What is a rainforest?
Most of us can recognise a rainforest, despite the fact that rainforests come in a wide variety of structures and complexity. We may not know individual species, but we recognise the shady canopy of dense trees, the misty dampness and humidity, the vines and epiphytes, the mosses and liverworts, and the moist layer of leaf litter (see rainforest structure for more information).
Trying to classify rainforests into types is not an easy task. Many other types of forest are classified by the main types of trees, but rainforest is distinguished by a multitude of species spread throughout the forest. Scientists have found it most useful to classify rainforests by their leaf size and different structural characteristics and complexity. Leaf size in rainforest trees is closley related with the wetness, temperature, fertility and altitude of a site. The simplest way of recognising the three main types of rainforest is:
Rainforest complexity includes forest height, the number of tree storeys, the presence of emergents and the abundance of lianes, palms, ferns and epiphytes and plank buttressing. Complexity increases with factors such as soil fertility, rainfall volume and frequency, temperature. For instance, in tropical rainforests there are three or more tree layers, with or without emergents. The tree layers become reduced to two and eventually to one distinct layer at higher altitudes.
Rainforests reach their peak development as complex mesophyll vine forests on the very wet and wet lowlands and foothills on soils which include basalts, basic volcanics, mixed colluvia and riverine alluvia. Mesophyll rainforests have an uneven canopy ranging from 20 to 40 metres in height. There are distinct layers of vegetation and many of the tallest trees (emergents), with large spreading crowns, poke prominently through the top of the canopy. There is a very rich variety of species - the most complex of any vegetation type found on the continent. Plank buttressing is common, robust woody lianes, vascular epiphytes and palms are typical, and fleshy herbs with wide leaves (such as gingers and aroids) are prominent. Exposed sites such as foothill ridges and seaward slopes often show signs of cyclone disturbed, broken canopies with dense vine tangles.
Mesophyll rainforests form an extensive and relatively continuous belt along the foothills of the coastal escarpment and adjacent coastal lowlands between Mount Spec in the south and Ayton in the north with a broad range extending westward to near Ravenshoe on the Atherton Tablelands. These forests occur from sea level to 1000m, although they are more common below 400m.
Three types of mesophyll forests are extremely rare. These include:
Palm leaf vine forests
Palm leaf forests are a very distinctive type of mesophyll rainforest where the upper canopy consists primarily of feather palms (usually Archontophoenix alexandrae) or fan palms (Licuala ramsayi var. ramsayi) - leaf size is not classified here. Palm forests can be found mostly in swampy depressions on lowland coastal plains, with feather palms inhabiting swampier, more nutrient rich soils than fan palms. Palm forests can be found from Ayton to Ingham , particularly between Innisfail and Gordonvale, and around Mission Beach and the Daintree. Some are also found in upland areas such as Kuranda.
Notophyll rainforest is the most extensive rainforest formation in the bioregion. The canopy trees have leaves between 7.5 and 12.5cm long. The transition from mesophyll to notophyll forest is generally associated with drier, less fertile and cooler conditions. Notophyll rainforests include a diverse group of communities. They are characterised by a canopy range of 12 to 45 metres in height, rattans or palm lianes, strangler figs, frequently conspicuous epiphytes and variable amounts of ferns, walking stick palms and fleshy herbs. Notophyll rainforests can be complex, simple, evergreen, semi-evergreen and semi-deciduous.
Notophyll rainforests occur mostly from foothills to uplands. They can be found on small areas of basic volcanic soils on cool wet uplands and highlands and on a range of drier sites at various elevations - on the western and northern fringes of the main rainforest masiff from Cardwell to Julatten, the Hann Tableland and to the north of Bloomfield, on sandy beach ridges in drier coastal zones and exposed sites backed by foothills north of Cairns. Complex notophyll vine forests occur from cloudy uplands and highlands on basalt to alluvial lowland flood plains to very wet granite boulder fields on the foothills. The threatened mabi forests represent the maximum development of deciduous forests in the Wet Tropics, a result of fertile basalt soils and a decreasing rainfall gradient to the west on the Tablelands.
Three types of notophyll forests are considered rare vegetation communities. These include:
Microphyll fern forests
Microphyll rainforests include all rainforest where the dominant canopy leaf size is less than 7.5cm in length. Microphyll leaves generally mean that the canopy trees are under an environmental stress such as wind exposure, salt or low soil fertility. Microphyll forests tend to be simpler, but can still contain a large variety of plants depending on the climate, soil and other environmental factors. Simple microphyll fern forests dominate on the summits and upper slopes of the higher peaks (Bartle Frere, Bellenden Ker, Carbine Tableland and Thornton Peak) which are frequently covered by cloud and often exposed to strong winds. Aerially suspended mosses are often found here - these forests are also referred to as 'cloud' or 'wet montane' forests. Microphyll vine thickets can occur in drier areas exposed to salt and wind such as coastal slopes.
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