Sustainable grassland herbage production under drought stress - the role of plant species number and functional group composition

Abstract

Producing grassland herbage with high yield and good nutritive value is a prerequisite of an efficient ruminant livestock production. Due to climate change, precipitation patterns are expected to vary and the frequency of extreme weather events like droughts are supposed to increase. Productive grassland requires a particularly sufficient and regular water supply during the growing season. Thus, grassland production, yield stability and nutritive value will be affected by drought. Because of this, adaptation strategies are necessary to ensure a sustainable future herbage production from grassland. Increasing plant biodiversity has been proposed as a way to improve ecosystem functions like productivity and nutritive value in grassland. There is an ongoing discussion on species richness' reactions to stress, especially drought stress, and on how productivity, nutritive value and water utilization are affected. Other investigations have stressed species identity and composition of functional groups as important factors for productivity and nutritive value. We, therefore, conducted a drought stress experiment in a vegetation hall from July 2009 to June 2011. The climatic conditions followed the normal seasonal pattern with frost in winter and higher temperatures in summer. Different drought stress conditions were performed over three periods in two growing seasons. Drought stress was induced by temporarily ceasing the watering of the containers after initial watering while soil water availability could be controlled. We chose productive agricultural species of temperate grasslands. Plants were sown in monoculture and three- and five-species mixtures and included the three functional groups legume (Trifolium repens L.), grass (Lolium perenne L., Dactylis glomerata L.) and forb (Plantago lanceolata L., Taraxacum officinale F.H. Wigg. agg.). This range of species number has been shown to affect productivity in biodiversity experiments. Effects of species richness and functional composition on yield, yield stability, water utilization and nutritive value (crude protein, water-soluble carbohydrates, neutral detergent fibre and acid detergent fibre) were examined. As indicators for the performance of yield and particularly agronomic water use efficiency (relation of yield and water use), we used nitrogen yield and concentration of swards as well as signatures of δ13C under not limited water supply and drought. Furthermore, in 2009, we conducted a short term field experiment at the experimental farm of the University of Goettingen at Relliehausen on old grassland. Here, influence of drought and species richness on yield and water utilization were also investigated. Our data show that drought stress lowered production and influenced water utilization. Both increased with the severity of the stress. Agronomic water use efficiency was unchanged or slightly increased under moderate stress but decreased under strong drought stress. Nitrogen yield and concentration were suitable indicators for agronomic water use efficiency while δ13C was less appropriate. Agronomic water use efficiency was increased by nitrogen. There was no or minor immediate impact of drought on nutritive value. Seasonal effects on nutritive value were more distinct. Generally, yield decrease of grassland herbage seems to be more important than changes in nutritive value. Species richness did not influence nutritive value or yield stability over the growing season. The occasional positive influence of species richness on yield and agronomic WUE and decrease of the influence under drought could be explained by a sampling effect. Well performing but drought sensitive legume increased with species richness. There was a hint that species richness also increased the rapidity of water use. Results of the field experiment support these data concerning drought effects, yield and water utilization. The “insurance hypothesis” that species richness can stabilize ecosystem functioning against environmental changes could not be confirmed. But functional groups composition of swards was an important determinant of performance in both unlimited water supply and drought stress. Especially legume had a positive influence on yield, agronomic water use efficiency and crude protein concentration but increased water use and seasonal variability. Grass stabilized yield and water use and increased water-soluble carbohydrates and fibre components while yield and agronomic water use efficiency decreased under the nitrogen limited conditions of our experiment. The functional group forb showed quite similar results regarding yield and water utilization but increased crude protein. Our results indicate that the predicted increase of droughts will reduce production and lead to a change in water utilization in productive grassland. Alteration in nutritive values will not be as important as decrease in yield. The investigated species richness seems to be less important than functional composition of grassland swards for production, water utilization and nutritive value. Thus, to cope with future climate change, an adapted sward composition might be one possibility to ensure ruminant livestock production from grassland.