Raphael von Büren (Basel University)
Vegetation survey at a wind-exposed microsite (2805 m asl), with view to the Rhone Valley (Valais, Switzerland). Foto: Raphael von Büren
Raphael von Büren's Master thesis was awarded with the Prize for the Best Plant Science Master Thesis, which was carried out at Basel University in the year 2021
Title: Contrasting habitat requirements of the abundant alpine tussock graminoids Carex curvula and Nardus stricta
Winter frost (due to missing snow cover) and different frost resistance explain the occurrence of Carex curvula and Nardus in the alpine belt. This extends the understanding of the fundamental niche of the two most abundant graminoid species in the Alps.
Aim: “Where and why does a species exist” represents a fundamental question in plant ecology. Nevertheless, the actual physiological range limits of alpine plant species remain largely unexplored. I aim at identifying the cold range limits of the two most abundant tussock-forming graminoid species on acidic soils above the climatic treeline in the European Alps: Carex curvula ssp. curvula (Cyperaceae) and Nardus stricta (Poaceae).
Methods: Soil temperatures (-3 cm) and other environmental variables were measured in situ at high spatial-temporal resolution, resulting in 115 well-characterized microsites (thermal conditions, snow cover duration, soil chemistry, vegetation composition, Landolt indicator values). By combining these observational data with freezing resistance (electrolyte leakage, tetrazolium vital staining, regrowth capability) at 38 of these microsites, I tried to explain the two graminoids’ distribution mechanistically.
Results: Carex and Nardus clearly segregated along different microsites. Neither soil chemistry (pH, C/N-ratio, phosphorus), nor growing degree hours and duration of the growing season (thus snowmelt date) played a decisive role. The occurrence was strongly affected by low soil temperature minima in winter. Carex occurred at sites with and without protecting snow cover and resisted low soil temperatures (-13 °C). Nardus was absent at microsites with short snow cover duration and soil minimum temperatures below -5 °C. Electrolyte leakage analyses unveiled higher freezing resistance of Carex leaves (mean LT50: -16.1 °C) compared to Nardus leaves (-13.3 °C) during the growing season. Towards the end of the growing season, foliar hardening was observed in both species. Tetrazolium vital staining revealed higher freezing resistance in young (belowground) shoots of Carex compared to Nardus, with shoot apices tolerating lowest temperatures. However, a vital shoot apex alone cannot ensure regrowth after winter. Crucial were intact vascular tissues (phloem, xylem) and roots, all less tolerant to freezing than apical tissues.
Main conclusions: Cold range limits were defined by thermal extreme values (sharp thresholds for survival) and not through gradual effects of soil temperature (thermal growth constraints). The study highlights the importance of incorporating (I) ground-truth microclimatic data in topographically diverse alpine environments as well as (II) a species’ freezing resistance to explore its survival limit at the cold edge of the fundamental niche.
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Raphael von Büren conducted this work at the Department of Environmental Sciences in the Physiological Plant Ecology PPE research group (Prof. Dr. Ansgar Kahmen) under the supervision of Dr. Erika Hiltbrunner.
> to image tetrazolium-stained shoots of both grasses