Cushion plant

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Silene acaulis, moss campion Silene acaulis - Mosscampion.jpg
Silene acaulis , moss campion

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. [1] 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. [2]

Contents

Description

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. [2] [3]

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. [3] [4] 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. [5]

Ecology

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. [2] [3]

A cushion plant growing on Mount Ossa, Tasmania. Cushion-plant-atop-Mount-Ossa.jpg
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 can photosynthesize. 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 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. [7]

The cushion plant may have flowers that are large and showy for such a small perennial, or sometimes hundreds of small flowers. [6] 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. [3]

Diversity

Donatia novae-zelandiae Donatia nz.jpg
Donatia novae-zelandiae

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. [2] [3] [8] [9]

Related Research Articles

<span class="mw-page-title-main">Alpine tundra</span> Biome found at high altitudes

Alpine tundra is a type of natural region or biome that does not contain trees because it is at high elevation, with an associated harsh climate. 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.

<span class="mw-page-title-main">Fellfield</span>

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.

<span class="mw-page-title-main">Yareta</span> Species of plant

Yareta or llareta is a velvety, chartreuse cushion plant in the family Apiaceae which is native to South America. It grows in the Puna grasslands of the Andes in Peru, Bolivia, northern Chile and western Argentina at altitudes between 3,200 and 5,250 metres.

<span class="mw-page-title-main">Alpine plant</span> Plants that grow at high elevation

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 taxa 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, wind, drought, poor nutritional soil, and a short growing season.

<i>Silene acaulis</i> Species of flowering plant in the carnation family Caryophyllaceae

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 and in high mountains of Eurasia and North America. It is an evergreen perennial flowering plant in the carnation family Caryophyllaceae.

<i>Diapensia lapponica</i> Species of flowering plant

Diapensia lapponica, the pincushion plant, is a plant in the family Diapensiaceae, the only circumboreal species in the genus Diapensia, the others being mainly in the Himalaya and on mountains in southwestern China. This species likely became circumboreal-circumpolar [Arctic–alpine] after it jumped to arctic habitat from North China and Russia. The most likely candidate for ancestor is a white-flowered D. purpurea The plants grow on exposed rocky ridges that are kept free from snow by high winds. Diapensia lapponica is extremely slow and low-growing and cannot compete with plants that overtop it. The plant is very sensitive to higher temperatures and so is often in misty foggy habitat. It usually dies when transplanted to lowland gardens and so this is not recommended. Cold-treated or wild and winter-collected seed will germinate indoors. The seed and leaves are high in lipids.

<span class="mw-page-title-main">Rosette (botany)</span> Botany term for a circular arrangement of leaves

In botany, a rosette is a circular arrangement of leaves or of structures resembling leaves.

<span class="mw-page-title-main">Hydrosere</span>

A hydrosere is a plant succession which occurs in an area of fresh water such as in oxbow lakes and kettle lakes. In time, an area of open freshwater will naturally dry out, ultimately becoming woodland. During this change, a range of different landtypes such as swamp and marsh will succeed each other.

<span class="mw-page-title-main">Arctic–alpine</span>

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.

<span class="mw-page-title-main">Sierra Nevada subalpine zone</span>

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.

<i>Donatia novae-zelandiae</i> Species of flowering plant

Donatia novae-zelandiae is a species of mat-forming cushion plant, found only in New Zealand and Tasmania. Common names can include New Zealand Cushion or Snow Cushion, however Snow Cushion also refers to Iberis sempervirens. Donatia novae-zelandiae forms dense spirals of thick, leathery leaves, creating a hardy plant that typically exists in alpine and subalpine bioclimatic zones.

<i>Pterygopappus</i>

Pterygopappus is a genus of flowering plants in the tribe Gnaphalieae within the family Asteraceae. 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.

<i>Azorella macquariensis</i> Species of flowering plant

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.

<span class="mw-page-title-main">Montane ecosystems</span> Ecosystems found in mountains

Montane ecosystems are found on the slopes of mountains. The alpine climate in these regions strongly affects the ecosystem because temperatures fall as elevation increases, causing the ecosystem to stratify. This stratification is a crucial factor in shaping plant community, biodiversity, metabolic processes and ecosystem dynamics for montane ecosystems. Dense montane forests are common at moderate elevations, due to moderate temperatures and high rainfall. At higher elevations, the climate is harsher, with lower temperatures and higher winds, preventing the growth of trees and causing the plant community to transition to montane grasslands, shrublands or alpine tundra. Due to the unique climate conditions of montane ecosystems, they contain increased numbers of endemic species. Montane ecosystems also exhibit variation in ecosystem services, which include carbon storage and water supply.

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.

<span class="mw-page-title-main">Tasmanian cushion plants</span>

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 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.

<span class="mw-page-title-main">Flora of the Sierra Nevada alpine zone</span>

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.

<i>Abrotanella forsteroides</i> Species of plant

Abrotanella forsteroides, commonly known as the Tasmanian cushion plant, is an endemic angiosperm of Tasmania, Australia. The plant is a dicot species of the daisy family Asteraceae and can be identified by its bright green and compact cushion like appearance. 

<i>Oreobolus pumilio</i> Species of herb

Oreobolus pumilio, commonly known as alpine tuftrush or Ibrang`rank, is a small mat forming herb which is distributed throughout the Australasian region. It is a relative of the sedge. It is often found in cushion plant communities, in alpine environments, where it is a dominant species. As a cushion plant, it is an ecological engineer and enables other species to grow in the alpine herblands to which it is native to.

References

  1. ↑ Malcolm, Bill; Nancy Malcolm (1988). New Zealand's Alpine Plants Inside and Out. Wellington, NZ: Kel Aiken Printing Company. pp. 61–68. ISBN   0-908802-04-8.
  2. 1 2 3 4 Went, F. W. (1971). Parallel evolution. Taxon, 20(2/3): 197-226.
  3. 1 2 3 4 5 Badano, 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 a Silene acaulis growth curve in the Canadian Rockies. Arctic and Alpine Research, 24(1): 50-55.
  5. ↑ Ralph, C. P. (1978). Observations on Azorella compacta (Umbelliferae), a tropical Andean cushion plant. Biotropica, 10(1): 62-67.
  6. 1 2 Alatalo, 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 a Donatia–Astelia cushion 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.
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