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

1. Individual effects of co‐occurring global change factors on net primary
productivity (NPP) have been widely studied; however, their interactive
effects remain highly debated. 2. 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. 3. 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. 4. Synthesis. Our results
suggest that 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.