Why sediments matter for mountain biogeomorphology

In mountain regions, present and future climate warming affects the spatio-temporal distribution, magnitude and frequency of geomorphic processes and vegetation response. Mountain biogeomorphology investigates the feedback between vegetation and geomorphic components and dynamics in alpine landscapes. Though sediment properties are known to provide important controls on vegetation colonization, species composition and provision of mountain microhabitats, they often receive considerably less attention than landform types and geomorphic processes. In this study, we aim to (i) elucidate biogeomorphic relationships between sediments and vegetation across landforms, (ii) identify key sediment and linked soil properties in mountain biogeomorphic systems and (iii) develop a workflow for routinely including sedimentological observations into mountain biogeomorphic research. We do so by investigating sediment granulometry and chemistry at four localities in the Swiss Alps, each dominated by different geomorphic processes (debris flow, soil erosion, solifluction, rock glacier creep), and integrating these data with vegetation information. Using multivariate statistics, we show that sediment properties, geomorphic processes and species composition are closely related across landforms, with different vegetation classes on stable terrain, coarse carbonate-rich sediments, active solifluction and active soil erosion. Our data suggest that these relationships can be explained by previously described geomorphic activity-related thresholds for biogeomorphic feedbacks, as well as possible thresholds related to sediment texture, with coarse grain sizes limiting plant colonization and biogeomorphic feedbacks. Sediment properties matter most for vegetation where geomorphic disturbances are low or absent. Based on our results, we propose that the median grain size of both coarse (>2 mm) and fine (≤2 mm) sediments, bulk P, K, Fe, Ca and N concentrations and total organic carbon (TOC) are key sediment and linked soil variables for mountain biogeomorphic research. We propose a workflow to effectively incorporate sediment data into mountain biogeomorphology and suggest directions for further research into sediment-vegetation interactions.