Interactive effects of global change factors on terrestrial net primary productivity are treatment length and intensity dependent

Individual effects of co-occurring global change factors on net primary productivity (NPP) have been widely studied; however, their interactive effects remain highly debated. Here, we conducted a global meta-analysis based on 919 multifactor observations from 120 published studies to examine the interactive effects on NPP of global change factors including elevated [CO2], warming, nitrogen addition, irrigation, drought and changes in species diversity. On average, of the factors studied, six pairs of factors had additive and two pairs had synergistic interactions. Importantly, some of those interaction types changed over time and with treatment intensity. The synergistic interaction between elevated [CO2] and nitrogen addition became additive at high nitrogen addition rates, whereas the synergistic interaction between irrigation and warming diminished at higher temperatures. Over time, the additive effect between elevated [CO2] and increased species richness switched to synergistic. Other global change factor pairs-including elevated [CO2] and warming, nitrogen addition and increased richness, irrigation and N addition, as well as drought and increased richness-remained additive regardless of their treatment intensity or experimental duration. Interaction types of those global change factor pairs did not vary with ecosystem types assessed in our study. Synthesis. Our results suggest that the assumptions of static effects through time or ignoring treatment intensity effects will provide inaccurate predictions of the interactive effects of global change factors on terrestrial NPP. Understanding the context-dependent nature of interactive effects is crucial for validating Earth system models and predicting future NPP responses to co-occurring global change drivers.