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. 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. 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. 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.

中文翻译：

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风和种子：垫层植物 Azorella Selago 形状形成的概念模型

亚南极垫层植物，Azorella selago，小的时候通常是半球形的，大的时候通常是新月形的。检查风速和空气传播的种子和沉积物沉积的空间变化，以确定风冲刷和沉积模式是否有助于垫层植物中非半球形状的发展。利用马里恩岛 A. selago 栖息地的数据，对半球形和新月形垫层植物参数化模型进行了计算流体动力学分析。数值数据与实地观察相结合，以得出形状发展的概念模型。气流模型表明，在植物的迎风侧，通常共生的草 Agrostis magellanica 的风冲刷和种子沉积更大。相比之下，较重的沉积物颗粒主要沉积在植物背风侧，导致背风侧 A. selago 茎被掩埋。这种沉积物的积累可能通过增加植物背风边缘的茎死亡率来启动半球形植物新月形的发展。一旦形成，新月形状可能会自我增强，因为它会产生更大的空气再循环（和更低的空气速度），从而促进麦哲伦草在新月背风处的进一步沉积和建立。因此，概念模型由三个阶段组成，即（1）可忽略的空气再循环，（2）沉积物沉积和草建立，以及（3）不同的垫层生长。