Conservation of semi-natural grasslands : Effects of different management methods on biodiversity
Abstract: Species-rich semi-natural grassland are valuable habitats in the agricultural landscape as they may contain a high diversity of both plant and animal species, as well as provide essential ecosystem services like pollination. To keep these habitats open and to maintain the biodiversity in them, management like grazing or mowing is necessary. Due to changed agricultural practices many semi-natural grasslands have been lost, e.g. due to secondary succession after abandonment or use of more intense management practices. As limited resources are available for the management and restoration of semi-natural grasslands, research is needed to find the best available management method that maintains biodiversity at a low cost. Therefore, the overall aim of this thesis was to use existing data to compare effects of different management methods and explore their effect on the biodiversity of semi-natural grasslands. More specifically, effects of grazing vs. mowing, different mowing frequencies and different mowing techniques were investigated using data from Swedish, European and global studies, with the aim of using the results to formulate management recommendations.Paper I investigated the effect of mowing using a sickle bar mower or a grass trimmer at a 5-cm or 0-cm cutting height on plant indicator species of good management for species richness, poor management and excess nitrogen. Analyses of data from a Swedish 11-year field experiment revealed that over time, there was no difference in the odds of finding indicators of any type for any of the mowing techniques. This suggest that there is a similar effect of mowing using a sickle bar mower and a grass trimmer on the floristic composition of semi-natural grasslands.Paper II and III investigated effects of different management methods using a Swedish long-term, multi-site field experiment. Paper II compared effects of grazing vs. mowing and paper III compared effects of annual mowing and mowing every third year. Both papers used meta-analyses to compare effect on plant indicator species of good management for species richness and poor management. Additionally, paper II examined effects of indicators of excess nitrogen, while paper III also examined effects on plant species richness and diversity and species-wise responses. For paper II, analyses revealed an increase in the odds of finding indicators of good management for species richness in mowed plots and increased odds of finding indicators of excess nitrogen in grazed plots. For paper III, meta-analyses revealed a small drop in plant species richness and diversity, and an increase in woody and tall-grown species, when mowing every third year. However, there were no difference in the odds of finding indicators of good management for species richness or poor management between mowing frequencies. These results suggest that mowing is a better long-term management method compared to grazing in seminatural grasslands in southern Sweden, while annual mowing have a slightly more positive effect compared to mowing every third year.Paper IV and V investigated management effects on a larger scale, by systematically reviewing studies comparing effects of grazing vs. mowing or different mowing frequencies on grassland flora and fauna, and analysing the data using meta-analysis. The included mowing frequencies were annual mowing vs. mowing every fifth, third or second year, or mowing two, three or four times a year. Paper IV analysed data from 35 studies from across the globe, and revealed a slightly more positive effect of grazing on the biodiversity of seminatural grasslands. However, results differed for different grassland characteristics like e.g. geography, grassland type and previous management. Paper V analysed data from 29 European studies, and revealed a more positive effect of mowing more frequently than once per year. However, the differences were small and varied for different grassland characteristics. For example, a higher mowing frequency was better in more productive grasslands. These results suggest that grazing is slightly better than mowing and that mowing less frequently, e.g. every other year, can be a viable management method in less productive grasslands, as a way of using limited funds more efficiently.The results from the papers included in this thesis suggest that different management methods have different effects on the biodiversity of semi-natural grasslands. However, in many cases these differences are relatively small. Thus, to a certain extent one of the most important factors for semi-natural grassland biodiversity seems to be the fact that management occurs at all, while the actual management methods is of less importance. Hence, choosing the most suitable management method for a certain situation should be based on conservation aims, grassland conditions, the previous management practice used, the cost of management, the availability of different management methods and previous research.
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