Diploma Thesis
Effects of extreme weather events – drought and heavy rainfall – on aboveground productivity of plant communities and competitive balance among various species
Kerstin Grant (01/2008)
Support: Anke Jentsch, Hermann Heilmeier
Summary:
The effects of extreme weather events, which are increasing in frequency and magnitude, on vegetation are mostly unknown. As part of the European event-focused climate change experiment EVENT, this study determined the impacts of local 100-year recurrent drought and heavy rainfall events on the aboveground productivity of experimental grassland communities and the competitive balance among five species.
An increase in productivity was expected for plant communities exposed to heavy rainfall and a decrease for the ones under water stress. Furthermore, buffering effects by higher species and functional diversity, the presence of legumes and different growth forms were assumed.
Aboveground biomass was harvested twice during the year 2007. Additionally, vegetation cover was detected once before and twice after manipulations. To determine the influence of rainout shelters artefacts on the results, a microclimatic survey was run including measurements of the photosynthetically relevant factors wind speed, photosynthetic photon flux density, temperature and vapour pressure. For quantifying the competitive balance of the single species, the mathematical indices relative neighbour effect (RNE), relative crowding (Dr’) and interaction strength (I) were calculated on base of biomass data. Analyses of variances and linear mixed effects models were chosen to reveal differences between the weather manipulations.
Overall productivity of the grassland communities remained stable in face of extreme drought and heavy rainfall. Increasing species or functional diversity as well as the presence of legumes showed no buffering against extreme weather events. Plant communities of different growth forms (here: experimental heathland) reacted in the same way and were not affected by drought or heavy rainfall. A response to extreme events was visible on the species level. Evidences for changing communities composition were found. Arrhenatherum elatius showed a kind of overcompensative growth after water stress but was not resistant to heavy rainfall. Holcus lanatus tended to increasing biomass production under heavy rain and compensated the biomass loss of Arrhenatherum elatius. The competitive balance of the five grassland species was altered by extreme weather events. No significant evidences for facilitation under drought were found but there was a trend to higher competitive neighbourhood under heavy rain. The rainout shelter did significantly change the microclimate but this alteration could be seen as inherent to natural droughts.
The stable productivity of grassland communities but varying response on species level support the insurance hypothesis that species might compensate an eventual failure of single species. Furthermore, it might have reflected the influence of former extreme events (previous manipulation and a natural drought in spring 2007). The results contribute to the idea of ecosystem resilience by species of the same functional group but with varying environmental requirements and tolerances which maintain the function of the plant community under changing weather conditions and they emphasise the importance of species diversity for ecosystem stability under extreme weather events.