Computational Ecology and Environmental Science Group, Microsoft Research, Cambridge, UK.
Glob Chang Biol. 2013 May;19(5):1504-17. doi: 10.1111/gcb.12152. Epub 2013 Feb 26.
The role of tree mortality in the global carbon balance is complicated by strong spatial and temporal heterogeneity that arises from the stochastic nature of carbon loss through disturbance. Characterizing spatio-temporal variation in mortality (including disturbance) and its effects on forest and carbon dynamics is thus essential to understanding the current global forest carbon sink, and to predicting how it will change in future. We analyzed forest inventory data from the eastern United States to estimate plot-level variation in mortality (relative to a long-term background rate for individual trees) for nine distinct forest regions. Disturbances that produced at least a fourfold increase in tree mortality over an approximately 5 year interval were observed in 1-5% of plots in each forest region. The frequency of disturbance was lowest in the northeast, and increased southwards along the Atlantic and Gulf coasts as fire and hurricane disturbances became progressively more common. Across the central and northern parts of the region, natural disturbances appeared to reflect a diffuse combination of wind, insects, disease, and ice storms. By linking estimated covariation in tree growth and mortality over time with a data-constrained forest dynamics model, we simulated the implications of stochastic variation in mortality for long-term aboveground biomass changes across the eastern United States. A geographic gradient in disturbance frequency induced notable differences in biomass dynamics between the least- and most-disturbed regions, with variation in mortality causing the latter to undergo considerably stronger fluctuations in aboveground stand biomass over time. Moreover, regional simulations showed that a given long-term increase in mean mortality rates would support greater aboveground biomass when expressed through disturbance effects compared with background mortality, particularly for early-successional species. The effects of increased tree mortality on carbon stocks and forest composition may thus depend partly on whether future mortality increases are chronic or episodic in nature.
树木死亡在全球碳平衡中的作用是复杂的，这是由于通过干扰导致碳损失的随机性而产生的强烈的时空异质性。因此，描述死亡率（包括干扰）的时空变化及其对森林和碳动态的影响对于理解当前的全球森林碳汇以及预测未来它将如何变化至关重要。我们分析了美国东部的森林清查数据，以估算九个不同森林地区的每个地块的死亡率（相对于个体树木的长期背景速率）变化。在每个森林地区的 1-5%的地块中观察到至少在大约 5 年内使树木死亡率增加四倍的干扰。干扰的频率在东北部最低，随着火灾和飓风干扰变得越来越常见，沿大西洋和墨西哥湾海岸向南增加。在该地区的中部和北部，自然干扰似乎反映了风、昆虫、疾病和冰暴的弥漫组合。通过将随时间估计的树木生长和死亡率的共变与数据约束的森林动态模型联系起来，我们模拟了死亡率随机变化对美国东部长期地上生物量变化的影响。干扰频率的地理梯度导致了最不干扰和最干扰地区之间的生物量动态明显不同，死亡率的变化导致后者的地上林分生物量随时间发生更强的波动。此外，区域模拟表明，与背景死亡率相比，给定的长期平均死亡率增加将通过干扰效应支持更大的地上生物量，特别是对于早期演替物种。因此，树木死亡率增加对碳储量和森林组成的影响可能部分取决于未来死亡率的增加是慢性的还是突发性的。
