A new generation of climate change experiments: Events, not trends

Intensification of weather extremes is currently emerging as one of the most important facets of climate change. Research on extreme events has increased in recent years and in 2004 amounted to a fifth of the experimental climate change studies published.  Numerous examples ranging from microbiology and soil science to biogeography demonstrate how extreme weather events can accelerate shifts of species compositions and distributions, thereby facilitating changes in ecosystem functioning and services. However, assessing the significance of extreme events for vegetation dynamics and ecological processes poses major challenges because of their very nature: the effects of these events are out of proportion to their short duration. We propose that extreme events can be characterized by their statistical extremity, their timing, and their abruptness relative to the life cycles of the organisms affected. Here, we present first results of an experiment, in which planted grassland and heath communities in central Europe were exposed to a single drought event, or a heavy rainfall event. The magnitude of manipulations imitated the local 100-year weather extreme according to extreme value statistics. Our results reveal that, for example, overall productivity of both plant communities remained stable in face of drought, despite significant effects on tissue die-back. Interestingly, effects of extreme weather events on community tissue die-back were modified by species composition.