Individual tree carbon stock estimates typically rely on allometric scaling relationships established between field-measured stem diameter (DBH) and destructively harvested biomass. The use of DBH-based allometric equations to estimate the carbon stored over larger areas therefore, assumes that tree architecture, including branching and crown structures, are consistent for a given DBH, and that minor variations cancel out at the plot scale. We aimed to explore the degree of structural variation present at the individual tree level across a range of size-classes. We used terrestrial laser scanning (TLS) to measure the 3D structure of each tree in a 1 ha savanna plot, with coincident field-inventory. We found that stem reconstructions from TLS captured both the spatial distribution pattern and the DBH of individual trees with high confidence when compared with manual measurements (R${}^2$ = 0.98, RMSE = 0.0102 m). Our exploration of the relationship between DBH, crown size and tree height revealed significant variability in savanna tree crown structure (measured as crown area). These findings question the reliability of DBH-based allometric equations for adequately representing diversity in tree architecture, and therefore carbon storage, in tropical savannas. However, adoption of TLS outside environmental research has been slow due to considerable capital cost and monitoring programs often continue to rely on sub-plot monitoring and traditional allometric equations. A central aspect of our study explores the utility of a lower-cost TLS system not generally used for vegetation surveys. We discuss the potential benefits of alternative TLS-based approaches, such as explicit modelling of tree structure or voxel-based analyses, to capture the diverse 3D structures of savanna trees. Our research highlights structural heterogeneity as a source of uncertainty in savanna tree carbon estimates and demonstrates the potential for greater inclusion of cost-effective TLS technology in national monitoring programs.

中文翻译：

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用陆地激光扫描探索热带稀树草原结构异形的变异性

单个树木的碳储量估计值通常取决于在实地测得的茎径（DBH）与破坏性收获的生物量之间建立的异速生长比例关系。因此，使用基于DBH的异速方程估算在较大区域上存储的碳，假设对于给定的DBH，包括分支和树冠结构在内的树木结构是一致的，并且小幅变化会在样地比例上抵消。我们的目标是探索各种尺寸类别下单个树级别上存在的结构变异程度。我们使用陆地激光扫描（TLS）来测量1公顷热带稀树草原图上每棵树的3D结构，并具有一致的现场清单。${}^2$= 0.98，RMSE = 0.0102 m）。我们对DBH，树冠大小和树高之间关系的探索表明，稀树草原树冠结构（以树冠面积衡量）存在显着变化。这些发现质疑基于DBH的异速方程的可靠性，以充分表示树木的多样性以及热带稀树草原的碳储量。但是，由于可观的资金成本，在环境研究之外采用TLS的速度一直很慢，并且监视程序经常继续依赖子图监视和传统的等速方程。我们研究的一个主要方面是探索通常不用于植被调查的低成本TLS系统的实用性。我们讨论了基于TLS的替代方法的潜在优势，例如树结构的显式建模或基于体素的分析，捕获稀树草原的各种3D结构。我们的研究突出表明结构异质性是热带稀树大树碳估算中不确定性的来源，并表明了在国家监测计划中进一步包含具有成本效益的TLS技术的潜力。