Department of Plant Sciences, University of California Davis, Davis, California, USA.
Jet Propulsion Laboratory California Institute of Technology, Pasadena, California, USA.
Glob Chang Biol. 2023 Oct;29(19):5652-5665. doi: 10.1111/gcb.16867. Epub 2023 Jul 27.
More frequent and severe droughts are driving increased forest mortality around the globe. We urgently need to describe and predict how drought affects forest carbon cycling and identify thresholds of environmental stress that trigger ecosystem collapse. Quantifying the effects of drought at an ecosystem level is complex because dynamic climate-plant relationships can cause rapid and/or prolonged shifts in carbon balance. We employ the CARbon DAta MOdel fraMework (CARDAMOM) to investigate legacy effects of drought on forest carbon pools and fluxes. Our Bayesian model-data fusion approach uses tower observed meteorological forcing and carbon fluxes to determine the response and sensitivity of aboveground and belowground ecological processes associated with the 2012-2015 California drought. Our study area is a mid-montane mixed conifer forest in the Southern Sierras. CARDAMOM constrained with gross primary productivity (GPP) estimates covering 2011-2017 show a ~75% reduction in GPP, compared to negligible GPP change when constrained with 2011 only. Precipitation across 2012-2015 was 45% (474 mm) lower than the historical average and drove a cascading depletion in soil moisture and carbon pools (foliar, labile, roots, and litter). Adding 157 mm during an especially stressful year (2014, annual rainfall = 293 mm) led to a smaller depletion of water and carbon pools, steering the ecosystem away from a state of GPP tipping-point collapse to recovery. We present novel process-driven insights that demonstrate the sensitivity of GPP collapse to ecosystem foliar carbon and soil moisture states-showing that the full extent of GPP response takes several years to arise. Thus, long-term changes in soil moisture and carbon pools can provide a mechanistic link between drought and forest mortality. Our study provides an example for how key precipitation threshold ranges can influence forest productivity, making them useful for monitoring and predicting forest mortality events.
更频繁和更严重的干旱正在推动全球森林死亡率的上升。我们迫切需要描述和预测干旱如何影响森林碳循环，并确定引发生态系统崩溃的环境压力阈值。在生态系统层面上量化干旱的影响是复杂的，因为动态的气候-植物关系可能导致碳平衡的快速和/或长时间的变化。我们采用 CARbon DAta MOdel fraMework（CARDAMOM）来研究干旱对森林碳库和通量的遗留影响。我们的贝叶斯模型-数据融合方法使用塔观测到的气象强迫和碳通量来确定与 2012-2015 年加利福尼亚干旱相关的地上和地下生态过程的响应和敏感性。我们的研究区域是南塞拉山脉的一个中高山混合针叶林。与仅受 2011 年数据约束相比，受 2011-2017 年总初级生产力（GPP）估计值约束的 CARDAMOM 显示 GPP 减少了约 75%，而 GPP 变化可以忽略不计。2012-2015 年的降水量比历史平均水平低 45%（474 毫米），导致土壤水分和碳库（叶片、易变、根系和凋落物）的级联枯竭。在特别紧张的一年（2014 年，年降雨量为 293 毫米）中增加 157 毫米，导致水和碳库的枯竭程度较小，使生态系统远离 GPP 临界点崩溃的状态，从而恢复。我们提出了新的基于过程的见解，这些见解证明了 GPP 崩溃对生态系统叶片碳和土壤水分状态的敏感性——表明 GPP 响应的全部程度需要几年时间才能显现。因此，土壤水分和碳库的长期变化可以为干旱和森林死亡率之间提供一个机制联系。我们的研究为关键降水阈值范围如何影响森林生产力提供了一个例子，使它们成为监测和预测森林死亡率事件的有用工具。
