Proglacial geomorphic disturbances drive the spatial distribution of the rare alpine species Trifolium Saxatile All.

Glacier retreat has increased in the European Alps during the last four decades. This affects proglacial geomorphic processes and the spatial distribution of plants. We assessed how the type and the magnitude of geomorphic disturbance control the spatial distribution of a rare alpine plant species: Trifolium saxatile All., whose main habitats are proglacial forelands and alluvial bars and banks, within the Glacier Blanc et Glacier Noir catchment area (Ecrins National Park, French Alps). Based on field observations during the summers of 2020 and 2021, we produced a map combining geomorphological deposits and presence-absence of T. saxatile to detect which geomorphological units are relevant for our study. We determined the magnitude and frequency of superficial sediment reworking between the 2020 and 2021 summers using Radio Frequency IDentification experimentation on moraines slopes and glacio-fluvial deposits on which populations of T. saxatile were observed. Abundance and functional traits (stem height, length and number of inflorescences) were also measured. To analyze the potential impacts of geomorphic disturbances on T. saxatile presence, abundance and intra-specific trait variability, generalized linear mixed models (GLMM) and factor analyses of mixed data (FAMD) were used. Results showed heterogeneity in the geographical distribution of T. saxatile within the proglacial landscape. Clovers preferentially established over three geomorphological units: (i) nearly stabilized lateral moraines, (ii) active braided floodplains and (iii) the exposed young alluvial bars. It was totally absent in purely gravitational landforms (e.g. scree, talus cones). The largest individuals of T. saxatile with many inflorescences were mostly found within the active torrential channel where geomorphic disturbances are intense and frequent but this was however balanced by a low abundance of individuals. On lateral moraines and alluvial terraces, which are almost stabilized and exhibit a lower level of geomorphic disturbances than the active braided floodplain (in terms of magnitude and frequency of movement of RFID tracked sediment particles), individuals were much more abundant, but they were smaller and with less inflorescences compared to the active torrential channel. This study shows the importance of geomorphic processes as key drivers of the spatial distribution, and potentially traits expression, of alpine species living in glaciated catchment areas.