Wind and seed: a conceptual model of shape-formation in the cushion plant Azorella Selago

Abstract

Aims The sub-Antarctic cushion plant, Azorella selago, is usually hemispherical when small but frequently crescent-shaped when larger. Spatial variation in wind speed and in air-borne seed and sediment deposition is examined to determine if wind scouring and deposition patterns could contribute to the development of non-hemispherical shapes in cushion plants. Methods Computational fluid dynamic analyses were conducted for hemispherical and crescent-shaped cushion plants parameterizing models with data from A. selago habitats on Marion Island. Numerical data were contextualized with field observations to arrive at a conceptual model for shape development. Results Airflow modelling showed that both wind scouring and seed deposition of the commonly co-occurring grass Agrostis magellanica are greater on the windward side of the plant. By contrast, heavier sediment particles are predominantly deposited on the leeward side of plants, leading to burial of lee-side A. selago stems. This sediment accumulation may initiate the development of the crescent-shape in hemispherical plants by increasing stem mortality on the plant’s leeward edge. Once developed, the crescent-shape is probably self-reinforcing because it generates greater air recirculation (and lower air velocities) which enhances further deposition and establishment of A. magellanica grasses in the lee of the crescent. The conceptual model consists therefore of three stages namely, (1) negligible air recirculation, (2) sediment deposition and grass establishment, and (3) differential cushion growth. Conclusion This conceptual model of plant shape development may explain the occurrence and orientation of crescent-shaped cushion plants and highlights how predicted changes in wind patterns may affect vegetation patterns.

Morphological variability of cushion plant Lyallia kerguelensis (Caryophyllales) in relation to environmental conditions and geography in the Kerguelen Islands: implications for cushion necrosis and climate change

“…Our data indicated lower occurrence of circular cushions on the southeast aspect, less exposed to wind, suggesting that the circular shape might be associated with wind exposure in L. kerguelensis. This finding is in agreement with the recent air flow dynamics model on A. selago where cushions with more pronounced crescent shapes were in habitats with less airflow turbulence (Combrinck et al 2020).…”

“…Azorella compacta (Apiaceae) plants present a larger surface area on the cushion part with northern exposure side which maximizes solar radiation (Kleier et al 2015). (Combrinck 2008;Combrinck et al 2020).…”

Islands in the ice: Potential impacts of habitat transformation on Antarctic biodiversity

“…Another example, at Hurd Glacier, on Livingston Island, found community composition of microbial colonizers differed based on substrate (rock or soil; Garrido‐Benavent et al, 2020). There is also still much to be understood regarding the stages of colonization and community succession, including the role that nurse plants and soil colonizers play in community establishment, and their interplay with environmental drivers (Combrinck et al, 2020; Klanderud, 2005; Le Roux et al, 2013). Currently, ice‐free vegetation is dominated by lichen assemblages, but if these are slow to establish, then other faster species may come to dominate communities.…”

Exposing wind stress as a driver of fine‐scale variation in plant communities

“…Wind stresses and disturbances may vary greatly over short distances (Chiras, 2017; Katsaprakakis & Christakis, 2012), and could therefore potentially contribute to fine‐scale variation in ecological communities. For example, wind velocity can vary around individual plants (Combrinck et al., 2020) and models that do not account for fine‐scale variation in wind patterns do not accurately predict seed dispersal (Lönnell et al., 2015). Variability in wind speed has also, for instance, been related to fine‐scale differences in plant height, species richness, growth form diversity and community type (Whitehead, 1954, 1959; Wilson, 1959).…”

“…In addition, the reciprocal effect of vegetation on wind patterns has also attracted limited attention (although see, e.g. Combrinck et al., 2020).…”