Assessment of terrain sensitivity on high plateaux: a novel approach based on vegetation and substrate characteristics in the Scottish Highlands

Background: High plateaux in the Scottish Highlands are vulnerable to disturbance and erosion, but there is a lack of quantitative measurements of terrain sensitivity.

Aims: To apply new quantitative methods to assess the sensitivity of such terrain to physical stress.

Methods: We investigated two components of the mechanical properties of the terrain on 10 plateaux underlain by several different lithologies: the tensile strength of the vegetation mat and underlying root zone, and the shear and compressional strengths of the substrate.

Results: Significant differences in tensile strength occur amongst plant communities, but there is also large within-site and between-site variation for particular communities. A significant component of such variability is attributable to the proportional representation of co-dominant species within communities, and inter-site variability is partly explained by substrate granulometry: particular communities exhibit lower strength characteristics when rooted in sandy substrates derived from coarse-grained lithologies than the same communities on silt-rich soils derived from fine-grained lithologies.

Conclusions: Terrain sensitivity to physical stress is conditioned by the interaction of vegetation and substrate characteristics. Generally, grass-dominated (particularly Nardus-dominated) communities tend to be most robust, and communities dominated by bryophytes and prostrate Calluna vulgaris are typically most sensitive. We identify a continuum of substrate strength: peat is the most sensitive substrate type, followed by other organic-rich soils, sandy matrix-supported substrates and silt-rich matrix-supported substrates. Clast-supported substrates and openwork blockfields are the most robust substrate types. Because the near-surface layers of mineral substrates are weakest, erosion is likely to remove these to expose the underlying robust but sterile clast-supported layers, altering soil status and inhibiting plant recolonisation on eroded substrates.