To date it is unknown if the resilience of plant communities to environmental stress will be increased or decreased in a CO₂-enriched world. With more CO₂-substrate, plants may recover more quickly from stress such as heat waves or pest outbreaks. On the other hand, if some species start to dominate via a pronounced positive response to elevated CO₂, communities might loose diversity and become less resilient. One way of addressing the question as to how elevated CO₂ concentrations affect the resilience of grasslands is long-term monitoring under elevated CO₂. The Giessen FACE experiment in Germany is located in a high diversity permanent grassland. Aboveground biomass of different functional groups and species composition are measured twice a year since 1993. Since 1998 CO₂ concentrations are elevated by 20% above ambient CO₂ year round. In 2003, the area was subject to a severe drought that significantly reduced biomass production. The objective of this study was to investigate the response of the vegetation to this drought and compare it to pre-drought vegetation dynamics.

Results/Conclusions

During the 2003 drought, biomass production was reduced by 26% with no significant difference in reduction between CO₂ treatments and controls. By the following year biomass production had recovered to at least pre-drought levels irrespective of treatment. However, whereas before 2003 the CO₂ treatments had on average around 6% more biomass production than the control plots, this response to elevated CO₂ was not observed until 3 years after the drought. This delay in the responsiveness to elevated CO₂ after drought was partially due to a shift in the dominance of the vegetation, with some of the dominant grass species significantly reduced during the drought. Furthermore, the recovery was faster in plots with higher species richness in the period before the drought. These results illustrate that diversity effects were more important than elevated CO₂ in the response to drought. They suggest that under elevated CO₂ grasslands may become more vulnerable to environmental stress through species loss.