Drivers of interannual variability in CO2 effects on productivity across long-term experiments

Despite three decades of long-term field manipulations, there remains no clear consensus as to what regulates the magnitude of the CO2 stimulation of plant productivity across ecosystems and through time. A large body of work suggests that availability of nutrients or water, the factors that most strongly control NPP in most ecosystems, also control the magnitude of the CO2 effect on NPP. On the other hand, several recent studies have suggested that indirect CO2 effects, mediated through soil moisture savings, may hold importance in explaining results from individual sites. To determine how water availability may drive CO2 stimulation of productivity we collected annual NPP and rainfall data from 12 long-term CO2 studies. We found that the CO2 effect size was not consistently related to interannual variability in NPP under ambient conditions, indicating that the factors controlling interannual variability in NPP do not control the magnitude of CO2 effects on NPP in a consistent manner. Across sites and years, the CO2 effect size was negatively related to the number of rain days during the growing season, indicating that CO2 may have engendered important indirect effects on productivity that were manifested during under drying intervals. While the drivers underlying direct CO2 effect size remain elusive, this analysis suggests that indirect effects may exhibit generality across sites.