| In this dissertation, experimental herbaceous plant communities were exposed to different scenarios with extreme at and drought events (conducted in a full factorial design). We investigated: - The impact of an extreme drought or heat event compared to their combined occurrence (heat + drought), - The seasonal effects of heat and/or drought events (when occurring in spring, summer or autumn), - The recovery after extreme events and their vulnerability to plant invasions, - The effects of successive extreme events.;Heat waves had only limited impact but when combined with drought, negative effects observed in single factor ought treatments were exacerbated through intensified soil drying and increased leaf temperatures.;Our plant communities proved more sensitive during summer compared with spring and autumn. Yet in some years we also observed little impact of summer drought and heat, especially when the extremes induced only mild ranges in soil moisture. The latter was influenced by factors such as species composition and the water demand of the community. Recovery after single extremes was usually rapid but varied between individual species. Species with the highest productivity were the least resilient to drought and heat. Communities exposed to extreme drought heat events became more vulnerable to invasion by new species in the following year, albeit shortly. The chanced invasibility disappeared after two years.;When multiple extremes occur in the same growing season, a preceding drought + heat extreme increased the sensitivity to a subsequent drought + heat extreme if the interval period was 2 weeks. With a longer recovery between extremes, the sensitivity was not affected.;However, in several experiments, end-of-season community biomass after exposure was greater than in unexposed controls. Here, indirect processes occurring after the extreme events proved to be more important than the direct effects during the extremes. Especially heat waves had an important influence, through their effect on nutrient mineralization.;We conclude that the timing of extreme events is critical to their impact. Temperate herbaceous communities are especially affected by combined heat and drought, but effects on end-of-season biomass are not necessarily negative. |
