Responses of Arnica montana L. and montane Nardus grasslands to climate change in Central German Uplands

Species-rich montane Nardus grasslands and their characteristic and highly valued plant species arnica (Arnica montana L.) are two elements of low-intensively-used, species-rich mountain grassland in Central German Uplands. In times of present and future global change, new stressors, like those related to climate change, will come into action, and are predicted to lead to a further decline of Arnica montana and Nardus grasslands. This is of specific concern in lower mountain ranges, where species and habitats are naturally restricted in their altitudinal distribution and where possible refugee areas at higher elevations are limited. As one significant impact of climate change, drought events of different intensity within the growing period of grasslands in spring and summer are projected to be a major threat for semi-natural mountain grasslands and their already endangered plant species, like Arnica montana, that have a pronounced montane distribution. Species and, subsequently, the grassland community need to compensate negative impacts of climate change by adapting to the new environment in their current occurrences and by having a high resilience against these impacts, respectively. However, little is known about the functional trait responses and possible adaptations of the threatened mountain grassland plant species Arnica montana with regard to their intraspecific variability and about the community responses of Nardus grasslands to climate-change-related droughts. In this cumulative PhD thesis present, responses and the adaptability (adaptive plasticity) of Arnica montana via plant functional traits and the community level responses and resilience of Nardus grasslands to drought stress related to climate change were investigated. Chapter 1 investigates changes in the functional trait performance and variability of Arnica montana in its adult life stage, along a climate gradient from lower elevations with higher- to higher elevations with lower summer aridity and in relation to the grassland management. Results showed a strong positive relationship of most functional traits with reduced summer aridity at higher elevations, which indicates a higher trait performance level at montane sites with less summer aridity. The variability of traits decreased steadily with decreasing summer aridity, suggesting less environmental stress for Arnica montana at higher elevations. Management factors, however, had only a small influence on both performance and variability of Arnica’s functional traits. In Chapter 2 I evaluated in a greenhouse experiment the impact of drought-induced stress on Arnica montana seedlings in their early life stage establishment phase. Together with survival, as a closely linked measure for fitness of an individual, above- and below-ground functional traits were identified that, on the one hand, are related to the species’ fitness decline and, on the other hand, may act as early-warning indicators for reduced fitness. Seedlings showed a high resistance to moderate drought, and senescence was higher and consecutive survival declined only under strong and extreme drought conditions. Based on the trait-survival relationships identified, declines in leaf length, leaf width, and leaf number were identified as sensitive response-indicator traits that indicated a fitness decline prior to a substantial increase in mortality. Chapter 3 studies the effects of extreme droughts on the community composition, diversity, Ellenberg indicator scores, and diagnostic species groups in species-rich Nardus grasslands, and investigates the associated mechanisms of vegetation change. Drought effects were investigated in three consecutive years by a rainout shelter experiment with different levels of rainfall reduction. Nardus grasslands responded with significant changes in species abundance and community structures, which also triggered changes in Ellenberg indicator scores and several species groups. Changes were largely driven by the high ambient drought level but occurred with a considerable time lag. Taken together, the findings of this thesis enhance our knowledge about the responses of Arnica montana and montane Nardus grasslands, as a representative mountain grassland type, to climate change, and about their relationship to management strategies. Specifically, Arnica montana has the potential to adapt to altered environmental condition in the short term via changes in plant functional traits, but these changes constitute, at the same time, a lower performance level of fitness-related traits, which can lead to a long-term decline in fitness at the individual and population level. The findings at the community level of Nardus grasslands, moreover, indicate a certain resilience of the community to climate-change-related droughts. Overall, the insights gained from the studies can contribute to the development of climate-change-adapted conservation concepts, and can inform habitat- and species-management measures.