The influence of drought stress on yield and forage quality of grasslands differing in functional composition

The present study aimed at unraveling the interacting influences of functional group composition, drought stress and N fertilization on sward productivity and forage quality in old semi-natural grassland in Northern Germany. The study was conducted in the framework of the KLIFF project at the University of Goettingen, which focused on climate impact and adaptation research in Lower Saxony. The experimental sites were located in a semi-natural permanent grassland in the Leine valley, in the north-eastern lowlands, and in the Solling mountain range, each with a decades-long history of extensive agricultural use. The three experimental factors (sward composition, fertilization, and drought stress) led to 8 different treatments and were set in a completely randomized block design with 4 replications of each treatment at each site. In the first chapter the impact of drought stress on resistance and resilience of grasslands, measured by their productivity, was investigated. Drought resistance in swards that were grass-dominated was larger than in functionally diverse swards. Grasses determined the drought resistance potential of swards. The presence of forbs and legumes impaired their resistance. Fertilization increased the resistance to drought stress of swards either through direct positive effects on the productivity or indirect effects through changes of functional sward composition. The presence of dicots was not important for sward resilience. Grasses and whole swards were resilient to drought stress only if previously fertilized. Sward resistance and resilience was shaped by the presence of the grass functional group. Regulating mechanisms on grass performance of the presence as well as of the biomass share of the forb and legume functional group were identified. The differences among the functional groups’ share of the total sward biomass might be important determinants of productivity changes under and after drought stress. In the second chapter the impact of drought stress on forage quality parameters of grassland was investigated. Forage quality was determined by sward functional composition, nitrogen fertilization level and time of the drought stress period (spring or summer), which all also influenced the drought stress quality response of swards. Drought stressed diverse swards and grass-dominated swards that were fertilized had increased crude protein contents. Drought stress did not induce major changes of neutral and acid detergent fibre contents. Water-soluble carbon contents were larger in drought stressed swards than in controls. The results of the first two chapters, and mainly the apparent importance of grass performance for productivity and quality, led to a more thorough investigation on how the grass functional group was influenced by forbs and legumes. The third chapter examines how the forage quality of grasses (not whole swards) was shaped by the presence and biomass share of the forb and legume functional groups. Grass quality was not generally modified by the presence of other functional groups, but effects were visible in combination with drought stress and N fertilization. Grass crude protein and water soluble carbohydrate contents were modified by the presence of dicots, while effects on fibre contents were negligible. Facilitation by legumes and competition for N with forbs were identified as the main drivers of these influences on grass quality parameters. Grasses with a good N supply had a competitive advantage over dicots, leading to large resistance of grasses against drought stress. The results showed that grasslands had a high resistance and resilience towards drought stress. This resulted in stable yields and stable, if not increased forage quality during and after drought stress. Sustainable management has to consider N availability. N availability in grassland can be assessed via functional diversity as well as N fertilization level. Therefore, functional diversity is a helpful concept for sward management. The biomass share of the grass functional group largely determines productivity and forage quality. It can be influenced by managing functional diversity, and thus the forb and legume share, as well as by directly managing the N availability by fertilization. Depending on functional diversity and N availability status, future drought stress might pose no threat to the yield and quality of grassland forage