On the need to use proper metrics to detect experimental drought treatments

Rainfall manipulation experiments are a powerful tool to investigate how drought in isolation or combined with other stresses and disturbance drivers (e.g., fire, herbivory, heatwaves) affects diverse ecological processes. Using passive rainout shelters (intercepting a fixed amount of precipitation) coupled with control plots (receiving ambient precipitation), hundreds of studies have greatly advanced our understanding of drought impacts on plant and soil communities. Despite the existence of guidelines of how to properly design and implement experimental droughts, methodological issues still hinder a correct interpretation of some rainfall manipulation studies. Because of the use of improper metrics to detect drought intensity, it is sometimes unclear whether drought plots really experienced dry conditions and whether control plots experienced near-average mean annual precipitation throughout the experimental period. Here, we reanalyzed three recently published rainfall manipulation studies to illustrate how multi-scalar drought indices (such as SPEI) can be used to better quantify the intensity of the experimental drought imposed and to place it in the historical climate context of each studied area. We also provide additional guidelines to improve the experimental design of future rainfall manipulation studies.