Internal stem decay as a potentially underestimated source of error in biomass estimates

森林生态系统是重要的陆地碳汇，从大气中吸收大量的二氧化碳。封存的碳被分配在树木生物量和土壤中。对树木生物量中有机碳储量的估算通常基于异速生长生物量函数，该函数使用树干直径和树高数据来计算树木生物量。因此，这些生物量功能完全依赖于树木的外部形状，而忽略了内部茎腐和树腔。这个过程当然是合理的许多树木个体，如果他们缺乏大的空洞或部分减少木材密度由于内部茎腐烂。然而，正如卢茨及其同事最近在2018年的一项研究中指出的那样，老的大直径树木对森林总生物量的贡献程度严重不成比例。在全球范围内，最大的1%的树木被认为包含了大约50%的地上树木生物量。在这种背景下，大型老树的内部茎腐和树洞很重要，因为它们可能导致对森林总碳储量的高估。在本项目中，我们对Lutz和同事的结论提出了挑战，因为对于林分中最古老和最大的树木的具体情况，忽略生物量估算中的内部茎干腐烂和树腔可能会导致重大误差，因为可以认为内部茎干腐烂在这类树木中比在大多数其他森林树木中更为常见。因此，我们怀疑仅仅依靠树木的外部形状来评估其生物量是合理的，只要在所有树木中，由于内部茎干腐烂而导致木材密度和空洞减少的可能性最大的个体被认为对确定整个森林的有机碳储量最重要。因此，在我们的项目中，我们将仔细研究不同大小类别的树木的内部茎腐烂对树木生物量的影响。利用声波层析成像、电阻率层析成像和木芯采样的结合方法，我们希望通过从树木生物量中减去假设的质量损失，在生物量估计中整合由内部树干衰变造成的质量损失，这是由传统的生物量函数确定的。这些调查将在选定的温带和北方森林生态系统中进行，以验证以下假设：如果将内部茎腐和树洞造成的质量损失纳入生物量估算，那么最大的1%树木的地上生物量将减少到显著低于林分地上生物量的50%。

Forest ecosystems are important terrestrial carbon sinks sequestering large amounts of CO2 from the atmosphere. The sequestered carbon is allocated both in the tree biomass and the soil. Estimates of the organic carbon stocks in the tree biomass are commonly based on allometric biomass functions, which use stem diameter and tree height data for the calculation of tree biomass. Hence, these biomass functions completely rely on the out-er shape of the trees and ignore internal stem decay and tree cavities. This procedure is certainly justified for many tree individuals, if they lack large cavities or sections with re-duced wood density due to internal stem decay. However, as pointed out in a recent study of 2018 by Lutz and co-workers, old large-diameter trees contribute to a strongly dispropor-tionate degree to the total biomass of forests. On a global scale, the largest 1 % of trees were concluded to contain approximately 50 % of the total above-ground tree biomass. Against this background, internal stem decay and tree cavities in large old trees matter, since they might lead to an overestimation of the total carbon stock of forests.In the present project, we challenge the conclusion of Lutz and co-workers, because for the specific case of the oldest and largest trees of a forest stand, neglecting internal stem de-cay and tree cavities in the biomass estimates may result in a significant error, as internal stem decay can be assumed to be much more common in this subset of trees than in most other forest trees. Therefore, we doubt that merely relying on the outer shape of the trees for assessing their biomass is justified, as long as of all trees the individuals with the great-est probability for having large reduced wood density and cavities due to internal stem de-cay are regarded to be most significant for determining the organic carbon storage of the whole forest. In our project we will, therefore, scrutinize the effect of internal stem decay for tree biomass in trees of different size classes.Using a combined approach of sonic tomography, electric resistivity tomography and wood-core sampling, we want to integrate the mass loss by internal stem decay in the biomass estimates by subtracting the assumed mass loss from the tree biomass, which was deter-mined with conventional biomass functions. These investigations will be carried out in se-lected temperate and boreal forest ecosystems for testing the hypothesis that the above-ground biomass of the largest 1 % of trees is reduced to significantly less than 50 % of the stand above-ground biomass, if the mass loss by internal stem decay and tree cavities is integrated in the biomass estimates.