Introduction to Australian Orchidaceae

Native Orchid Habitats

Native orchids, with the exception of highly localised species, are rarely habitat-specific and usually extend over a range of habitats. Various factors affect their distribution and influence the habitats they grow in. Although there is often some overlap, for the most part terrestrial orchids occupy quite different habitats to the epiphytes and each group can be considered separately.

Terrestrial Orchid Habitats

Australian terrestrial orchids reach their optimum development in the sclerophyll forests of the southern parts of the continent. Here they occur in a range of forested habitats including wet sclerophyll, open forest, woodland, shrubby forest and coastal scrubs. Many species also occur in heathland and shrubland and some specialised species grow within the canopy of rainforests or along its margins. In some eastern states, grasslands and grassy forests are the favoured habitat of specialised genera, particularly Diuris and Prasophyllum. Similarly other specialised genera such as Burnettia, Epiblema and Hydrorchis occupy swampy habitats (see below).

Influence of moisture: Orchids need free moisture to survive and generally those areas which have moderate to high reliable rainfall support a greater diversity of species and larger orchid populations. All of the 115 terrestrial genera found in Australia occur in coastal or near-coastal localities, whereas only 71 genera extend to inland localities where rainfall is lower and less reliable. Within any habitat some species favour particular microclimates such as those which develop on southern slopes, gullies and near watercourses where mesic conditions predominate in winter and spring. Low-lying sites that are moist to wet for at least part of the year often support a suite of species different from those found on well-drained sites. Permanent swamps and ephemeral swamps or low-lying sites subject to periodic inundation, especially those which are wet in winter and spring, can be a significant habitat for some orchids with the plants sometimes partially submerged at flowering time. In drier inland areas or rainshadow sites orchids are often found growing around the margins of rock outcrops, where the run-off from limited rainfall is concentrated. This is particularly apparent in Western Australia where large granite outcrops are a significant orchid habitat, often surrounded by stunted forest or shrubland where few orchid species survive. Sometimes inland gorges, especially those surrounded by mountainous terrain, have a higher rainfall than adjacent districts and are able to support relict populations of orchids.

Influence of climate: Australian climates are strongly seasonal and exhibit great fluctuations in moisture and temperature. Southern areas commonly have cool mesic conditions between autumn and spring and hot, dry conditions over summer. The vast majority of terrestrial orchids have adapted to these conditions by adopting a life cycle based on subterranean storage organs (mostly tubers). Plants die back to a tuber over summer to avoid the stresses of heat and dryness and the tuber sprouts in autumn or winter when conditions are cooler and soils are moist. The reverse situation occurs in much of the tropical areas of northern Australia when the monsoonal rains of the wet season occur over summer (sometime between November and March) to be followed by a long period of dryness until the next monsoon season. The terrestrial orchids have adapted to this climate by growing and flowering during the wet (which corresponds to the southern summer) and becoming dormant during the dry season (which more or less corresponds to the southern autumn, winter and spring).

Influence of soils: The majority of Australian terrestrial orchids grow in soils of very low fertility. Most species occur in freely draining soils, such as sands, gravels and laterite, but in much of southeastern Australia, well-drained shallow clay loams predominate and support good orchid populations. Areas containing fertile loams have been cleared for farming over most of south-eastern Australia, but remnant sites indicate the presence of good orchid numbers and high species diversity. Most species grow in acid soils but calcareous soils developed on ancient dune systems and shallow soils containing limestone rubble or terra rossa soils over sheet limestone can also support terrestrial orchids where rainfall is sufficient.

Influence of fire: Bushfires are a significant environmental factor in Australia and have a major influence on the species composition and density of plant habitats. Fire-prone habitats, particularly those with a dominant shrub layer, become more dense with increasing intervals between burns and this increase in density causes a significant reduction in the number of terrestrial orchids that flower. Following a hot summer burn in southern Australia there is frequently a dramatic increase in the total number of flowering orchid plants as well as the diversity of species. Some terrestrial species have become adapted to regular burns and their response indicates that fire has become an integral part of their life cycle. A few species are so highly adapted that they will only flower following a hot summer fire. Although the responses by orchids to fire in temperate regions can be dramatic, there is no evidence that similar effects occur in the woodlands and forests of tropical areas.

Influence of altitude: All of the 115 native terrestrial genera are represented by species occurring in coastal districts. As altitude increases it is a general rule that the number of orchid genera decreases. For example 86 genera occur in the intermediate zones between 500 and 1000 m altitude, 46 genera are found in the montane zone between 1000 and 1500 m altitude and only 18 genera extend to subalpine zones above 1500 m altitude.

Subalpine habitats: A specialized group of terrestrial orchids has adapted to the woodland, meadows, moors, fens, bogs, marshes and feldmark of the subalpine regions of southeastern Australia. The prevailing climate is greatly influenced by the high altitudes resulting in a cooler climate overall with heavy winter frosts and annual snowfalls. Soils range from well-drained clay loams and stony loams to heavy clays and peats which are wet for most of the year. Living cushions and mounds of sphagnum moss also provide suitable substrate for some species.

Epiphytic/lithophytic Orchid Habitats

By contrast to the terrestrial species, the native epiphytes prefer the warm humid conditions of the tropics. Although two species of native epiphytic orchids do extend as far south as Tasmania, the vast majority of epiphytes are found within the tropics.

Influence of climate: Epiphytic orchids favour situations where warmth, humidity and air movement combine to create suitable conditions for their growth and reproduction. More than 90% of Australian epiphytic orchids occur in northeastern Queensland. Here the rainfall is high, generally reliable and less seasonal than other northern tropical areas because of sporadic periods of rain falling outside of the monsoon seasons. Although epiphytic orchids often occur in regions of high rainfall, the plants are perched on branches or rocks and must be able to withstand periods of extreme water stress between rainfall events. They are able to survive this periodic stress by mechanisms such a velamen cells in the roots which allow rapid sponge-like uptake of water and water vapour, development of storage organs such as pseudobulbs, thick cuticles on exposed leaf surfaces and the adoption of Crassulacean Acid Metabolism (CAM) allowing diurnal storage of malic acid and the nocturnal uptake of atmospheric carbon dioxide, thereby greatly reducing water loss via transpiration.

Significance of rainforest: Epiphytic orchids are mainly found in rainforest because this habitat provides most of their growing requirements. The significance of this habitat for epiphytes is emphasized by the fact that all 77 native genera of epiphytic orchids are represented by at least one species occurring in rainforest. Rainforest is a general term for a range of forests with a closed canopy including tropical mangroves that fringe the large coastal rivers and estuaries, lush dense jungles of the coastal lowlands, tall forests that clothe the ranges and tablelands at intermediate to high altitudes, stunted montane forest, drier types of rainforest developed in rain shadow areas and isolated monsoonal thickets and vine scrubs. Within the rainforest itself selection pressures are at work with the orchids surviving where they receive suitable light, sufficient moisture and unimpeded air movement. Some species favour the trunks and larger branches of rainforest trees, whereas others grow on the small outer branches and twigs (these are often termed twig epiphytes). Many of the epiphytic species which occur in rainforest also grow as lithophytes on boulders, rock outcrops and cliff faces within the rainforest itself or along its margins.

Host specificity: Within any rainforest some species of tree make suitable hosts for epiphytes, whereas other species are apparently unsuitable and do not support epiphytes. The reasons for this variation in host suitability are probably complex and involve such factors as structure of the bark, the presence of phytotoxic chemicals in the bark and the presence of suitable mycorrhizal fungi for orchid seed germination and plant establishment.

Influence of altitude: Various species of Australian epiphytic orchids are distributed from sea level to intermediate altitudes on the ranges and tablelands, some also extending high into the mountains. In tropical parts of Queensland this includes mountain peaks at about 1600 m altitude, where some 10 genera of native epiphytic orchids occur. The suite of species growing in the coastal lowlands of the tropics is usually quite different from those found at high altitudes on the ranges and tablelands. Whereas the lowlands are hot and humid for much of the year, especially in summer, the tablelands and mountain peaks experience cooler summers and cold winters, which can include frosts. The high peaks also attract cloud development that can form on a daily basis and provide extra moisture for epiphytes. Frequent spells of fogs, mists and drizzly rain (mizzle) also sustain epiphytes which thrive in the extra moisture and air movement that accompanies these events.

Influence of fire: Epiphytic and lithophytic orchids are destroyed by fires and in sites that are burnt regularly these orchids are restricted to branches and boulders away from the reach of the flames.

Other epiphyte habitats: Epiphytic orchids are not restricted to rainforests and can be found wherever suitable conditions of shade, warmth, humidity, air movement and rainfall occur. They are commonest in coastal districts and much less common in areas well inland from the coast. Some species frequent open forest, especially humid forests on the ranges and tablelands and those that fringe rainforest communities. Others occur on trees in or around swamps, beside watercourses and in deep gullies and gorges. Lithophytic species grow on rock outcrops, boulders, cliffs and escarpments, sometimes in shade, also in very exposed situations.