Local landscape morphology controls soil temperature and moisture dynamics at an alpine treeline ecotone

Global warming is particularly pronounced in mountainous alpine regions like the Swiss Alps, with consequences on local to global ecosystems. Within alpine regions, the climatically sensitive treeline ecotone is situated between the timberline, where the forest canopy is connected, and the unvegetated alpine zone. This ecotone is comprised mostly of shrubs and grasses, with smaller trees. The treeline ecotone is thus characterized by marked small-scale spatial variability in landform, rock, soil, and vegetation, making it challenging for generalizing and modelling landscape changes. In this regard, highly resolved spatial and temporal landscape assessment is of utmost importance in assessing the response of such sensitive, yet, dynamic ecotones to global warming.

Here, we investigate how the amount of solar radiation and temporal extent of snow cover influence soil temperature and moisture at two topographical positions: (i) depression and (ii) ridge. We hypothesized that topographical features, as well as soil composition are key factors influencing soil moisture dynamics, and thermal exchange. These two sites were selected within a single landform on a glacially shaped alpine meadow to minimize the effect of other ecosystem factors that were not of interest to this study. Geophysical measurements were used to characterize the subsurface structure of the landform between the two sites. A soil profile up to a depth of 80 cm at the depression and 50 cm at the ridge was opened, described and sampled. Each profile was further equipped with microclimate sensors for in-situ measurements of soil temperature, moisture, and matric potential over a period of one and a half years. The profile soil samples were analyzed for texture, porosity, and organic matter content.

The results indicated that the extent of snow cover shapes the dynamics of soil temperature and moisture.  The duration of snow cover was substantially influenced by local topography, as observed in snow persisting for four weeks longer in the depression compared to the ridge during summer. This, in turn, affected soil thermal behavior and contributed to a longer growing season on the ridge than in the depression. Temperature and moisture variability were more pronounced on the ridge, with soil temperature interquartile ranges of 0.2°C to 2.4°C in the depression and 0.3°C to 5.4°C on the ridge, highlighting greater temperature variability on the ridge. Similarly, soil moisture content showed unexpected patterns, with a median of 0.38 m³ m⁻³ in the depression and 0.46 m³ m⁻³ on the ridge. This result contrasts with expectations based on the higher clay and silt content in the depression, which typically promotes moisture retention, and merits further examination.

Our findings highlight the critical influence of snow cover and topography on soil temperature and moisture dynamics within the alpine treeline ecotone. Unexpectedly higher moisture levels on the ridge location and pronounced thermal variability emphasize the need to account for localized soil and microclimatic interactions. These results underscore the challenges in generalizing ecosystem responses to climate change and the importance of small-scale assessments in sensitive alpine landscapes.