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

Abstract:

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.