A cushion plant is a compact, low growing, mat forming plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

 that is found in alpine

Alpine climate

Alpine climate is the average weather for a region above the tree line. This climate is also referred to as mountain climate or highland climate....

, subalpine

Subalpine

The subalpine zone is the biotic zone immediately below tree line around the world. Species that occur in this zone depend on the location of the zone on the Earth, for example, Snow Gum in Australia, or Subalpine Larch, Mountain Hemlock and Subalpine Fir in western North America.Trees in the...

, arctic, or subarctic

Subarctic climate

The subarctic climate is a climate characterized by long, usually very cold winters, and short, cool to mild summers. It is found on large landmasses, away from the moderating effects of an ocean, generally at latitudes from 50° to 70°N poleward of the humid continental climates...

 environments around the world. The term "cushion" is usually applied to wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

y 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

Parallel evolution

Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...

 or convergent evolution

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

 with species from many different plant families

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 on different continents converging on the same evolutionary adaptations to endure the harsh environmental conditions.

Description

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

Apical dominance

In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets....

 that terminate in individual rosettes

Rosette (botany)

In botany, a rosette is a circular arrangement of leaves, with all the leaves at a single height.Though rosettes usually sit near the soil, their structure is an example of a modified stem.-Function:...

. Each stem grows at a consistent rate so that no one rosette is more exposed than the rest of the cushion. Observations on senescence

Senescence

Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...

 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 (0.0236220472440945 in) to 1.82 cm (0.716535433070866 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

Peru

Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

 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.

Ecology

Cushion plants commonly grow in rapidly draining rocky or sandy soils in exposed and arid

Arid

A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...

 subalpine, alpine, arctic, subarctic or subantarctic

Subantarctic

The Subantarctic is a region in the southern hemisphere, located immediately north of the Antarctic region. This translates roughly to a latitude of between 46° – 60° south of the Equator. The subantarctic region includes many islands in the southern parts of the Indian Ocean, Atlantic Ocean and...

 feldmark

Feldmark

Feldmark, also spelt fjaeldmark , is a plant community characteristic of sites where plant growth is severely restricted by extremes of cold and by exposure to wind, typical of alpine tundra and subantarctic environments.-Description:...

 habitats. In certain habitats, such as peat

Peat

Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

y fen

Fen

A fen is a type of wetland fed by mineral-rich surface water or groundwater. Fens are characterised by their water chemistry, which is neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients...

s or bogs, cushion plants can also be a keystone species

Keystone species

A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and...

 in a climax community

Climax community

In ecology, a climax community, or climatic climax community, is a biological community of plants and animals which, through the process of ecological succession — the development of vegetation in an area over time — has reached a steady state. This equilibrium occurs because the climax community...

. As such, the plants are often colonizers of bare habitat with little or no soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

. Due to their role as initiators of primary succession

Primary succession

Primary succession is one of two types of biological and ecological succession of plant life, occurring in an environment in which new substrate devoid of vegetation and usually lacking soil, such as a lava flow or area left from retreated glacier, is deposited...

 in alpine habitats, the plants have specific adaptations to the desiccation and mechanically harsh environment of windy alpine slopes.
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 in the limited season where enough warmth and sunlight are available for photosynthesis, but may begin this cycle prior to the snow melting. During the summer or warm months or weeks, the plant's form is well adapted to trapping air within its body to extend the time during which it can photosynthesize

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

.

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 small alpine forget-me-not

Myosotis alpestris

Myosotis alpestris or Alpine Forget-me-not is a herbaceous perennial plant of the genus Myosotis.The Alpine Forget-me-not is the county flower of Westmorland in the United Kingdom and the state flower of Alaska....

 may stand only inches above the ground, but its taproot

Taproot

A taproot is an enlarged, somewhat straight to tapering plant root that grows vertically downward. It forms a center from which other roots sprout laterally.Plants with taproots are difficult to transplant...

 can 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 the epidermis

Epidermis (botany)

The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...

, reducing the rate of water loss. Additionally, many cushion plants have small and fleshy leaves which reduce the surface area of the plant, which reduces transpiration

Transpiration

Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

 and conserves water. In alpine environments well above the tree line, cold is a limiting factor for growth, 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. 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.

The cushion plant may have flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

s that are large and showy for such a small perennial. This is necessary to attract pollinator

Pollinator

A pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain...

s over long distances, and in the short season of growth.

Cushion plants have been described as ecosystem engineer

Ecosystem engineer

An ecosystem engineer is any organism that creates or modifies habitats. Jones et al. identified two different types of ecosystem engineers:...

s because of their ability to locally maintain increased moisture and soil temperatures below the cushion ±15 °C (±27 °F) relative to adjacent soil temperatures. Some, specifically Mulinum leptacanthum and Oreopolus glacialis, have been positively identified as species that alter the macronutrient

Nutrient

A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

 concentrations in the soil. These attributes allow other species to more easily colonize the harsh environments that cushion plants inhabit. Species richness

Species richness

Species richness is the number of different species in a given area. It is represented in equation form as S.Species richness is the fundamental unit in which to assess the homogeneity of an environment. Typically, species richness is used in conservation studies to determine the sensitivity of...

 is therefore demonstrably increased where cushion plants have colonized.

Diversity

The cushion plant form is not endemic to any single area or plant family. About 338 species worldwide in 78 genera in areas ranging from Tasmania

Tasmania

Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...

, New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, and Tierra del Fuego

Tierra del Fuego

Tierra del Fuego is an archipelago off the southernmost tip of the South American mainland, across the Strait of Magellan. The archipelago consists of a main island Isla Grande de Tierra del Fuego divided between Chile and Argentina with an area of , and a group of smaller islands including Cape...

 to the arctic tundra of Svalbard

Svalbard

Svalbard is an archipelago in the Arctic, constituting the northernmost part of Norway. It is located north of mainland Europe, midway between mainland Norway and the North Pole. The group of islands range from 74° to 81° north latitude , and from 10° to 35° east longitude. Spitsbergen is the...

 have convergently evolved

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

 the same plant form in response to similar environmental conditions. Thirty four diverse plant families such as Apiaceae

Apiaceae

The Apiaceae , commonly known as carrot or parsley family, is a group of mostly aromatic plants with hollow stems. The family is large, with more than 3,700 species spread across 434 genera, it is the sixteenth largest family of flowering plants...

, Asteraceae

Asteraceae

The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...

, Caryophyllaceae

Caryophyllaceae

The Caryophyllaceae, commonly called the pink family or carnation family, is a family of flowering plants. It is included in the dicotyledon order Caryophyllales in the APG III system, alongside 33 other families, including Amaranthaceae, Cactaceae and Polygonaceae...

, Donatiaceae, and the Stylidiaceae

Stylidiaceae

The family Stylidiaceae is a taxon of dicotyledonous flowering plants. It consists of five genera with over 240 species, most of which are endemic to Australia and New Zealand. Members of Stylidiaceae are typically grass-like herbs or small shrubs and can be perennials or annuals...

include cushion plant species.