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= Cushion Plant Communities at Mount Field National Park, Tasmania =

The term 'cushion plant' refers to a group of plants that form ground-hugging mats. Generally, a cushion mat consists for three or four different species. Cushion plants can occur in both alpine and subalpine habitats. This article concentrates on the cushion plants found at Mount Field National Park, Tasmania.



Description
Cushion plants are a common part of alpine and subalpine vegetative habitats in Tasmania. Plants can grow either individually or form mosaics composing of more than one species. Data collected from Newdegate Pass, Tasmania suggests that cushion plants are slow growing and may occupy the same vegetative site for a long time period. The current cushion community at Newdegate Pass is estimated to be at least around 850 years old.

Diversity
There are seven species of true cushion plants found on Mt Field, Tasmania, with six found on Newdegate Pass. Three of the seven species are found in the family Asteraceae and then one each in Donatiaceae, Epacridaceae, Loganiaceae and Stylidiaceae. Certain monocots such as Oreobolus pumilio, Lepidosperma inops, Carpha rodwayi, Centrolepis monogyny and Gaimardia fitzgeraldi, while not true cushion plants, have cushion forms. Other plants such as Erigeron trigonus, Sprengelia incarnata, Drosera arcturi, Epacris serpyllifolia, Montitega dealbata and Pentachondra pumila are found amongst cushion species and appear to benefit from cushion ecosystem functions.

Ecology
Due to their mosaic growth pattern, one square metre may contain hundreds to thousands of individual plants.

Ecosystem Function
Cushion species provide a number of key ecosystem functions.

Firstly, certain cushion species which form dense mounds can be considered a keystone species as they are necessary for the germination of other plants, serving as seed beds.

Secondly, they can be seen as nurse plants and can affect the abundance of non-cushion plants. In certain areas, non-cushion species have been found at a greater abundance compared to cushion species in cushion areas, when compared to open areas. This data suggests that many non-cushion species depend upon cushion species in order to survive the harsh climate in which they reside.

Thirdly, certain Tasmanian cushion species have been found to have a surface temperature up to 10 degrees Celsius higher than the surrounding environment, indicating that they potentially play a role in allowing surrounding plants to survive in the same condition.

Evolution
Due to the restricted evolution of organisms in alpine environments, there are only a few ways to evolve, which satisfy both biological and physiological demands. Cushion species evolved as a result of convergent evolution. Convergent evolution is a phenomenon whereby organisms from different lineages come to resemble on another, and is generally the result of inhabiting similarly harsh environments, such as the wind-trimmed regions of alpine areas.

Threats
Mount Field National Park is protected under the Tasmanian Wilderness World Heritage Area. However, this does not stop natural threats such as fire and disease or the accidental trampling via a hikers foot. The greatest threat to alpine communities is fire, with many (including cushion plants) taking several hundred to several thousand years to regain their former community to its previous state.