How does biotic weathering work? Influence of alpine plants on rock temperature and rock moisture

Abstract

Rock temperature and moisture are critical factors influencing rock weathering. In alpine environments, these parameters are determined by both macroscale factors, including climate, and microscale factors, including vegetation cover. We investigate the effects of alpine plant species with distinct architectures-among others Dryas octopetala L., Primula auricula L. and Carex firma Scop.-on rock temperature and moisture at rocky limestone slopes. The Arnspitze massif (German/Austrian border) was affected by severe wildfires in the 1940ies so that wide slope areas in the subalpine belt are still characterised by limestone outcrops free from forest cover. Rock temperature and electrical resistivity (as a proxy for moisture) were monitored over three months at hourly resolution, complemented by small-scale electrical resistivity tomography (ERT) and microwave sensing (MW). Bare rock, soil-covered rock (<10 cm), and plant-covered rock with different species were compared. Plant cover was found to reduce the mean daily temperature amplitudes in the rock by 3.2 to 5.2 K and change rates by up to 7 K h−1 compared to uncovered rock. Soil cover effects vary, influenced by soil thickness and microtopographic exposure. Varying rock temperature dynamics are attributed to plant architecture, with shading, canopy heating, decoupling from atmospheric conditions and rock moisture content hypothesized as key factors. Rock moisture increases under soil and plant cover, with reduced evaporation and altered drainage patterns assumed as driving mechanisms. ERT measurements reveal high spatial heterogeneity in rock moisture at the microscale, which is influenced by plant cover, and which is providing favourable sites for vegetation establishment. MW measurements show heightened moisture content under plants at shallow depth (few cm), while with further increasing depth, rock moisture decreases in plant covered rock, suggesting possible plant water uptake with different responses depending on species and functional types. Regarding biotic rock weathering we hypothesize that plant cover generally mediates direct temperature cracking by reducing temperature extremes, but enhances chemo-physical subcritical cracking through increased moisture levels. This underscores how sparse alpine vegetation potentially influences microscale weathering processes.