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Cushion plant

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Silene acaulis,moss campion

Acushion plantis a compact, low-growing, mat-formingplantthat is found inalpine,subalpine,arctic,orsubarcticenvironments around the world. The term "cushion" is usually applied towoodyplants that grow as spreading mats, are limited in height above the ground (a few inches at most), have relatively large and deeptap roots,and have life histories adapted to slow growth in a nutrient-poor environment with delayed reproductivity and reproductive cycle adaptations.[1]The plant form is an example ofparallelorconvergent evolutionwith species from many different plantfamilieson different continents converging on the same evolutionary adaptations to endure the harsh environmental conditions.[2]

Description[edit]

Cushion plants form large, low-growing mats that can grow up to 3 m (10 ft) in diameter. The typical form is a compact mass of closely spaced stems with minimalapical dominancethat terminate in individualrosettes.Each stem grows at a consistent rate so that no one rosette is more exposed than the rest of the cushion. Observations onsenescencehave concluded that cushion plants typically dieen masserather 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.[2][3]

Cushion plants grow very slowly. In the case ofSilene 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.[3][4]A study onAzorella compactain southernPerudetermined 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.[5]

Ecology[edit]

Cushion plants commonly grow in rapidly draining rocky or sandy soils in exposed andaridsubalpine, alpine, arctic, subarctic orsubantarcticfeldmarkhabitats. In certain habitats, such aspeatyfensor bogs, cushion plants can also be akeystone speciesin aclimax community.As such, the plants are often colonizers of bare habitat with little or nosoil.Due to their role as initiators ofprimary successionin alpine habitats, the plants have specific adaptations to the desiccation and mechanically harsh environment of windy alpine slopes.[2][3]

A cushion plant growing on Mount Ossa,Tasmania.

The establishment of a new cushion plant on a windy slope, or freshly exposed Arctic tundra is not a common event. The established plants may be hundreds of years old, although they extend only a few inches above the surface. The plants are spreading and are wider than they are tall, but they are not extensive above the ground. The plant will grow for many years before it is ready to begin its first reproductive cycle. The plant actively grows only during the limited period when enough warmth and sunlight are available for photosynthesis, but may begin this cycle prior to the snow melting. The plant's form is well adapted to trapping warm summer air within its body to extend the time during which it canphotosynthesize.Cushions at higher elevation are typically smaller and denser.[6]

Plants growing in the alpine or subalpine regions face the challenge of obtaining and retaining water. One solution for obtaining water is the growth of an extensive root system. A smallalpine forget-me-notmay stand only inches above the ground, but itstaprootcan extend for a couple of feet below the soil surface. The long taproot is necessary because of both the limited precipitation in many alpine and arctic environments, mostly as snowfall, and because of the rapid drainage of a newly formed and shallow soil. Besides obtaining water, the plant must also retain moisture to survive in a dry and desiccating environment. The compact growth form of cushion plants reduces air flow over the surface of theepidermis,reducing the rate of water loss. Additionally, many cushion plants have small and fleshy leaves which reduce the surface area of the plant, which reducestranspirationand conserves water. In alpine environments well above the tree line, cold is a limiting factor for growth. So, by having tightly packed stems and foliage, cushion plants are able to convert and trap heat from sunlight, causing them to warm several degrees above the ambient air temperature and extend their short growing season. Many alpine cushion plants also have thick matted hairs that warm up and heat the air trapped in between the hairs when the sun shines. These hairs also act as a greenhouse by preventing the warmer air from rising away from the plant, and they also act as wind breaks, preventing the wind from blowing away the trapped heat.[7]

The cushion plant may haveflowersthat are large and showy for such a small perennial, or sometimes hundreds of small flowers.[6]This is necessary to attractpollinatorsover long distances, and in the short season of growth.

Cushion plants have been described asecosystem engineersbecause of their ability to locally maintain increased moisture and soil temperatures below the cushion ±15 °C (±27 °F) relative to adjacent soil temperatures. Some, specificallyMulinum leptacanthumandOreopolus glacialis,have been positively identified as species that alter themacronutrientconcentrations in the soil. These attributes allow other species to more easily colonize the harsh environments that cushion plants inhabit.Species richnessis therefore demonstrably increased where cushion plants have colonized.[3]

Diversity[edit]

See also:Tasmanian cushion plants
Donatia novae-zelandiae

The cushion plant form is notendemicto any single area or plant family. About 338 species worldwide in 78 genera in areas ranging fromTasmania,New Zealand,andTierra del Fuegoto the arctic tundra ofSvalbardhaveconvergently evolvedthe same plant form in response to similar environmental conditions. Thirty-four diverse plant families, such asApiaceae,Asteraceae,Caryophyllaceae,Donatiaceae,and theStylidiaceae,include cushion plant species.[2][3][8][9]

References[edit]

  1. ^Malcolm, Bill; Nancy Malcolm (1988).New Zealand's Alpine Plants Inside and Out.Wellington, NZ: Kel Aiken Printing Company. pp. 61–68.ISBN0-908802-04-8.
  2. ^abcdWent, F. W. (1971). Parallel evolution.Taxon,20(2/3): 197-226.
  3. ^abcdeBadano, E. I., Jones, C. G., Cavieres, L. A., and Wright, J. P. (2006). Assessing impacts of ecosystem engineers on community organization: a general approach illustrated by effects of a high-Andean cushion plant.OIKOS,115: 369-385.
  4. ^McCarthy, D. P. (1992). Dating with cushion plants: establishment of aSilene acaulisgrowth curve in the Canadian Rockies.Arctic and Alpine Research,24(1): 50-55.
  5. ^Ralph, C. P. (1978). Observations onAzorella compacta(Umbelliferae), a tropical Andean cushion plant.Biotropica,10(1): 62-67.
  6. ^abAlatalo, J.M. and Molau, U. 1995. Effect of altitude on the sex ratio in populations of Silene acaulis. – Nordic Journal of Botany. 15: 251-256. doi: 10.1111/j.1756-1051.1995.tb00150.x
  7. ^Adams, J. M. 2007. Vegetation-climate interaction how vegetation makes the global environment. Berlin: Springer. p. 82.
  8. ^Heusser, C. J. (1995). Palaeoecology of aDonatia–Asteliacushion bog, Magellanic Moorland-Subantarctic Evergreen Forest transition, southern Tierra del Fuego, Argentina.Review of Palaeobotany and Palynology,89: 429-440.
  9. ^Corbett, C. (1995). Pollination Ecology in a Tasmanian Alpine Environment, BSc Honours thesis. School of Geography and Environmental Studies, University of Tasmania, Australia.

External links[edit]

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