Data from: Rising variability, not slowing down, as a leading indicator of a stochastically driven abrupt transition in a dryland ecosystem

Complex systems can undergo abrupt state transitions near critical points. Theory and controlled experimental studies suggest that the approach to critical points can be anticipated by critical slowing down (CSD), i.e., a characteristic slowdown in the dynamics. The validity of this indicator in field ecosystems, where stochasticity is important in driving transitions, remains unclear. We analyzed long-term data from a dryland ecosystem in the Shapotou region in China and show that it underwent an abrupt transition from a nearly bare to a moderate grass-cover state. Prior to the transition, the system showed no (or weak) signatures of CSD, but exhibited expected increasing trends in the variability of the grass cover, quantified by variance and skewness. These surprising results are consistent with the theoretical expectation of stochastically driven abrupt transitions that occur away from critical points; indeed, a driver of vegetation - annual rainfall - showed rising variance prior to the transition. Our study suggests that rising variability can potentially serve as a leading indicator of stochastically driven transitions in real world ecosystems.

复杂系统在临界点附近可发生突变性状态跃迁。理论与受控实验研究表明，临界点的趋近可通过临界慢化（Critical Slowing Down, CSD）——即动力学过程的特征性放缓——予以预判。但该指标在以随机性为跃迁驱动因素的野外生态系统中的有效性仍不明确。我们针对中国沙坡头地区旱地生态系统的长期监测数据开展分析，结果显示该系统曾发生从近乎裸荒到中度草被覆盖状态的突变性跃迁。跃迁发生前，该系统未表现出（或仅表现出微弱的）临界慢化特征，但正如预期，其草被覆盖度的变异性——以方差和偏度量化——呈现上升趋势。上述出人意料的结果与“随机性驱动的突变跃迁可发生于临界点之外”的理论预期相符；实际上，作为植被驱动因子之一的年降雨量，其方差在跃迁发生前同样呈现上升趋势。本研究表明，变异性上升可作为现实生态系统中随机性驱动跃迁的潜在早期预警指标。