Cushion plant

A cushion plant is a compact, low growing, mat forming plant that is found in alpine, subalpine, arctic, or subarctic environments around the world. The term "cushion" is usually applied to woody plants that grow as spreading mats, are limited in height above the ground (a few inches at most), have relatively large and deep tap roots, and have life histories adapted to slow growth in a nutrient poor environment with delayed reproductivity and reproductive cycle adaptations. The plant form is an example of parallel or convergent evolution with species from many different plant families on different continents converging on the same evolutionary adaptations to endure the harsh environmental conditions.

Cushion plants form large, low-growing mats that can grow up to 3 metres (10 ft) in diameter. The typical form is a compact mass of closely spaced stems with minimal apical dominance that terminate in individual rosettes. Each stem grows at a consistent rate so that no one rosette is more exposed than the rest of the cushion. Observations on senescence have concluded that cushion plants typically die en masse rather than individual rosettes dying at separate times. Underneath the living rosettes, the plants typically produce nonphotosynthetic material or allow previous leaves to die, creating an insulating effect.

Cushion plants grow very slowly. In the case of Silene acaulis, growth rates have been measured at 0.06 cm (0.02 in) to 1.82 cm (0.72 in) per year. Coinciding with this impeded growth is increased longevity, with the largest cushions of some species attaining ages of up to 350 years. A study on Azorella compacta in southern Peru determined that, based on a growth rate of 1.4 mm per year, individual plants in the study area were upwards of 850 years old with occasional specimens approaching 3,000 years old.

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