Sensitivity of temperate grassland to extreme precipitation variability – biomass production, forage quality and biotic interactions
Kerstin Grant (04/2009-12/2015)
Support: Anke Jentsch
The Earth’s climate is increasing in its variability as a consequence of global warming. Therefore, it is important to develop a better understanding how ecosystems, especially grassland, may respond to increased climate variability. The following thesis presents potential impacts of increased intra-annual climate variability on the productivity, forage quality, and plant community composition of mesic temperate grassland. Possible interactions of multiple climate factors – precipitation variability and seasonal warming – are studied and underlying processes that alter the resistance and resilience of mesic temperate grassland are investigated. In particular, this thesis tests management practises, namely fertilisation, cutting date, and cutting frequency, as potential tools to adapt grassland to the projected increase in climate variability. All investigations were done using the following experimental approach: precipitation variability was manipulated in two long-term field experiments, EVENT-1 and EVENT-2, located in the Ecological-Botanical Garden of the University of Bayreuth. In EVENT-1, artificial, mesic temperate grassland with varying community composition was subjected to annually recurrent, extreme weather events (drought, heavy rainfall). In EVENT-2, semi-natural, mesic temperate grassland was subjected to altered intra-annual precipitation variability (low, medium, high). The ‘high variability’ treatment included either a spring or a summer drought event combined with heavy rain pulses to test for seasonality effects. Precipitation variability was tested in interaction with management practises (fertilisation, cutting regime) or seasonal warming (winter, summer) was studied. The results of this thesis show that high intra-annual precipitation variability is decreasing the aboveground productivity of mesic temperate grassland – particularly when it coincides with low annual precipitation amounts. The seasonal occurrence of the climatic factors, early vs. late drought events or winter vs. summer warming, was neither amplifying nor buffering the observed precipitation effects on productivity. Increasing intra-annual precipitation variability led to changes in the grassland community composition favouring forbs over grasses. In contrast to findings on productivity, the interaction of the precipitation and temperature variability was an important driver for the shift in community composition, A more variable precipitation regime with spring or summer droughts coinciding with higher summer temperatures reduced species evenness. Furthermore, the changes in community composition due to a more extreme precipitation pattern and plant senescence were found to be responsible for alterations of forage quality. Increased precipitation variability caused a short-term decrease in forage quality of single plants, in terms of lower leaf nitrogen and protein concentrations. However, a general increase in the forage quality was observed at the community level in the face of high intra-annual precipitation variability. The results of this thesis further reveal that single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa, and the nature of the shifts varied depending on the particular community composition. Moreover, a facilitative legume effects on neighbouring plant growth was found under ambient weather conditions and heavy rainfall events but it disappeared under extreme drought. The observed changes in plant-plant interaction and the presence of legumes could be seen as possible mechanisms that stabilise grassland community productivity in the face of increased precipitation variability. In this thesis, management practises, which are fast and easily applicable, are tested for the first time for their suitability as adaptation tools to overcome the influence of altered intra-annual climate variability on mesic temperate grassland. More frequent cutting (4 vs. 2 cuts per year) neither buffered, nor amplified the adverse effects of rainfall variability on productivity, but made forage quality in early summer more responsive to altered rainfall patterns. In general, fertilisation enhanced forage quality, while a 10-day delayed harvest reduced forage quality. However, these practises were only partially successful at sustaining grassland productivity under increased intra-annual precipitation variability. Only fertilisation reduced losses in grassland annual yield caused by an extreme precipitation regime. The effectiveness of these practises was negatively influenced by a later seasonal occurrence of extreme weather events. Adaptation strategies have yet to be found which are also flexible enough to respond to the seasonality of extreme weather events. The alteration of the grassland communities due to increased precipitation variability indicated that a higher diversity in species and functional traits seem to have the potential to partly buffer negative effects. In addition, this thesis includes a critical view on precipitation change experiments and gives recommendations on how to improve the integration of data from these experiments into meta-analyses and climate change models. The application of routine measurements of basic variables and an experimental design jointly developed by field ecologists and modellers would bring the largest benefit for the global change research. Altogether, this thesis shows that mesic temperate grassland already experiences biomass losses and vegetation shifts due to increased climate variability. These changes in turn modify ecosystem services beyond productivity, for example forage quality. The mechanisms behind the resistance and resilience of this grassland are not easily identified because of the complexity of abiotic and biotic factors. This simplified experiment reveals that biotic interactions, such as competition and facilitation, and functional diversity play a key role in the response of mesic temperate grassland to the alterations in precipitation and warming pattern. Future research, such as on the assistance of ecosystem adaptation to the projected climate change, now faces the challenge to capture not only the frequency and magnitude, but also the seasonality of multi climatic factors.