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

Madeleine L Combrinck,Thomas M Harms,Janine Schoombie,Melodie A Mcgeoch,Peter Christiaan Le Roux

doi:10.1007/s11104-020-04665-3

Madeleine L Combrinck, Thomas M Harms + Show 3 more

Open Access

https://doi.org/10.1007/s11104-020-04665-3

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Abstract

AimsThe 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.MethodsComputational 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.ResultsAirflow 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.ConclusionThis 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.

Highlights

Strong and prevailing winds can have considerable effects on the growth and survival patterns of individual plants
Airflow modelling showed that both wind scouring and seed deposition of the commonly cooccurring grass Agrostis magellanica are greater on the windward side of the plant
Even though the compact and hemispherical cushion growth form is considered an adaptation to windy conditions (Hauri 1912) (see (Hauri and Schröter 1914) and (Spomer 1964)), there has been surprisingly little research examining the impact of wind on cushion plants (e.g. (Ashton and Gill 1965; Fitzgerald and Kirkpatrick 2017; Lynch and Kirkpatrick 1995; Whitehead 1951))

Summary

Introduction

Strong and prevailing winds can have considerable effects on the growth and survival patterns of individual plants. The cushion growth form may reflect responses to other stressors, such as higher ultraviolet radiations and lower temperatures, which are prevalent at higher altitudes (Ruffier-Lanche 1964; Ruthsatz 1978). In these abiotically-stressful environments individual cushion plants of the same species can exhibit a range of different shapes, as they grow larger (Huntley 1972; Pyšek and Liška 1991), despite a circular circumference (i.e. a hemispherical shape) being optimal for heat and moisture retention (Gauslaa 1984), see (Zotz et al 2000). The presence of rocks and other plants can cause irregular growth of cushions (Pyšek and Liška 1991), while cushions established on

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