Mountains, Slopes and Plains
The Australian Capital Territory comprises open, rolling country - valleys and plains - that grade gently to low hills to the south and west. These gradually rise to the mountains in the west. The topography has been described as "Mountains, Slopes and Plains".
The city of Canberra is situated in rolling plains and river valleys in the north east quarter of the Territory, at an altitude of around 600 metres (2000 feet). The region is known as the Southern Tablelands. It is open, dry terrain, with a rainfall of around 635 mm (25 inches) per year. The prevailing winds are from the north-west, year round. The weather is cold and frosty for three months of the year, and hot and dry for three more.
To the south and west of the city, the plains give way to hills that rise eventually to the Tidbinbilla Range and the Brindabella Mountains. These reach heights of nearly six thousand feet. They are snow covered for weeks at a time in winter. These ranges determine the northward flow of the wild Cotter River, which runs for most of the length of the Territory. The rainfall may reach over 1500 mm (60 inches) or more at points along the Brindabellas. It increases the further west you go into the mountains.
"Droughts and Flooding Rains"
Apart from the average rainfall, the dominant characteristic of the region - indeed, of the whole Australian continent - is the variability of the rainfall. By now, most of the world is aware of the phenomenon of El Niño, which generates extreme swings of climate - droughts or floods - around the globe. In Australia we are well aware of droughts. These may last for several months when there is no rainfall, and a year or more with much diminished rain.
Extensive research has been conducted into the variability of our climate. One of the best measures of the onset of an El Niño event is the Southern Oscillation Index (SOI). An apparently simple measurement relating the average barometric pressure in Darwin in the north of Australia to that in Tahiti in the central South Pacific, it gives a reliable indication of the movement of the warm surface waters across the Pacific Ocean that cause the climate shifts. The SOI has been charted for over a hundred years. You can find a graph of annual SOI values here.
The implication of drought for the vegetation in the Canberra Region is that it must be able to tolerate years of much lower than average rainfall. This has contributed to the hardiness of everything that lives around here.
The vegetation in and around the city is known as dry sclerophyl, or dry hard-leaf (eucalypt) forest. There are many areas of bush within the precincts of the city.
As you travel west from the city into the mountains, the vegetation changes from dry to wet sclerophyl, and in places may resemble temperate rainforest. The dry sparse bush around the city turns to steep slopes with giant eucalypts and understory of acacia, leptospermum and ferns. At higher altitudes, the vegetation changes to a more alpine type, with Snow Gum forest and alpine meadow.
In the south eastern quarter of the Territory there is the Naas Range. This range is sheltered from the prevailing westerly winds and has a lower rainfall as a result of the rainshadow from the ranges to the west. The peaks of the range rise over 1200 metres (four thousand feet). The vegetation is mainly dry sclerophyl, with some wet sclerophyl. Ferns are relatively scarce compared to further west.
The tree canopy
The dominant forest canopy
tree throughout eastern Australia is the Eucalypt.
In the Territory there are dozens of species that are favoured in different
microclimates. They vary from the Stringy-Bark Brittle Gum complex (E.
macroryncha - E. mannifera maculosa)
in the dry country to the Ribbon Gums (E. viminalis)
in the higher river valleys, to the giant Mountain Gums (E.
dalrympliana) and Alpine Ash (E.
delegatensis) on the mountain slopes. On the mountain tops,
the canopy is dominated by the Snow Gums, E.
pauciflora. In the exposed areas the trees give way altogether
to Alpine Meadow and Sphagnum Bog.
With the prevailing tree types, there are also prevailing fern species. You can often predict what types of ferns you are likely to find under any specific tree canopy type.
In the drier bushland, on the slopes among rocky outcrops you will frequently find Cheilanthes austrotenuifolia, the Rock Fern. In drier, colder, more exposed areas it is replaced by a relative, Cheilanthes distans, the Bristly Cloak Fern. In slightly moister spots, little clumps of Necklace Fern, Asplenium flabellifolium poke out tentatively from under the rocks.
In protected gullies in the drier areas, such as on the south west quarter of Black Mountain, you can find quite a diverse range of fern species. In some areas, Common Maidenhair Fern, Adiantum aethiopicum, grows in such profusion it crowds out the grasses. With it, clumps of Sickle Fern, Pellaea falcata, and in spots even Gristle Fern, Blechnum cartilagineum find a place.
As you travel into the hills to the west of Canberra, you also move into the regions with higher rainfall, and, naturally, ferns become increasingly common. The first to notice is Common Bracken. This occurs in thickets along the roadside. In moister areas you will see other ground ferns such as the Hypolepis species, Rainbow Fern Calochlaenia dubia, Tender Brake, Pteris tremula, and, higher up, Mother Shield Fern, Polystichum proliferum. These are all ferns of the open forest floor and hillsides.
The best fern country, as might be expected, is to be found in the ranges where the rainfall is highest and the water flow permanent. In these well-watered valleys, several species of water fern abound, particularly Blechnum nudum, Blechnum minus and Blechnum wattsii. These occur generally in sheltered valleys under the tree canopy. The are often accompanied by dense stands of the Soft Tree Fern Dicksonia antarctica. Rarer but present in places is the Rough Tree Fern Cyathea australis.
Along the permanent creeks in the ranges, the water ferns grow in thickets. In the wettest and most protected areas, you can see some of the rarer ferns to be found in the Territory, such as Bat's wing Fern, Histiopteris incisa, Filmy Ferns, and Kangaroo Fern, Microsorium diversifolium.
At the top of the ranges, associated with the sphagnum swamp and Snow Gums (E. pauciflora), there are extensive thickets of Blechnum penna-marina, the Alpine Water Fern. This fern, and the sphagnum moss it grows among, is adapted to several weeks under snow cover each winter.
This brings us to the wider picture, the understanding of the ecology of an area. Ferns are an important part of that ecology.