Drought field experiments: how to adapt rainout shelters to agroforestry?

Abstract Agroforestry could be a major strategy to adapt agriculture to climate change, thanks to the microclimate effects of trees and improved infiltration. However, the experimental validation of these claims is scarce. In this methodological review, we discuss options for the experimental simulation of drought conditions in agroforestry field experiments, comparing it with strategies adopted in natural, agricultural, or forestry ecosystems. We classify rainout shelters used in field experiments according to mobility, completeness of rain interception and height of rainout shelter. We show that specificities of agroforestry systems create constraints and require compromises in the design and operation of rainout shelters. We conclude that large rainout shelters, which induce drought for both the trees and the crops while limiting artifacts and biases, would be most relevant for studying the resistance of agroforestry systems to drought. Unfortunately, the review of rainout shelters already used in agroforestry systems reveals a lack of rainout shelters capable of intercepting rain on both trees and crops, achieving total rain interception, while being relatively low-cost and manageable by a small team. Therefore, we benchmark three novel rainout shelter designs that we tested in a mature agroforestry system under Mediterranean climatic conditions. We discuss their advantages and disadvantages in terms of both scientific and operational aspects. While compromises had to be done between experimental design, risks of artifact/bias, effectiveness, ease of installation, operation and maintenance, and agricultural management, these prototypes are starting points for achieving well-performing rainout shelters and testing the effects of drought in agroforestry experiments.