DOI: 10.5061/dryad.7d7wm3803
title: Similar vegetation-geomorphic disturbance feedbacks shape unstable glacier forelands across mountain regions

description: 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.
description: 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 <em>Racomitrium</em> moss species with dominant cover (up to 90%) in the Mueller glacier foreland (cf. Burrows 1973). In the Turtmann glacier foreland, moss cover was &lt; 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.