Spatial scale, plant identity and management effects on the diversity-productivity relationship in a semi-natural grassland

Present study was aimed at examining the effects of spatial scale, plant identity and management on the relationship between diversity and productivity in an old semi-natural grassland in the Solling uplands, Germany. The study was conducted in the framework of the Grassland Management (GrassMan) experiment which is a part of the Excellence cluster „Functional Biodivesity Research“ at the University of Goettingen. The experimental field is a Lolio-cynosuretum semi-natural permanent grassland with more than a hundred-year old history of extensive agricultural use. The three experimental factors (sward composition, fertilization and cutting frequency) results in 12 different treatments and are set in Latin Rectangle, comprising 6 replications of each treatment. Experimental approach that we used, the so called „removal experiment“, is aimed at studying the effects of removal itself and recovery of the vegetation after disturbance, as well as the different aspects of ecosystem functioning In the first chapter we investigate the effects of sampling scale on the relationship between species diversity and productivity. So far, many observational studies, conducted in semi-natural grasslands, explored the relationship between species diversity and productivity at the common size of vegetation surveys of 1 m² or larger, selected according to the species minimum areal. Experimental studies, on the other hand, referred to the small-scale effects of diversity and productivity relationship which often caused the problem of extrapolating and generalizing of their results to more natural plant communities. We studied the effects of spatial scale on the biomass production and diversity relationship by selecting four spatial scales: small (0.04 m² and 0.16 m²), medium (1 m²), large (9 m²), and very large (225 m²) and comparing the power of this relationship, including the effects of agricultural management. We found that the relationship between diversity and productivity of a semi-natural grassland differed across the scales of sampling and that harvesting of the biomass at small spatial scales did not always fully reflect the relationship between the two variables (which often turned into insignificant at larger spatial scales). The most common size of plots for vegetation surveys, being 1 m², in this study showed high variation, both in vegetation composition and harvested biomass. Management system established at the field seemed to play a role in the direction of this relationship (positive or negative). So, plots cut three times a year, becoming more homogeneous (more even) in vegetation composition showed a positive relationship between diversity and productivity. We suggest that selecting an appropriate spatial scale is therefore very important in heterogeneous natural grasslands, also those agriculturally managed. While in more homogeneous environments rather small spatial scale is adequate for describing the composition and many aspects of ecosystem functioning, in more heterogeneous habitats it is important to include this parameter in the analysis. In the second chapter we present the results of a study on the effects of management intensification in a permanent grassland and the response of overall and dominant species diversity. A removal experiment in the Solling uplands, Germany (three sward types: control, dicot-reduced and monocot-reduced) employed four different levels of management intensity resulting from a combination of two factors: fertilization (no and 180-30-100 kg ha -1 year -1 of N-P-K, respectively) and cutting frequency (cut once and three times a year). This study was conducted over two years (2010, 2011), starting with a third year after introducing the management treatments. We defined species diversity by species number per plot as well as evenness and identified dominant species, making up about 80% share of the yield. We collected information on several plant functional traits for each of the dominant species: plant height, leaf dry matter content, stem dry matter content, leaf specific area, green leaves / total leaves ratio, stem specific density, and calculated additionally the ratio of stem specific density and plant height. Further measures of functional diversity included functional group shares, functional diversity index, defined as the total branch length of the traits-species cluster diagram, and aggregated plant functional traits for each plot. We found that management intensification did not affect the total species number, but affected species evenness and functional diversity of dominant species, including their number and identity. Correlations of above-ground biomass and several dominant species’ traits were responsible for fertilization effects on above-ground productivity in this grassland. This indicates the importance of monitoring not only species richness but also other measures of diversity, as well as including management aspects in studies of plant functional traits in grasslands. In the third chapter of the thesis we present the results from the whole investigation period and summarize the findings of the GrassMan experiment regarding the relationship between species richness and productivity, as well as the changes in species number over time and the main determinants of productivity. We analyzed the overall effects of species diversity expressed in species number, functional group composition and species identity effects on the above-ground biomass production. We found that the effects of species richness on the productivity were rather weak while the functional group diversity was a better predictor of productivity in some years. Intensifying the management, however, caused higher above-ground biomass production. It also affected species composition and evenness: increasing cutting frequency increased the evenness while increasing fertilization decreased it. We suggest that functional group richness might be important for better use of available resources. We conclude that existing species composition under appropriate agricultural management seems to have a potential for sustainable forage production without significant species losses, when not used and fertilized too intensively, and without the need of being converted to arable land or manipulating the species composition. The changes in species diversity should, however, be monitored, including not only species number but also other parameters, such as vegetation composition and functional group shares. We finally discuss that our findings do not necessarily support the evidence from experimental studies on sown grasslands which often found that species richness had a defining role in biomass production. While overall species richness was of relatively less importance than management in this grassland, species composition was changing beyond just the number of species. We thus underline the importance of bringing biodiversity experiments to the „real-world“ ecosystems and suggest that thorough consideration of spatial aspects of the diversity-productivity relationship, as well as incorporating multiple measures of diversity in the experiments, conducted in agricultural grasslands under appropriate management strategies, might give better insights in their functioning and serve as motivation for farmers to conserve existing species diversity. Apart from the number of important ecosystem functions, providing fodder for herbivores and ruminants, conserving natural vegetation composition contributes to delivering further ecosystem services, which could support cultural and biodiversity benefits of the agricultural landscapes.