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 m (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.
Cushion plants commonly grow in rapidly draining rocky or sandy soils in exposed and arid subalpine, alpine, arctic, subarctic or subantarctic feldmark habitats. In certain habitats, such as peaty fens or bogs, cushion plants can also be a keystone species in a climax community. As such, the plants are often colonizers of bare habitat with little or no soil. Due to their role as initiators of primary succession 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 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 can photosynthesize. Cushions at higher elevation are typically smaller and denser.
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 may stand only inches above the ground, but its taproot 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, 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 and 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.
The cushion plant may have flowers that are large and showy for such a small perennial, or sometimes hundreds of small flowers.This is necessary to attract pollinators over long distances, and in the short season of growth.
Cushion plants have been described as ecosystem engineers 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 concentrations in the soil. These attributes allow other species to more easily colonize the harsh environments that cushion plants inhabit. Species richness is therefore demonstrably increased where cushion plants have colonized.
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, New Zealand, and Tierra del Fuego to the arctic tundra of Svalbard have convergently evolved the same plant form in response to similar environmental conditions. Thirty-four diverse plant families, such as Apiaceae, Asteraceae, Caryophyllaceae, Donatiaceae, and the Stylidiaceae, include cushion plant species.
In physical geography, tundra is a type of biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sámi word тӯндар meaning "uplands", "treeless mountain tract". Tundra vegetation is composed of dwarf shrubs, sedges and grasses, mosses, and lichens. Scattered trees grow in some tundra regions. The ecotone between the tundra and the forest is known as the tree line or timberline.
Alpine tundra is a type of natural region or biome that does not contain trees because it is at high elevation. As the latitude of a location approaches the poles, the threshold elevation for alpine tundra gets lower until it reaches sea level, and alpine tundra merges with polar tundra.
A fellfield or fell field comprises the environment of a slope, usually alpine or tundra, where the dynamics of frost and of wind give rise to characteristic plant forms in scree interstices.
A subshrub or dwarf shrub is a short woody plant. Prostrate shrub is a related term. "Subshrub" is often used interchangeably with "bush".
Alpine plants are plants that grow in an alpine climate, which occurs at high elevation and above the tree line. There are many different plant species and taxon that grow as a plant community in these alpine tundra. These include perennial grasses, sedges, forbs, cushion plants, mosses, and lichens. Alpine plants are adapted to the harsh conditions of the alpine environment, which include low temperatures, dryness, ultraviolet radiation, and a short growing season.
Silene acaulis, known as moss campion or cushion pink, is a small mountain-dwelling wildflower that is common all over the high arctic and tundra in the higher mountains of Eurasia and North America,. It is an evergreen perennial.
There are 164 vascular plant species on the Norwegian Arctic archipelago of Svalbard. This figure does not include algae, mosses, and lichens, which are non-vascular plants. For an island so far north, 164 species constitutes an astonishing variety of plant life. Because of the harsh climate and the short growing season, all the plants are slow growing. They seldom grow higher than 10 cm.
In botany, a rosette is a circular arrangement of leaves or of structures resembling leaves.
An Arctic–alpine taxon is one whose natural distribution includes the Arctic and more southerly mountain ranges, particularly the Alps. The presence of identical or similar taxa in both the tundra of the far north, and high mountain ranges much further south is testament to the similar environmental conditions found in the two locations. Arctic–alpine plants, for instance, must be adapted to the low temperatures, extremes of temperature, strong winds and short growing season; they are therefore typically low-growing and often form mats or cushions to reduce water loss through evapotranspiration.
The Sierra Nevada subalpine zone refers to a biotic zone below treeline in the Sierra Nevada mountain range of California, United States. This subalpine zone is positioned between the upper montane zone at its lower limit, and tree line at its upper limit.
Lobelia deckenii is a species of giant lobelia of the mountains of East Africa. It grows in moist areas, such as valley bottoms and moorland, in contrast to Lobelia telekii which grows in a similar but drier habitat. These two species produce occasional hybrids. Lobelia deckenii plants usually produce multiple rosettes. Each rosette grows for several decades, produces a single large inflorescence and hundreds of thousands of seeds, then dies. Because individual plants have multiple rosettes, they survive to reproduce repeatedly, and plants with more rosettes flower more frequently. It is iteroparous.
Hylocomium splendens, commonly known as glittering woodmoss, splendid feather moss, stairstep moss, and mountain fern moss, is a perennial clonal moss with a widespread distribution in Northern Hemisphere boreal forests. It is commonly found in Europe, Russia, Alaska and Canada, where it is often the most abundant moss species. It also grows in the Arctic tundra and further south at higher elevations in, for example, northern California, western Sichuan, East Africa, Australia, New Zealand and the West Indies. In Scotland it is a characteristic species of the Caledonian Forest. Under the UK's national vegetation classification system, pinewood community W18 is named as "Pinus sylvestris-Hylocomium splendens woodland", indicating its significance in this ecosystem.
Pterygopappus is a genus of flowering plants in the pussy's-toes tribe within the daisy family. There is only one known species Pterygopappus lawrencii which is endemic to alpine Tasmania. It forms thick, light blue/green mats with densely packed leaves. It is most common in the mountains of the northeastern part of the island. It is a slow grower and prefers cool, moist environments.
Azorella macquariensis, also known as Macquarie azorella or Macquarie cushions, is a species of cushion plant endemic to Australia’s subantarctic Macquarie Island. It was referred to the more widely distributed Azorella selago until 1989, when it was described as a separate species.
The Southern Andean steppe is a montane grasslands and shrublands ecoregion occurring along the border of Chile and Argentina in the high elevations of the southern Andes mountain range.
Montane ecosystems refers to any ecosystem found in mountains. These ecosystems are strongly affected by climate, which gets colder as elevation increases. They are stratified according to elevation. Dense forests are common at moderate elevations. However, as the elevation increases, the climate becomes harsher, and the plant community transitions to grasslands or tundra.
Draba graminea is a species of flowering plant in the mustard family known by the common names Rocky Mountain draba and San Juan Whitlow-grass. It is endemic to the state of Colorado in the United States, where it is limited to the San Juan Mountains.
Alpine vegetation refers to the zone of vegetation between the altitudinal limit for tree growth and the nival zone. Alpine zones in Tasmania can be difficult to classify owing to Tasmania's maritime climate limiting snow lie to short periods and the presence of a tree line that is not clearly defined.
Tasmanian cushion plants are low growing, highly compact, woody, spreading mats that can grow up to 3 m in diameter, located mainly on the island of Tasmania. These mats are made up of tightly packed stems that grow at the same rate so that no apical rosettes protrude above the rest. The term cushion plant refers to a characteristic growth habit adopted by various species from a range of families in order to adapt to alpine and subalpine environments and areas of high latitude. They are adapted to grow in low nutrient areas and typically have deep taproots. Cushion plants are very slow growing and do not grow high above ground; mounds typically remain under 30 cm high. Underneath the living surface of the cushion, the plants either allow dead leaves to persist or produce non-photosynthetic material, resulting in an insulating effect.
The flora of the U.S. Sierra Nevada alpine zone is characterized by small, low growing, cushion and mat forming plants that can survive the harsh conditions in the high-altitude alpine zone above the timber line. These flora often occur in alpine fell-fields. The Sierra Nevada alpine zone lacks a dominant plant species that characterizes it, so may or may not be called a vegetation type. But it is found above the subalpine forest, which is the highest in a succession of recognized vegetation types at increasing elevations.