The mass of carbon contained in trees is governed by the volume and density of their wood. This represents a challenge to most remote sensing technologies, which typically detect surface structure and parameters related to wood volume but not to its density. Since wood density is largely determined by taxonomic identity this challenge is greatest in tropical forests where there are tens of thousands of tree species. Here, using pan-tropical literature and new analyses in Amazonia with plots with reliable identifications we assess the impact that species-related variation in wood density has on biomass estimates of mature tropical forests. We find impacts of species on forest biomass due to wood density at all scales from the individual tree up to the whole biome: variation in tree species composition regulates how much carbon forests can store. Even local differences in composition can cause variation in forest biomass and carbon density of 20% between subtly different local forest types, while additional large-scale floristic variation leads to variation in mean wood density of 10–30% across Amazonia and the tropics. Further, because species composition varies at all scales and even vertically within a stand, our analysis shows that bias and uncertainty always result if individual identity is ignored. Since sufficient inventory-based evidence based on botanical identification now exists to show that species composition matters biome-wide for biomass, we here assemble and provide mean basal-area-weighted wood density values for different forests across the lowand tropical biome. These range widely, from 0.467 to 0.728 g cm−3 with a pan-tropical mean of 0.619 g cm−3. Our analysis shows that mapping tropical ecosystem carbon always benefits from locally validated measurement of tree-by-tree botanical identity combined with tree-by-tree measurement of dimensions. Therefore whenever possible, efforts to map and monitor tropical forest carbon using remote sensing techniques should be combined with tree-level measurement of species identity by botanists working in inventory plots.

中文翻译：

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物种很重要：木材密度在多个尺度上影响热带森林生物量

树木中所含的碳质量取决于木材的体积和密度。这对大多数遥感技术来说是一个挑战，这些技术通常检测与木材体积相关的表面结构和参数，但不检测与木材密度相关的参数。由于木材密度很大程度上取决于分类特性，因此在拥有数万种树种的热带森林中，这一挑战最为严峻。在这里，利用泛热带文献和亚马逊地区的新分析以及具有可靠识别的图块，我们评估了与物种相关的木材密度变化对成熟热带森林生物量估计的影响。我们发现，从单棵树到整个生物群落的所有尺度上的木材密度，物种对森林生物量都有影响：树种组成的变化调节着森林可以储存的碳量。即使成分的局部差异也会导致森林生物量和碳密度在略有不同的当地森林类型之间出现 20% 的差异，而额外的大规模植物区系差异会导致整个亚马逊流域和热带地区的平均木材密度出现 10-30% 的差异。此外，由于物种组成在各个尺度上、甚至在林分内的垂直方向上都存在差异，我们的分析表明，如果忽略个体身份，总是会产生偏差和不确定性。由于现在有足够的基于植物鉴定的库存证据表明物种组成对整个生物群落的生物量很重要，因此我们在这里汇总并提供低地和热带生物群落不同森林的平均底面积加权木材密度值。这些范围很广，从 0.467 到 0.728 g cm−3，泛热带平均值为 0.619 g cm−3。我们的分析表明，绘制热带生态系统碳图总是受益于本地验证的逐棵树植物特性测量与逐棵树尺寸测量相结合。因此，只要有可能，利用遥感技术绘制和监测热带森林碳的努力应与在清查地块工作的植物学家对物种身份的树木水平测量相结合。