General diversity patterns and possible observer effects
With an average of 47 species per 4 m2 plot, the subalpine grasslands in our research area can be considered very species rich, compared with the average of 43 species in 10 m2 subalpine grassland plots of comparable subalpine grassland in Switzerland (see Elyno-Seslerietea class in GrassPlot Diversity Explorer: https://edgg.org/databases/GrasslandDiversityExplorer; N = 350; Biurrun et al. 2021). However, we found an overall increase in species numbers over time in all treatments, which can likely be attributed to an observer training effect. In a review of observer effects in vegetation sampling, Morrison (2016) reported this common phenomenon that prior experience and training can increase the precision in detection and identification of species during vegetation surveys, which in turn can result in higher species numbers when a plot is surveyed by an expert than by a beginner. As mean ecological indicator values have been shown to be relatively robust to various sampling-related shortcomings, including pseudoturnover, i.e. observer differences in species lists, and training effects (Futschik et al. 2020; Boch et al. 2022), we consider these values the most reliable estimate of ecological changes including temporal shifts in vegetation composition, particularly in species-rich vegetation types like in our experiment on Schynige Platte. Nevertheless, we believe that our approach of comparing changes in species numbers between treatments over time gives a reliable representation of changes because the training effect can be considered being equivalent among treatments.
Response of plant species richness, mean ecological indicator values, and species composition to abandonment and modified haymaking methods
After six years, both of the treatments combining traditional haymaking and scraping tended to have positive effects on species richness and habitat quality, as indicated by the changes in mean ecological indicator values. We are aware that six years is a rather short experiment duration to find clear treatment effects under alpine climatic conditions with a very short vegetation period. Although the vegetation in the abandoned and only scraped plots, i.e. plots with no biomass removal, changed very obviously visually, our vegetation analysis revealed few effects of the treatments on the vegetation composition. Thus, we found only few transitions among vegetation types. However, traditional haymaking combined with scraping in autumn did promote the development towards the originally present vegetation type, while the abandoned and only scraped plots showed negative developments, pointing towards a gradually degrading status of the vegetation.
In Western Europe, most grasslands below the alpine zone are secondary grasslands, originating from timber harvesting, forest clearing to establish arable fields, or grazing of woodlands (Boch et al. 2020). Without humans, the temperate to warm and humid climate in Western Europe would lead to the development of different forest types, which would cover most of the region as potential natural vegetation. In contrast, only about 2% of the area below the timberline would constitute natural grasslands that could persist without human interventions on azonal and extrazonal sites (Bohn et al. 2004; Boch et al. 2020). The abandonment of traditional land use has therefore been identified as one of the major threats for grassland plant biodiversity, particularly in secondary grasslands of temperate regions (Visconti et al. 2018; Dengler et al. 2020). With abandonment, successional dynamics lead to the promotion of a small number of competitive and often dominant plant species. Increasing competition for light can in turn hinder the germination and establishment of light-demanding grassland species and cause the competitive exclusion of small-statured, specialized and often threatened species in the sub-canopy, leading to decreasing plant species richness (Maurer et al. 2006; Deák et al. 2016; Valkó et al. 2018; Boch et al. 2019a; Zehnder et al. 2020; Elliott et al. 2023). In calcareous grasslands in the UK, van den Berg et al. (2011) found increased mean nutrient indicator values after land-use extensification. Similarly, Boch et al. (2019a) found abandonment and wood encroachment to be positively related to mean nutrient indicator values and negatively related to the proportion of specialist species. These patterns were also reflected in our findings: in comparison to the treatments combining traditional haymaking and scraping, abandonment tended to negatively affect species richness and increase mean nutrient and competition indicator values. This points to a loss of habitat quality and a shift in the plant community composition (Landolt et al. 2010; Küchler et al. 2018). In addition, the vegetation change towards tall-grass communities after land-use abandonment could increase the probability of natural hazards on steep slopes, such as landslides and (until shrub encroachment) avalanches (Tasser et al. 2003; Stöcklin 2007). Thus, our results suggest that land use by humans is essential to maintain the high diversity in these subalpine grasslands, to retain their cultural heritage, and to prevent natural hazards (MacDonald et al. 2000; Maurer et al. 2006; Stöcklin 2007).
In the subalpine grasslands on Schynige Platte, traditional land use consisting of one cut per year in autumn (by scythe) seems to be inadequate for maintaining the original species richness and composition, as species numbers have decreased and the vegetation community has changed towards more nutrient-rich grasslands during the last decades (Hegg and Schaffner 2012). This might be due to altered environmental conditions, such as climate change and nitrogen deposition. For instance, climate warming might lead to species becoming dominant because the conditions are more suitable for them or to the establishment of new dominant species as upward range shifts occur (Steinbauer et al. 2018). Besides climate warming, nitrogen deposition has been suggested to have had strong effects on species composition and turnover in Swiss grasslands in the past decades (Roth et al. 2019). During the six years of our experiment, we found little evidence of detrimental effects of traditional land use on habitat quality, as indicated by the non-significant effects on the mean ecological indicator values, and the even positive developments in comparison to the abandonment treatment.
Regarding species richness, we found a stronger increase in the plots combining haymaking and scraping than in the other treatments. Scraping can be seen as a moderate top-soil disturbance, removing remove more biomass and nutrients than haymaking alone, reducing inter-specific competition to some degree, and creating open patches which can in turn promote seed germination and species establishment. However, the increase in species numbers in this treatment cannot be attributed to the establishment of ruderal species that are favored by disturbance as we found no significant differences among the treatments in the mean indicator values for ruderality. In line with our findings, Müller et al. (2014) and Klaus et al. (2018) found higher species richness after experimental top-soil disturbance than in undisturbed controls in German grasslands. In addition, as indicated by the development of the mean ecological indicator values in our study, traditional haymaking combined with scraping tended to have positive effects on habitat quality, towards nutrient-poor and high-light conditions that are suitable for specialized species. Although to a lesser degree, this positive development was also visible in the species composition, as haymaking and scraping in autumn promoted development towards vegetation composition which has been attributed to the Caricion ferrugineae alliance, i.e. the originally present vegetation type. However, as the vegetation period is rather short in subalpine grasslands and vegetation responses are therefore slow, our results might have been more pronounced if the treatments had been maintained for a period longer than six years. Furthermore, whether such a disturbance measure needs to be applied over several years or only once when e.g. litter and moss cover has been built and whether a similar development could also be achieved by e.g. extensive grazing combined with haymaking, which would be even more practical in some cases, needs to be investigated in future studies.