Anthropocene trajectories of high alpine vegetation on Mont-Blanc nunataks

ABSTRACT

Climate warming causes dramatic glacier retreat and intense vegetation changes in alpine regions. High-elevation nunataks, that is bedrock islands protruding from glaciers with uppermost flowering plants, are no exception. Yet the consequences of climate change on nunatak vegetation remain relatively unexplored. Here, we report findings from a re-visit of historical plant surveys carried out on six nunataks situated between 2180 m a.s.l. and 3509 m a.s.l. amidst the glaciers of the Mont-Blanc range (France). We compared vegetation surveys conducted in 2020 to those made 150 years before, and performed remote sensing analyses to depict changes in vegetation productivity during recent decades. We report an increase in plant species richness for the lowest and least isolated nunataks, which contributed to a strong signal of vegetation greening over the last 35 years. This trend was due to the upward migration of competitive species, but also due to species colonization from neighboring high alpine areas into recently unglaciated microsites. We also highlight striking ecological trajectories that have been little discussed so far, such as stable vegetation composition in the highest and most isolated nunataks, an increase of plant species associated with non-permanent snow-cover and water run-off, an increase of graminoids at lower elevations, and of phanerophytes at higher elevations. We argue that high alpine vegetation changes are not driven by the sole upward migration of lowland competitive species, and that careful monitoring of ongoing ecological changes over broad elevation gradients is necessary to better understand the rapid transformation of high alpine landscapes.

KEYWORDS:

• Historical botany
• high alpine sky-islands
• extreme environments
• colonization-extinction dynamics
• range shifts
• Mont-Blanc range

Acknowledgments

The authors are grateful to Colin van Reeth, for his participation and help during the expedition to the Mer de Glace basin. CD acknowledges the Ecrins National Park for supporting original research outside its territory. The authors thank Matthieu Lefebvre, curator of the Herbarium of the Grenoble Museum of Natural History (GRM), and Rebecca Bilon, its director, for the reception and conservation of all herbarium samples of the present study. The authors are also grateful for assistance provided by the caretakers of the Grands Mulets, Couvercle and Leschaux huts.

Disclosure statement

No potential conflict of interest was reported by the authors.

Author contributions

CD had the original idea of the study; CD, BZC, and SL designed the study; NB, SL and CD performed the statistical analyses; BZC and AB performed the remote sensing analyses; CD, BZC, and SL wrote the first draft of the manuscript. CD, BZC, NB and SL contributed to the sample collection. CD coordinated and mounted the reference herbarium collection. The project as a whole was coordinated by CD and SL.

Additional information

Funding

This work was supported by the long-term research site Zone Atelier Alpes, a member of the LTSER network. The project participants were paid by the CREA Mont-Blanc (BZC), the CNRS (SL), the UGA (AB, CD) and the PNE (CD)