In tropical forests, the high proportion of trees showing irregularities at the stem base complicates forest monitoring. For example, in the presence of buttresses, the height of the point of measurement (H_POM) of the stem diameter (D_POM) is raised from 1.3 m, the standard breast height, up to a regular part of the stem. While D_POM is the most important predictor for tree aboveground biomass (AGB) estimates, the lack of harmonized H_POM for irregular trees in forest inventory increases the uncertainty in plot-level AGB stock and stock change estimates. In this study, we gathered an original non-destructive three-dimensional (3D) data set collected with terrestrial laser scanning and close range terrestrial photogrammetry tools in three sites in central Africa. For the 228 irregularly shaped stems sampled, we developed a set of taper models to harmonize H_POM by predicting the equivalent diameter at breast height (DBH′) from a D_POM measured at any height. We analyzed the effect of using DBH′ on tree-level and plot-level AGB estimates. To do so, we used destructive AGB data for 140 trees and forest inventory data from eight 1-ha plots in the Republic of Congo. Our results showed that our best simple taper model predicts DBH′ with a relative mean absolute error of 3.7% (R² = 0.98) over a wide D_POM range of 17–249 cm. Based on destructive AGB data, we found that the AGB allometric model calibrated with harmonized H_POM data was more accurate than the conventional local and pantropical models. At the plot level, the comparison of AGB stock estimates with and without H_POM harmonization showed an increasing divergence with the increasing share of irregular stems (up to −15%). The harmonization procedure developed in this study could be implemented as a standard practice for AGB monitoring in tropical forests as no additional forest inventory measurements is required. This would probably lead to important revisions of the AGB stock estimates in regions having a large number of irregular tree stems and increase their carbon sink estimates. The growing use of three-dimensional (3D) data offers new opportunities to extend our approach and further develop general taper models in other tropical regions.

中文翻译：

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一种模拟不规则树茎锥度的 3D 方法：使地块生物量估计值在热带森林中具有可比性

在热带森林中，树干基部不规则的树木比例很高，这使森林监测变得复杂。例如，在存在扶壁的情况下，茎直径 ( D _{POM ) 的测量点 (} H _POM ) 的高度从标准胸高 1.3 m 升高到茎的规则部分。虽然D _POM是树木地上生物量 (AGB) 估计的最重要预测因子，但缺乏统一的H _POM森林清单中不规则树木增加了样地级 AGB 库和库变化估计的不确定性。在这项研究中，我们在非洲中部的三个地点收集了由地面激光扫描和近距离地面摄影测量工具收集的原始无损三维 (3D) 数据集。对于采样的 228 个不规则形状的茎，我们开发了一组锥形模型，通过从D _POM预测等效胸高直径 (DBH')来协调H _POM在任何高度测量。我们分析了使用 DBH' 对树级和地块级 AGB 估计的影响。为此，我们使用了来自刚果共和国 8 个 1 公顷地块的 140 棵树木的破坏性 AGB 数据和森林清单数据。我们的结果表明，我们最好的简单锥度模型在 17-249 cm 的宽D _POM范围内预测 DBH' 的相对平均绝对误差为 3.7% ( R ²  = 0.98) 。基于破坏性的 AGB 数据，我们发现用协调H _POM数据校准的 AGB 异速生长模型比传统的局部和泛热带模型更准确。在地块级别，比较有和没有H _{POM的 AGB 库存估计值}随着不规则茎的比例增加（高达-15％），协调显示出越来越大的差异。本研究中制定的协调程序可以作为热带森林 AGB 监测的标准做法实施，因为不需要额外的森林清单测量。这可能会导致对具有大量不规则树干的地区的 AGB 库估算进行重要修订，并增加其碳汇估算。越来越多地使用三维 (3D) 数据为扩展我们的方法和进一步开发其他热带地区的一般锥度模型提供了新的机会。