Taylor’s law predicts unprecedented pulses of forest disturbance under global change

Climate extremes are causing increasingly large pulses of forest disturbance across biomes, raising concerns that forests are pushed beyond their safe operating space. Predicting future disturbance pulses remains, however, a major challenge, because disturbance pulses are stochastic and driven by complex ecological and socio-economic processes. Here, we provide a tractable solution to this problem using Taylor's law, which relates the mean and variability of a system through a power law relationship (variance ~ mean^b). We test the hypothesis that forest disturbance dynamics can be described through Taylor’s law using high-resolution (30 m) annual disturbance maps of Europe’s forests going back 35 years and covering more than 1.6 million km² of forests. We find strong evidence for a positive power law relationship between mean disturbance rates and their temporal variability (R² of 0.95), indicating that increasing mean disturbance rates – as observed for Europe and many other parts of the globe – significantly amplify the probability of large pulses of tree mortality. The power law relationship was consistent across natural disturbance agents (bark beetle, fire, wind), spatial scales (100-25,600 km²) and biomes (boreal, temperate, Mediterranean) with a power law coefficient of b = 2.35, and also applied to human-dominated disturbances (R² of 0.84 but a lower power law coefficient of b = 1.86). Simulations based on the power law model illustrate how increasing mean disturbance rates cause annual disturbance rates to quickly become more variable, and thus more likely to include extremely large disturbance pulses: A long-term mean disturbance rate of 1 % yr.^-1 will lead to disturbance pulses with > 5 % annual mortality once every 112 years (0.9 % probability). Increasing disturbance rates to 1.5 % yr.^-1, however, we would expect a disturbance pulses of the same magnitude already every five years (19 % probability). Our findings thus challenge the assumption that extreme disturbance pulses as observed recently were unexpected or inherently unpredictable, providing a framework for the integration of future disturbance pulses into forest policy and management.