Climate and Ecosystem Sciences Department, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique, Toulouse, France.
Glob Chang Biol. 2020 Oct;26(10):5734-5753. doi: 10.1111/gcb.15254. Epub 2020 Jul 25.
Elevated atmospheric carbon dioxide (eCO ) is predicted to increase growth rates of forest trees. The extent to which increased growth translates to changes in biomass is dependent on the turnover time of the carbon, and thus tree mortality rates. Size- or age-dependent mortality combined with increased growth rates could result in either decreased carbon turnover from a speeding up of tree life cycles, or increased biomass from trees reaching larger sizes, respectively. However, most vegetation models currently lack any representation of size- or age-dependent mortality and the effect of eCO on changes in biomass and carbon turnover times is thus a major source of uncertainty in predictions of future vegetation dynamics. Using a reduced-complexity form of the vegetation demographic model the Functionally Assembled Terrestrial Ecosystem Simulator to simulate an idealised tropical forest, we find increases in biomass despite reductions in carbon turnover time in both size- and age-dependent mortality scenarios in response to a hypothetical eCO -driven 25% increase in woody net primary productivity (wNPP). Carbon turnover times decreased by 9.6% in size-dependent mortality scenarios due to a speeding up of tree life cycles, but also by 2.0% when mortality was age-dependent, as larger crowns led to increased light competition. Increases in aboveground biomass (AGB) were much larger when mortality was age-dependent (24.3%) compared with size-dependent (13.4%) as trees reached larger sizes before death. In simulations with a constant background mortality rate, carbon turnover time decreased by 2.1% and AGB increased by 24.0%, however, absolute values of AGB and carbon turnover were higher than in either size- or age-dependent mortality scenario. The extent to which AGB increases and carbon turnover decreases will thus depend on the mechanisms of large tree mortality: if increased size itself results in elevated mortality rates, then this could reduce by about half the increase in AGB relative to the increase in wNPP.
大气中二氧化碳浓度升高（eCO2）预计会增加森林树木的生长速度。增长幅度转化为生物量变化的程度取决于碳的周转率，因此也取决于树木的死亡率。大小或年龄相关的死亡率加上增长率的增加，可能导致树木生命周期的加速导致碳周转率降低，或者树木达到更大的大小导致生物量增加。然而，大多数植被模型目前都没有表示大小或年龄相关的死亡率，因此 eCO2 对生物量和碳周转率变化的影响是预测未来植被动态的主要不确定性来源之一。使用简化复杂度的植被动态模型——功能组装陆地生态系统模拟器，模拟理想的热带森林，我们发现，尽管在大小和年龄相关的死亡率情景中碳周转率都降低，但生物量仍有增加，这是对木质净初级生产力（wNPP）假设增加 25%的 eCO2 驱动的响应。在大小相关的死亡率情景中，由于树木生命周期的加速，碳周转率降低了 9.6%，但当死亡率是年龄相关时，碳周转率也降低了 2.0%，因为更大的树冠会导致更激烈的光竞争。当死亡率是年龄相关时，地上生物量（AGB）的增加幅度（24.3%）比大小相关时（13.4%）大得多，因为树木在死亡前达到了更大的大小。在背景死亡率不变的模拟中，碳周转率降低了 2.1%，AGB 增加了 24.0%，然而，AGB 和碳周转率的绝对值高于大小或年龄相关的死亡率情景。因此，AGB 的增加幅度和碳周转率的降低幅度将取决于大树死亡率的机制：如果树木的大小本身导致死亡率升高，那么相对于 wNPP 的增加，AGB 的增加幅度可能会减少一半。
