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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. [1] 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.
Six families, including seven species, of cushion plants occur in Tasmania, and one additional family and species occur on Macquarie Island. Three additional families, including seven species, can adapt to the bolster habit, although not exclusively. The cushion plant growth habit is linked with drainage and the plant's ability to redirect water flow due to its growth pattern. [2]
Family | Scientific name [3] | Common name [3] | Description |
---|---|---|---|
Asteraceae | Abrotanella forsteroides | tasmanian cushionplant | Endemic to Tasmania. Leaves are bright green, crowded, thick and sheathing from the base to a pointed tip. Flowers are a tiny white daisy held just above the leaves. [4] |
Pterygopappus lawrencei | sage cushionplant | Endemic to Tasmania. A compact sage blue/green cushion; leaves are tiny, thick and closely overlapping, broadly spathulate with pointed tips and a silky, hairy base. Flowers are a terminal single white daisy held just above foliage. [4] | |
Caryophyllaceae | Colobanthus pulvinatus | cushion cupflower | A compact, rigid cushion plant with stiff, prickly leaves with thickened margins, 2 – 5 mm long. Flowers are solitary with yellow/green cup-shaped sepals tucked in the foliage. [4] Listed as rare under the Threatened Species Protection Act 1995. [5] Also found in NSW. [6] |
Donatiaceae | Donatia novae-zelandiae | snow cushionplant | Bright green, hard and compact cushion. Leaves are linear, fleshy and pointed, 5 – 6 mm long, with fine hairs at the base. Solitary flowers have 5 white petals and grow at the same level as the leaves. Also found in New Zealand. [4] |
Epacridaceae | Dracophyllum minimum | claspleaf heath | Endemic to Tasmania. Densely branched, compact, hard, mounding cushion. Leaves are thick and rigid, broad, sheathing and slightly concave with a hard point. White, solitary tubular flowers sit directly on top of foliage. [4] |
Scrophulariaceae | Veronica ciliolate Previous name: Chionohebe ciliolate | Ben Lomond cushion plant | Endemic to Tasmania. Soft, compact, rounded cushion. Leaves are light green and in dense basal rosettes, 2.5 – 4 mm long, 2 mm wide, pointed, with a tuft of silky hairs at tip. Flowers have 5 white petals and two groups of stamen fused to the petal tube. [4] Veronica ciliolate subsp. Fiordensis is listed as vulnerable under both the Threatened Species Protection Act 1995 and the Environment Protection and Biodiversity Conservation Act 1999. [7] |
Stylidiaceae | Phyllachne colensoi | Yellow cushionplant | A dense, tufted cushion plant with a distinct yellowish colour. Leaves are wider at the base and narrow to a blunt tip with a glandular pore, 3 – 4 mm long, densely packed around the stem. Flowers are white, single and terminal with 5-6 spreading lobes. The column is held above the petals. Also found in New Zealand. |
Apiaceae | Azorella macquariensis | Macquarie cushions | Endemic and restricted to Macquarie Island. Leaf has reniform outline, with 3 or 5 lanceolate lobes, they are free to the base, acute and pungent, with a setose point. Flowers are bisexual and solitary; two leaf-like lanceolate bracts are fused at the base to form a cup. [8] Listed as endangered under the Threatened Species Protection Act 1995 and critically endangered under the Environment Protection and Biodiversity Conservation Act 1999. [9] |
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.
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. [8] 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. [1] This effect is due to their low and compact growth form that alleviates severe alpine conditions. [1]
Several Tasmanian cushion species have been found to have a surface temperature up to 10 °C greater than the ambient temperatures. [10] 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. [10] 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.
Three Tasmanian cushion plant species are listed as "under threat": Veronica ciliolata (subspecies fiordensis), Azorella macquariensis and 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:
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:
Colobanthus pulvinatus is listed as rare under the Threatened Species Protection Act 1995. [5]
Macquarie Island is an island in the southwestern Pacific Ocean, about halfway between New Zealand and Antarctica. Regionally part of Oceania and politically a part of Tasmania, Australia, since 1900, it became a Tasmanian State Reserve in 1978 and was inscribed as a UNESCO World Heritage Site in 1997.
Bolster heath or cushion moorland is a type of vegetation community that features a patchwork of very low growing, tightly packed plants found at the limits of some alpine environments. The cushion plants form a smooth surfaced 'cushions' from several different plants, hence the common name of cushion heath. The cushion growth habit provides protection against the desiccating wind and help keep the cluster warm.
Nothofagus gunnii, the tanglefoot or deciduous beech, is a deciduous shrub or small tree endemic to the highlands of Tasmania, Australia. It was described in 1847 by R.C Gunn N. gunnii is a small woody tree with a shrubby appearance known to grow up to 8 metres (26 ft). It lives only on mountains due to temperature limitations within the Tasmanian maritime climate and mainly grows at altitudes greater than 800 metres (2,600 ft) above sea level. It grows in alpine and sub-alpine regions in the central portions of the island. Though capable of reaching the size of a small tree, it is most common as a thick shrub or woody ground cover, hence its common name of "tanglefoot".
Diselma archeri is a species of plant of the family Cupressaceae and the sole species in the genus Diselma. It is endemic to the alpine regions of Tasmania's southwest and Central Highlands, on the western coast ranges and Lake St. Clair. It is a monotypic genus restricted to high altitude rainforest and moist alpine heathland. Its distribution mirrors very closely that of other endemic Tasmanian conifers Microcachrys tetragona and Pherosphaera hookeriana.
Athrotaxis cupressoides, commonly known as pencil pine, despite being a species of the family Cupressaceae and not a member of the pine family. Found either as an erect shrub or as a tree, this species is endemic to Tasmania, Australia. Trees can live for upwards of 1000 years, sustaining a very slow growth rate of approximately 12 mm in diameter per year.
Azorella selago is a species of cushion plant native to the sub-Antarctic islands of the Southern Ocean, including the Crozet Islands, the Possession Islands, the Heard Island and McDonald Islands, the Kerguelen Islands, and the Prince Edward Islands. The closely related Azorella macquariensis, which is endemic to Macquarie Island, was split from it taxonomically in 1989. A. selago is often a keystone species where it occurs and is well studied for its contribution to its native ecosystems.
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, 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.
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.
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.
Feldmark, also spelt fjaeldmark, is a plant community characteristic of sites where plant growth is severely restricted by extremes of cold and exposure to wind, typical of alpine tundra and subantarctic environments.
Orites revolutus, also known as narrow-leaf orites, is a Tasmanian endemic plant species in the family Proteaceae. Scottish botanist Robert Brown formally described the species in Transactions of the Linnean Society of London in 1810 from a specimen collected at Lake St Clair. Abundant in alpine and subalpine heath, it is a small to medium shrub 0.5 to 1.5 m tall, with relatively small, blunt leaves with strongly revolute margins. The white flowers grow on terminal spikes during summer. Being proteaceaous, O. revolutus is likely to provide a substantial food source for nectivorous animal species within its range.
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.
Gonocarpus montanus is a perennial, terrestrial herb in the family Haloragacae. It is native to N.S.W, Victoria, Tasmania and New Zealand. Common names include mountain raspwort and mat raspwort. Its synonym is Haloragis montana.
Astelia alpina called pineapple grass, silver astelia, or perching lily is a commonly found species in alpine and subalpine areas of Tasmania and the Australian Alps. It is a perennial herb that typically dominates its environment by growing in dense clusters, called mats, in alpine bogs. There are two subspecies: Astelia alpina var. novae hollandiae from New South Wales and Victoria and Astelia alpina var. alpina endemic to Tasmania. Both subspecies appear very similar to each other. The species was originally described by Robert Brown.
The vegetation in Tasmania's alpine environments is predominately woody and shrub-like. One vegetation type is coniferous shrubbery, characterised by the gymnosperm species Microcachrys tetragona, Pherosphaera hookeriana, Podocarpus lawrencei, and Diselma archeri. Distribution of these species is relevant with abiotic factors including edaphic conditions and fire frequency, and increasingly, the threat of climate change towards species survival exists. Conservation and management of coniferous shrubbery are necessary considering that the paleoendemic species, Microcachrys,Pherosphaera and Diselma, have persisted in western Tasmanian environments for millions of years.
Pherosphaera hookeriana, or Mount Mawson pine, is a dwarf conifer endemic to Tasmania, at altitudes above 600 meters. There are roughly 30 known sites, with population numbers in the tens of thousands. The species occurs in a range of habitats typically in areas near water bodies, mostly on dolerite derived soils. The species is highly fire sensitive and an increase in fire events associated with climate change may lead to local extinction and fragmentation of habitat.
Abrotanella forsteroides, commonly known as the Tasmanian cushion plant, is an angiosperm endemic to Tasmania, Australia. The plant is a dicot species in the daisy family Asteraceae and can be identified by its bright green and compact cushion like appearance.
Abrotanella scapigera is an endemic angiosperm of Tasmania, Australia. It is a member of the family Asteraceae, commonly found in alpine regions of northwest and south-central Tasmania. This species is named after its characteristic sparsely leaved flowering stem that distinguishes it from the other 18 species of the Genus.
Olearia ledifolia, commonly known as rock daisy bush, is a species of flowering plant of the family Asteraceae. It is endemic to Tasmania and found at higher altitudes where it grows as a low, compact bush with tough, leathery leaves and small white and yellow daisy-like "flowers" in summer.
Plantago gunnii is endemic to Tasmania and is commonly referred to as bolster plantain or bolster plantago. The etymology of the genus Plantago comes from the Latin words planta and the suffix –ago, which is represented by the leaves pressed flat against the ground. The species name gunnii is in honour of Ronald Campbell Gunn of Launceston, who first discovered the species.