Long-term vegetation change in semi-natural grasslands – disentangling trends from pseudo-turnover

Abstract

Grasslands cover a significant part of the Earth’s surface and support a large proportion of its biodiversity. Despite their importance, species-rich grasslands have been severely decimated and degraded over the past century. Resurvey studies can provide the necessary knowledge of current developments in grasslands to enable effective conservative action. Many insightful resurveys have been conducted in recent decades, however, grassland ecosystems have been under-represented in these studies. Data collected during fieldwork often contains a significant amount of noise due to inaccuracies in the data collection, inappropriate study design, and random variation in species composition. In the methodological part of this thesis, we aim to contribute to disentangling signal from noise in resurvey studies. Specific resurvey studies in Nardus grasslands and calcareous grasslands were conducted. We introduced the concept of “baseline turnover” to measure the extent of vegetation changes that occur regularly due to stochastic fluctuations in species composition. It is an important factor to consider when attempting to distinguish signal from noise in studies of vegetation change. With an article on the statistical phenomenon of “regression to the mean”, we aimed to raise awareness of this potential pitfall in the statistical analysis of resurveys and longitudinal experiments. A resurvey of Nardus grasslands in western Germany confirmed the trend of soil pH increase in acidic soils. However, the widespread eutrophication of Nardus grasslands was only weakly pronounced. We believe that this is due to the exceptionally consistent management of annual mowing over the previous 30 years. The initial survey of calcareous grasslands in Lower Saxony, Germany, confirmed that particularly warm and dry microclimates have had a positive effect on the characteristic species in the past. In the resurvey, the vegetation showed clear signs of the severe drought that had occurred. This can be interpreted as a warning that sites that previously favoured typical calcareous grassland species may develop conditions that are too extreme in the future.