Faculty of Environment, Science and Economy, University of Exeter, Exeter, Devon, EX4 4QF, UK.
Met Office Hadley Centre, Fitzroy Road, Exeter, EX1 3PB, UK.
Sci Rep. 2023 Oct 31;13(1):18712. doi: 10.1038/s41598-023-44860-7.
Carbon uptake by the land is a key determinant of future climate change. Unfortunately, Dynamic Global Vegetation Models have many unknown internal parameters which leads to significant uncertainty in projections of the future land carbon sink. By contrast, observed forest inventories in both Amazonia and the USA show strikingly common tree-size distributions, pointing to a simpler modelling paradigm. The curvature of these size-distributions is related to the ratio of mortality to growth in Demographic Equilibrium Theory (DET). We extend DET to include recruitment limited by competitive exclusion from existing trees. From this, we find simultaneous maxima of tree density and biomass in terms of respectively the ratio of mortality to growth and the proportion of primary productivity allocated to reproduction, an idea we call Demographic Optimality (DO). Combining DO with the ratio of mortality to growth common to the US and Amazon forests, results in the prediction that about an eighth of productivity should be allocated to reproduction, which is broadly consistent with observations. Another prediction of the model is that seed mortality should decrease with increasing seed size, such that the advantage of having many small seeds is nullified by the higher seed mortality. Demographic Optimality is therefore consistent with the common shape of tree-size distributions seen in very different forests, and an allocation to reproduction that is independent of seed size.
陆地碳吸收是未来气候变化的关键决定因素。不幸的是，动态全球植被模型有许多未知的内部参数，这导致对未来陆地碳汇的预测存在很大的不确定性。相比之下，在亚马逊地区和美国的观测森林清查都显示出明显的共同树木大小分布，指向一个更简单的建模范例。这些大小分布的曲率与生存竞争对生长的比例有关，这在动态平衡理论（DET）中有所体现。我们将 DET 扩展到包括由现有树木的竞争排斥限制的招募。由此，我们发现树木密度和生物量的同时最大值分别与死亡率与生长的比例以及分配给繁殖的初级生产力的比例有关，我们称之为“人口最优”（DO）。将 DO 与美国和亚马逊森林共有的死亡率与生长的比例相结合，预测大约有八分之一的生产力应该分配给繁殖，这与观察结果大致一致。该模型的另一个预测是种子死亡率应随种子大小的增加而降低，从而使具有许多小种子的优势被更高的种子死亡率所抵消。因此，人口最优与在非常不同的森林中看到的共同树木大小分布形状以及与种子大小无关的繁殖分配是一致的。
