Extreme natural drought enhances interspecific facilitation in semiarid agroforestry systems

Agroforestry is a widely advocated adaptation strategy for enhancing agricultural resilience to extreme climates. However, it is somewhat controversial and there is limited evidence regarding its efficacy in semiarid regions. Here we describe studies on jujube agroforestry regimes using two different cash crops: the annual fodder species rape (Brassica napus) and the perennial daylily (Hemerocallis fulva). The jujube agroforestry systems were characterized with respect to their interspecific water interactions and their responses to extreme natural drought using stable isotopic techniques and in situ soil water observations in China's semiarid Loess Plateau region. We found that agroforestry altered the jujube trees' water sources but its impact depended on soil wetness: compared to monocultures, jujube trees in agroforestry systems generally shifted to deeper water under dry conditions but to shallower water under wet conditions. Complementary water use between jujube trees and crops was observed on most sampling dates, but there was clear evidence of water competition in the shallow layer under relatively wet conditions. Furthermore, a simple index, Relative Difference in Soil Water content (RDSW), was developed to indicate interspecific water interactions under various wetness conditions. It was found that facilitative interspecific interactions were clearly enhanced in both agroforestry systems during extreme drought. In addition, jujube yields in agroforestry systems were clearly higher than in monoculture. These findings demonstrate that agroforestry is a climate-smart agricultural system and can increase the resilience of semiarid jujube plantations to extreme real-world drought.