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

Madeleine Combrinck, Thoman Harms, Melodie McGeoch, Janine Schoombie, Peter le Roux

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)
13 Downloads (Pure)

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.
Original languageEnglish
Pages (from-to)339-366
Number of pages28
JournalPlant and Soil
Volume455
Issue number1-2
Early online date29 Aug 2020
DOIs
Publication statusPublished - 1 Oct 2020

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