Native to Africa, Europe, Asia, New Zealand & Australia
Example Australian species:
Scleranthus biflorus is a cushion-bush found in Australia and New Zealand
Other common names include the knawel and two-flowered knawel or twin-flower knawel
A common plant in grassland, particularly at higher altitudes
It may be in the form of a mat or a multi branched, spreading perennial herb
As of 2015, this species was under review, and new taxa were planned
Description
A spreading, moss-like plant with dense, bright green, linear leaves about 4mm long
There are minute, double-headed green flowers in late spring
Propagation can be by division or by seed
The fungus Rhizoctonia can cause dead brown patches in this species
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
Description
Cushion plants form large, low-growing mats that can grow up to 3 m 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 non-photosynthetic 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 to 1.82 cm 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
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 colonisers 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 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 colonise the harsh environments that cushion plants inhabit
Species richness is therefore demonstrably increased where cushion plants have colonized
Diversity
See also: Tasmanian cushion plants
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
Evolution
The cushion plant is an example of convergent evolution
Unrelated species from a number of different families on separate continents have evolved the cushion form to adapt to similarly harsh environments
Ecosystem function
Cushion plants have been shown to be a keystone species in the ecosystem in which they occur
For example, Azorella macquariensis is the only cushion-forming plant that occurs on Macquarie Island and it plays a vital role in the ecosystem as it makes up the major structural component of feldmark vegetation on the island
Cushion plants are also often considered "nurse species", as their presence is necessary in order for many other alpine species to become established
Cushion plants have been found to enhance species richness in areas with inherent impoverished local diversity
This effect is due to their low and compact growth form that alleviates severe alpine conditions
Several Tasmanian cushion species have been found to have a surface temperature up to 10 °C greater than the ambient temperatures
The warmer temperatures allow the plants to continue to photosynthesize and grow in conditions that would otherwise inhibit such productivity
This substantially extends the growing season
In addition, a combination of the insulating properties of past growth under the surface of the cushion plants and the peaty soils in which they grow prevents temperatures around the roots dropping below -1 °C, which stops the roots from freezing
Surrounding plants species have adapted to not only benefit, but rely on these conditions the cushion plants provide to survive and grow, and are restricted to habitats in which cushions are present
Threats
Three Tasmanian cushion plant species are listed as "under threat":
Veronica ciliolata (subspecies fiordensis)
Azorella macquariensis
Colobanthus pulvinatus
Tasmanian endemic species, Veronica ciliolata subspecies fiordensis, is listed as vulnerable under both the Threatened Species Protection Act 1995 and the Environment Protection and Biodiversity Conservation Act 1999.[7] The total population of mature individuals is estimated to be from 1500 to 2000.[11] Threats include:
Ski field expansion and other inappropriate development of the Ben Lomond plateau, in which the population is restricted,
Climate change and an increase in temperatures that may negatively affect this species if suitable alpine habitat within possible dispersal range diminishes
Stochastic events including landslides, drought, pathogens and inadvertent damage. The species' low range has made it particularly vulnerable if such events occur.
Azorella macquariensis, which is endemic to Macquarie Island, is listed as endangered under the Threatened Species Protection Act 1995 and Critically Endangered under the Environment Protection and Biodiversity Conservation Act 1999.[9] Since 2008, this species has encountered substantial dieback and has had a catastrophic decline in population size.[8] The cause of the dieback is unknown and under investigation. Once dead, the cushions may erode and as a result could have adverse impacts on the rest of feldmark vegetation due to its vital structural role in the ecosystem. Potential factors that threaten Azorella macquariensis include:
Feldmark vegetation has been identified to be at risk to a change in climate if temperatures increase, drier soil conditions, less rainfall and increased frosts occur
The introduction of an alien species such as a plant, vertebrate, invertebrate or pathogen, could have detrimental effects on the species, either directly, through infection or competition, or indirectly, through habitat degradation
Colobanthus pulvinatus is listed as rare under the Threatened Species Protection Act 1995
xx
xx
xx
xx
xx
xx
xx
xx
xx
xx