Directionality in sediment patterns associated with a keystone plant species on Marion Island

Abstract

In recent years, an ongoing awareness of the importance of multidisciplinary research has lead to a large number of studies exploring the ways in which biotic and abiotic systems interact1. Amongst these studies, the field of biogeomorphology focuses on the interactions between species and geomorphologic landforms and processes2. On Marion Island an important interaction exists between the geomorphology of fellfield landscapes and the dominant vascular plant species, Azorella selago 3 4. For example, by influencing frost creep and other sediment movement processes, Azorella cushions play an important role in the formation of terraces and lobes3 4. Such interactions between plants and sediment movement, resulting in vegetation-banked terraces and lobes5 6 and sediment sorting7 are well-known from cold-climate regions. To be able to predict landscape evolution, an in-depth understanding of the interactions between this keystone species and the surrounding sediment is necessary. Here, to understand the consequences of Azorella plants for substrate movement and sorting, we quantified the grain size distribution of sediment surrounding Azorella cushions, using a combination of image analysis software. Mean grain size and variance in grain size was significantly greater on the upslope side of Azorella plants, while the number of particles was significantly higher on the downslope side of plants. Mean grain size, variance in grain size and the number of particles differed between cushions, although these differences were not related to cushion size. These results highlight the role of Azorella cushions in interacting with and shaping the surrounding landscape. Understanding these interactions between Azorella and the landscape is especially important in the light of recent climate change on the island8 9, as biogeomorphologic relations are predicted to be affected by climate change.