CUSHION PLANTS – MORE THAN JUST A PRETTY FACE
Tasmanian alpine plants are an unusual and interesting mix. One noteworthy group is the cushion plants, which occur above the tree-line on many Tasmanian mountains. Believe it or not, cushion plants look like flattened cushions hugging the ground, although if you go to sit on one you will discover why some people prefer the name ‘bolster heaths’ as the surface is quite hard and spiky. Mainland Australia has no true alpine cushion plants, but in Tasmania there are half a dozen species (this photo shows Abrotanella forsteroides - click for full image,50kb). Sometimes these grow as separate cushions but often different species intermingle to form a mosaic that from above, looks like a colourful road map.The cushion surface is formed by a method of growth whereby every stem elongates and produces new leaves at the same rate and it is impossible to tell which leaves belong to which branch. Indeed, the stems are hidden beneath the densely packed foliage of tiny (~5 mm long) leaves.Because the leaves are so densely packed the branches inside are protected from wind, snow and ice, and the core of the plant retains a fairly constant temperature. This form of growth is so useful in coping with the harsh and unpredictable weather experienced at higher altitudes that four species from four different plant families have converged in their evolution to such an extent that it is very difficult to tell them apart when they are not flowering. This closeup shows Pterygopappus lawrencei (pale green) and Abrotanella forsteroides (dark green - click for full image,58kb). Summer reveals that even in flower Donatia novae-zelandiae, Dracophyllum minimum and Phyllachne colensoi are in some ways more similar to each other than to other species within their own genera. The flowers that they produce are tiny, being only five or six millimetres long and a couple of millimetres wide, so a hand-lens will be useful for identifying them.A single cushion plant, which can cover several square metres of ground and be up to half a metre high, can be covered in several thousand flowers in mast (peak flowering) years. This makes a spectacular display, with the different species flowering sequentially from December to March. Despite the small size of each individual flower the overall effect - a white carpet – is appealing to photographers and enticing to insects looking for a good food source. Bugs (Hemiptera), beetles (Coleoptera), flies (Diptera) and grasshoppers (Orthoptera) visit cushion plant flowers to feed. When dipping their heads into the flowers it is likely that pollen gets caught around the insect’s mouthparts and legs, from where it may be transferred to another flower to allow cross-pollination. However, most of the pollen will be dropped on flowers of the same plant, as the insect moves over a single cushion to feed, rather than wasting energy flying or walking to another plant. It would appear that beetles in particular are important pollinators of Tasmanian alpine cushion plants, which is interesting because beetles have very seldom been reported as important alpine pollinators in other parts of the world. So, next time you are walking on a mountain in Tasmania, keep an eye out for the hard green carpets called cushion plants, try to distinguish a few different species and see if you can spot a wandering beetle or two.
[Source; C. Corbett, Pollination Ecology in a Tasmanian Alpine Environment, BSc Honours thesis, 1995. School of Geography and Environmental Studies, University of Tasmania, Australia. Photos courtesy C. Corbett and A. Dudley.]