Climate extremes, such as heat waves and extreme droughts, are predicted to become more frequent in a future atmosphere with more greenhouse gases. Temperature records show that heat extremes have already become significantly more frequent worldwide in the last 50 years. Development of methodology to artificially simulate extreme events in the field is now giving a boost to the research field of the ecological impact of extreme events, as it makes ecologists independent of the occurrence of natural extremes. The current contribution summarizes a series of experiments at the University of Antwerp on the impact of extreme heat and drought on single plants and plant communities. Results are presented on temperate grassland and arctic tundra. Heat waves were generated with controlled infrared irradiation in free-air conditions in the field, drought by excluding rainfall. We investigated the following questions: (1) is resistance to extremes in individuals plants (survival) coupled to specific plant traits? (2) does survival at the community level vary with the complexity of the community? (3) do climate extremes exert after-effects long after the event has passed (4) are plants grown at elevated temperature better protected against extremes? 5) do extreme events influence seed establishment? (6) are soil biota buffered against climatic extremes?

Results/Conclusions

The results suggest that current paradigms on stress sensitivity may not per se apply to extreme events. For example, plant traits that best explained plant resistance to extreme drought, were different from known indicators of resistance to moderate stress. Species-rich grasland mixtures were more sensitive to extremes than monocultures, contradicting theory that complex systems are more stable at community level. Faster depletion of the soil water profile due to greater complementarity of water use in these species-richer systems was at the basis of their enhanced sensitivity. Still in multi-species grassland communities, year-round exposure to moderate warming enhanced the sensitivity to severe drought, in spite of acclimation. Arctic ecosystems also exhibited surprising patterns, for example, heat waves alleviated plant stress during exposure, but this reversed when temperatures returned to normal. Transient heat peaks also hampered establishment from seed in several Arctic plant species, and even affected soil biota. The record of ecological responses to extreme events is currently too limited to generalize on their future impact. Assuming that extreme events will continue to shape the distribution of species both locally and globally, the rapid further development of this new field in ecology is needed.