便相对低矮的树木，也能在林冠层内或林冠层上方形成宽大且受光充足的树冠。尽管当前的异速生长模型（allometric model）与相关理论主要围绕胸径、木材密度与树高展开，但冠幅大小与结构的影响尚未得到充分研究。为探究纳入冠层参数对提升生物量估算精度的作用程度，我们在亚马逊西南部未开展过直接生物量测量的区域，对51株胸径介于11至169厘米的树木进行了砍伐称重。本研究涉及的树木，其枝条生物量占总地上生物量的近一半（44±2%），这凸显了纳入树冠参数的重要性。与我们的预测一致，现有主流的泛热带生物量估算方程虽纳入了树高参数，但未考虑冠幅维度，因此对全部51株树木的总生物量低估了11%至14%，这一偏差主要源于对多数大径级树木的严重低估。在我们构建的模型中，纳入冠幅半径参数可显著提升模型性能并降低估算误差，针对大径级树木的效果尤为显著。此外，在全部数据集内，冠幅半径对地上生物量变异的解释度（10.5%）略高于树高（6.0%）。树冠形态同样具有重要影响：具有单轴分枝构型（monopodial architectural type）的树木，其估算生物量较其他生长型树木低21%至44%。我们的分析表明，纳入冠层异速生长参数，可大幅提升热带森林树木生物量及其在原生林与退化林中分布格局的估算精度。