Types of tropical rainforests

Sub-tropical rainforests

Sub-tropical rainforests are generally found where the rainfall is more than 1300mm annually and growing in fertile eutrophic parent rocks (basalt and rich shales), you’ll most likely find subtropical rainforest favouring sheltered gullies from sea level to about 900 metres. There is normally a well developed multi layered canopy of between 10 and 60 species of trees, many of which will exhibit the buttressing commonly associated with rainforest trees. Strangler species, including the ubiquitous Strangler Fig, stands of Bangalow Palms, woody vines and large epiphytes such as Orchids, Birdsnest, Elk and Staghorn ferns will be obvious, and the ground cover will consist of ground ferns and large leafed herbs.

Littoral rainforest

Littoral Rainforest is similar to Subtropical Rainforest, but occurs when it is close to the sea and exposed to salt laden winds. Usually on nutrient enriched deep sands or soils derived from slates and basalts, it’s considered more as a distinctive series of communities rather than a subform of rainforest. Three other distinctive rainforest or semi rainforest communities occur in some regions but are not generally recognised as subforms. One of these, Palm Rainforests, is dominated by Bangalow or Cabbage Palm stands, and are often found in company of Melaleuca (Ti Tree) swamp forests. Although most have been filled in for residential development, some still occur in isolated pockets throughout the region.

Dry rainforest

Dry Rainforest types are distinguished from Subtropical rainforest by scattered emergent species such as Hoop Pine, Teak (Flindersia australis) and Lacebark (Brachychiton discolor) trees in the upper canopy, and 10 to 30 species in the lower canopy. Buttressing and palms are uncommon or absent. Very large vines are common, and a prickly shrub layer, with species sporting delightful common names like “Wait-a-While” and “Lawyer Vine”, is usually well developed. Ground cover is limited to leaf litter and sometimes a few species of large epiphytes. Dry rainforest is usually found on fertile eutrophic rock soils, and favours sheltered warm areas with rainfall around 600mm to 1100mm per year, marked by a dry spell.

Warm temperate rainforest

Found on poorer soils consisting rocks such as rhyolite, trachyte and slates in the Tweed (Wollumbin) Volcano region, and on the more fertile eutrophic rocks in southern cooler regions, Warm Temperate Rainforest requires rainfall over 1300mm per year. Distinguished by a two strata layer which creates a more even canopy of trees, only 3 to 15 species will be evident, with stranglers, palms, woody vines and buttressing rare or absent. The tree trunks tend to be slender and uniform in appearance, with distinct circular shaped communities of whitish lichens covering the bark. Tree and ground ferns are frequent, and epiphytes can be common but are not generally abundant in the numbers or species present.

Cool temperate rainforest

The last type of rainforest represented in CERRA, is that of the Cool Temperate Rainforest. It is noted for its commonest and most often only dominant species, the Southern or Antarctic Beech (Nothofagus moorei), which is testament to Australia’s being part of the southern supercontinent, Gondwanaland, more than 130 million years ago.

Found at altitudes of 900 to 1500 metres, Cool Temperate rainforests receive between 1750mm to 3000mm of rain annually, and are often shrouded in frequent mists when it’s not raining.  While they mostly display 2 strata, you will sometimes find just one, with a uniform canopy of just 2 or 3 species. Stranglers and palms are absent, as is plank buttressing, but the tree trunks can be of massive size. Large vines and epiphytes will be rare or absent, although thin wiry vines and a few small ferns and orchids may occur. Ground ferns and tree ferns are very common and mossy epiphytes and lichens are in greatest abundance here.

Did you know?

Cloud-strippers

How often we climb to the top of a mountain anticipating a spectacular view, only to find ourselves looking at the inside of a cloud instead? Indeed, how often do we look up and find peaks like Mount Bartle Frere, Mount Bellenden Ker or Thornton Peak obscured by mist?

Anyone who has stood inside one of those clouds knows that it is a moist place; water condenses on hair, clothes and vegetation. Recently, CRC Rainforest researchers decided to measure the amount of water which is obtained by upland forests through ‘cloud-stripping’, as the process is known, compared with that obtained directly from rainfall.

By putting collars around the stems and trunks of trees, and placing troughs below the canopy, they were able to measure the total amount of water reaching the ground. By comparing that total with the amount collected in a rain gauge, they were able to calculate how much extra water had been stripped from the clouds. They also measured it directly with an instrument composed of a mesh attached to a rain gauge which they set up under a roof to keep out the rain. Cloud moisture condensed on the mesh and dripped into the rain gauge.

Astonishingly, it was discovered that as much as 40 percent more water was harvested from the clouds than was measured, as rain, in the standard rain gauge (see graph). This has important implications for not only the upland forests but also for forests at lower altitudes – and for the people who inhabit the area. The upland forests use very little of the water they harvest. The leaves are almost constantly wet and clouds block off sunlight which would otherwise drive the transpiration process. However, these forests act as a reservoir, storing huge volumes of water which are released gradually, keeping our creeks and rivers flowing through the dry season.

Keep an eye out for...

Ancient twin kauri pines

One of the best know botanical features of our visit to Lake Barrine on our rainforest and waterfalls tours are the Ancient Twin Kaurie Pines ( Agathis Microstachya ). The Twin Kauri Pines have grown to an amazing 50 metres ( 165 feet ) and have trunks that are approximately 2 metres ( 6 1/2 feet ) around each.

They are aged at approximately 1000 years old and are believed to live up to 2000 years old and are almost identical to fossil kauris found in rocks 300 million years old.