The following section gives a short summary of the manuscripts forming this thesis with focus on the most important findings of each study including the declaration of personal contribution to each manuscript. The field work for Manuscripts 1 – 3 was carried out in the multifactorial EVENT-2 experiment designed to test the response of semi-natural, mesic temperate grassland to manipulated alterations in intra-annual precipitation variability (low, medium, 2 x high: with spring or summer drought events), management practices (cutting regime, fertilisation) and seasonal warming (winter, summer). The field work for Manuscript 4 and 5 took place in the two-factorial EVENT-1 experiment testing the response of artificial grassland communities to the manipulation of extreme weather events (drought and heavy rainfall) and community composition. For more details on the experiments please refer to the methods sections in the manuscripts. Manuscript 6 is a review which grew out of an organized oral session on precipitation manipulation experiments at the 96th Annual Meeting of the Ecological Society of America.
Manuscript 1
Title: Importance of seasonality for the response of a mesic temperate grassland to precipitation variability and warming
Authors: Kerstin Grant, Jürgen Kreyling, Carl Beierkuhnlein, Anke Jentsch Journal: Ecosystems, submitted 02.12.2015
Personal contribution: data collection: 30%, data analysis: 90%, writing: 90%, concept: 80%, discussion and editing: 80%, corresponding author
Manuscript 1 presented that the seasonal occurrence of the climatic factors, here early vs.
late drought events in the high intra-annual precipitation variability treatments, seemed to be an important driver of the shifts in community composition but not for decreases in aboveground net primary productivity (ANPP) compared to regular rainfall pattern throughout the entire growing season. Only the treatment with a high precipitation variability including spring drought events favoured forbs over grasses. Furthermore, the timing of chronic warming, here winter vs. summer, altered the direction of response of both, community composition and ANPP. Winter warming increased ANPP and favoured forbs while summer warming showed no significant effect on biomass but decreased species richness. The interaction of the precipitation and temperature variability treatments showed an interesting response: a more variable precipitation regime with spring or summer droughts coinciding with higher summer temperatures reduced species evenness and likely promoted the establishment of specialists and drought tolerant species. Because ecological effects and processes to the timing of climate factors might be divergent or hidden if not separately studied, future climate change experiments should include the seasonality and timing of climatic factors depending on the local climate projections.
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Title: Water stress due to increased intra-annual precipitation variability reduced forage yield but raised forage quality of a temperate grassland
Authors: Kerstin Grant, Jürgen Kreyling, Laura F.H. Dienstbach, Carl Beierkuhnlein, Anke Jentsch
Journal: Agriculture, Ecosystems & Environment 186 (2014) 11–22
Personal contribution: data collection: 40%, data analysis: 90%, writing: 90%, concept: 70%, discussion and editing: 80%, corresponding author
Manuscript 2 focused on how water stress due to increased intra-annual precipitation variability reduced forage yield (ANPP) but raised forage quality of mesic temperate grassland. It further presented how two land management strategies namely fertilization and alteration of harvest date (delay by 10 days) interact with altered precipitation regime. In agreement with Manuscript 1, this study showed that community composition was altered due to increased intra-annual precipitation regimes with spring drought events in terms of less biomass of grasses and more forbs. Fertilization during drought periods and harvest delay after drought periods were only partially successful as management strategies to sustain forage production under more extreme precipitation regimes. They proved to be less effective if the extreme weather event occurred later in the season. A nitrogen dilution effect (decreased plant nitrogen concentration with increasing shoot biomass) might have masked possible effects of precipitation variability on plant nitrogen and therefore on quality of grassland species. However, the results of this study emphasized the importance of seasonality in climatic factors for the responsiveness of grassland performance as changes in community composition and plant senescence were main drivers of forage quality response to changed precipitation patterns.
Manuscript 3
Title: Increased rainfall variability reduces biomass and forage quality of temperate grassland largely independent of mowing frequency
Authors: Julia Walter, Kerstin Grant, Carl Beierkuhnlein, Juergen Kreyling, Michael Weber, Anke Jentsch
Journal: Agriculture, Ecosystems & Environment 148 (2012) 1–10
Personal contribution: data collection: 30%, data analysis: 30%, writing: 10%, concept: 30%, discussion and editing: 50%
Manuscript 3 addressed a possible interaction between the land management practice cutting frequency and intra-annual precipitation variability on productivity and forage quality.
Increased precipitation variability caused a reduction of ANPP of temperate grassland and short-term decreases in forage quality, in terms of lower leaf N and protein concentrations.
An increase in cutting frequency (4 vs. 2 times a year) enhanced the root-shoot ratio and the concentration of leaf N. However, more frequent cutting neither buffered, nor amplified the adverse effects of higher precipitation variability on productivity, but caused a reduction in leaf N concentrations in early summer. This study also included a comparison of the response of ANPP to the alteration of changed inter- and intra-annual precipitation variability (first year of the study) vs. intra-annual precipitation variability with constant annual
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precipitation amount (second year). There was evidence that the combined changes in the amount, timing and frequency of precipitation enhanced plant water stress and can lead to higher impacts on agricultural yields.
Manuscript 4
Title: Extreme weather events and plant-plant interactions – Shifts between competition and facilitation among grassland species in the face of drought and heavy rainfall
Authors: Kerstin Grant, Jürgen Kreyling, Hermann Heilmeier, Carl Beierkuhnlein, Anke Jentsch
Journal: Ecological Research (2014) 29: 991–1001
Personal contribution: data collection: 75%, data analysis: 95%, writing: 90%, concept: 95%, discussion and editing: 80%, corresponding author
Manuscript 1 and 2 revealed changes in the community composition of grassland due to increased intra-annual rainfall variability and indicated that plant-plant interactions might have influenced the response of grassland to extreme weather events. Thus, Manuscript 4 looked at plant-plant interactions, namely facilitation and competition, between temperate grassland species in the face of extreme drought and rainfall events as part of altered rainfall variability. Species specific shifts in plant–plant interactions from facilitation to competition or vice versa were caused by single climatic extremes. The study revealed that the nature of the shifts varied depending on the particular community composition. Because drought did not affect ANPP of this experimental grassland community, the complementary response in competition intensity – shift from competition to facilitation for Arrhenatherum elatius and a contrasting shift for Lotus corniculatus – might indicate that changes in biotic interactions as one of the mechanisms for stable community productivity in the face of drought. Furthermore, the results on plant-plant interactions suggested that the presence of a legume in the plant communities appeared to have a key role in the response of competition intensity experienced by grasses to climate change.
Manuscript 5
Title: Climatic extremes lead to species-specific legume facilitation in an experimental temperate grassland
Authors: Mohammed Abu Sayed Arfin Khan, Kerstin Grant, Carl Beierkuhnlein, Jürgen Kreyling, Anke Jentsch
Journal: Plant and Soil (2014) 379:161–175
Personal contribution: data collection: 25%, data analysis: 5%, writing: 5%, concept: 10%, discussion and editing: 20%
Manuscript 5 addressed the role of nitrogen-fixing legumes, here Lotus corniculatus, in the response of grassland productivity to four years of yearly recurrent drought and heavy rainfall events. Aboveground net primary productivity of neighbouring non-legumes was facilitated by legumes under ambient weather conditions and when the experimental grassland communities were exposed to heavy rainfall events. However, this facilitative legume effects
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on neighbouring plant growth disappeared under extreme drought. Species identity was decisive for the response to legume effect on productivity but also on δ15N, leaf N concentration and N uptake. We argued that the missing legume effect on community productivity under drought was caused by reduced N-uptake of target species rather than reduced N-fixation by the legume. The findings emphasised the importance of legume presence for stabilizing grassland performance under future increased precipitation variability.
Manuscript 6
Title: Toward a better integration of biological data from precipitation manipulation experiments into earth system models
Authors: Nicholas G. Smith, Vikki L. Rodgers, Edward R. Brzostek, Andrew Kulmatiski, Meghan L. Avolio, David L. Hoover, Sally E. Koerner, Kerstin Grant, Anke Jentsch, Simone Fatichi, and Dev Niyogi
Journal: Reviews of Geophysics (2014) 52(3):412–434
Personal contribution: literature research 5%, writing: 0%, concept: 0%, discussion and editing: 5%
Manuscript 6 aimed to bridge the gap between precipitation manipulation experiments and Earth system models. It gave recommendation on how biological data from field experiments could be better integrated into models and should reignite the necessary dialog between the different research communities. The manuscript reviewed most recent projections for future terrestrial presentation, the structure and functioning of the current-generation of land surface models (LSM) and different approaches of precipitation change field experiments. It further pointed out routine measurements (e.g. soil moisture and micrometeorological data in high temporal and spatial resolution, soil water retention curves, respiration rates, plant functional types etc.) that would improve the incorporation of useful data and more refined processes into models. We further compared biological processes commonly measured in the field, namely carbon assimilation and productivity, phenology, soil organic matter decomposition and plan community dynamics with their model analogs. We found that often the measured processes are not compatible with the way they are represented in LSMs. Additionally we addressed the challenge of scaling from plot to globe. In conclusion, the global change community would benefit most from experiments designed by researchers of both communities – field ecologists and modelers.
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