Glacier forelands are among the most rapidly changing landscapes on Earth. Stable ground is rare as geomorphic processes move sediments across large areas of glacier forelands for decades to centuries following glacier retreat. Yet, most ecological studies sample exclusively on stable terrain to fulfil chronosequence criteria, thus missing potential feedbacks between geomorphic disturbances and vegetation colonization. By influencing vegetation and soil development, such vegetation-geomorphic disturbance feedbacks could be crucial to understand glacier foreland ecosystem development in a changing climate. We surveyed vegetation and environmental properties, including geomorphic disturbance intensities, in 105 plots located on both stable and unstable moraine terrain in two geomorphologically active glacier forelands in New Zealand and Switzerland. Our plot data showed that geomorphic disturbance intensities changed permanently from high/moderate to low/stable when vegetation reached cover values around 40%. Around this cover value, species with response and effect traits adapted to geomorphic disturbances dominated. This suggests that such species can act as ‘biogeomorphic’ ecosystem engineers that stabilize ground through positive feedback loops. Across floristic regions, biogeomorphic ecosystem engineer traits creating ground stabilization, such as mat growth and association with mycorrhiza, are remarkably similar. Non-metric multidimensional scaling revealed a linked sequence of decreasing geomorphic disturbance intensities and changing species composition from pioneer to late successional species. We interpret this linked geomorphic disturbance-vegetation succession sequence as ‘biogeomorphic succession’, a common successional pathway in unstable river and coastal ecosystems across the world. Soil and vegetation development were related to this sequence, and only advanced once biogeomorphic ecosystem engineer species covered 40–45% of a plot, indicating a crucial role of biogeomorphic ecosystem engineer stabilization. Different topoclimatic conditions could explain variance in biogeomorphic succession timescales and ecosystem engineer root traits between the glacier forelands. As glacier foreland ground is widely unstable, we propose to consider glacier forelands as ‘biogeomorphic ecosystems’ in which ecosystem structure and function are shaped by geomorphic disturbances and their feedbacks with adapted plant species, similar to rivers and coasts.

In the Mueller glacier foreland, we surveyed 55 plots with 125 species in total in February 2018, using the smartphone-based VegApp (www.vegapp.de) to note field observations. In the Turtmann glacier foreland, we surveyed 50 plots with 121 species in total in July and August 2015 using a paper-based survey. We visually determined individual species cover, as well as tree, shrub, herb, bare ground and total cover (Appendix S1: Table S2). Our survey focused on vascular plants, however, we included two prominent Racomitrium moss species with dominant cover (up to 90%) in the Mueller glacier foreland (cf. Burrows 1973). In the Turtmann glacier foreland, moss cover was < 10% and not included in further analyses. Nomenclature follows Lauber and Wagner (2018) for Turtmann and Breitwieser et al. (2010) for the Mueller glacier foreland.

Based on our observations of visible landforms and active sediment transport, we noted which geomorphic processes (gullying, debris flow, interrill erosion, debris sliding, wind erosion, wash, cryoturbation, solifluction) disturb or recently disturbed plot vegetation (Fig. 1c, Table 1). Using known magnitude and frequency for identified geomorphic processes in general (Rapp 1960, Flageollet 1996, Otto and Dikau 2004) and on moraine slopes (Curry et al. 2006, Woerkom et al. 2019), we assigned a geomorphic disturbance intensity to each plot based on the most intense geomorphic process that disturbed the plot (Table 1; Eichel et al., 2016). Thereby, geomorphic disturbance intensities became comparable across glacier forelands.