AUSTRALIAN RAINFORESTS

 

CLOSED-FOREST formation of Specht (1970) - pfc 70-100%

Characteristics

1. Canopy is closed and the trees closely spaced (i.e. high density of stems per unit area).
2. Trees are arranged in several more or less continuous strata. Uppermost strata may be uneven, due to emergent trees projecting through the canopy.
   Tropical and subtropical rainforests have 3 or more tree layers. The number of tree layers becomes reduced to 2 and eventually 1 distinct layer at higher latitudes and altitudes, in temperate and montane forest types.
3. Rainforests are distinguishable by characteristic life-forms, epiphytes, lianes, palms, certain stem and root structures (buttresses, cauliflory), absence of annual herbs on the forest floor.
4. Complex species composition.
   Under optimal conditions there is a great interdependence of niches. Lower spp. diversity results from poorer environmental conditions, e.g. decreases in moisture availability, temperature, nutrients, soil aeration, and reduction in habitat size (fragmentation) and stability.

Disturbances are an integral factor of the rainforest environment and they strongly influence spp. diversity. RF are composed of a mixture of species representing different stages of succession following different kinds of disturbance (e.g. death and fall of overmature trees, cyclones, landslips).

These successions ensure maximum saturation of a rainforest area in time as well as space.

The distribution of the main vegetation types (or formations) is determined primarily by climate and secondarily by soil types.

CLASSIFICATION OF CLIMATE (w.r.t. vegetation)

de Candolle (1855) - distribution of formations is determined by moisture (deserts and grasslands/prairies and steppes) and temperature - altitudinal and latitudinal variation (tundra, taiga/boreal conifer forest, temperate deciduous and evergreen forests, rainforests).

de Candolle (1874) - critical changes in heat regime at particular times of the year account for changes between formations.

- ordered formations according to heat- loving or drought-resistant properties.

• e.g. rainforest plants = megatherms = "most heat"
• deciduous forest plants = mesotherms = "middle heat"
• tundra & taiga plants = hekistotherms = v. low temps
• desert plants = xerophiles = drought-resistant.

Vladimir Köppen (1884, 1931) - used vegetation maps as the basis for climate maps (climate data aren't available for large areas)

• classified climate types based on temperature, precipitation, timing of dry season.
• developed and empirically-derived, quantitative model useful for making broad, world-wide comparisons of climate.
• useful for the broad interpretation of distributions of vegetation, fauna and soils, populations etc. with reference to climatic patterns.

Dick (1975) - published a climate map of Australia based on Köppen's system, using data from 2,104 stations.
- this shows an essentially simple arrangement of climatic types compared to other continents, because of:
(i) the maritime influence,
(ii) generally low elevation and subdued relief,
(iii) and relatively restricted latitudinal range.

80% of the continent is subject to dry climates (BS and BW).

• due to the expanse of the country in subtropical latitudes subject to subtropical anticyclones.
• the arid half (BW) of the continent is dominated by tropical continental air in all seasons.

20% of Australia has a moist (humid) climate; 75% of this is <150 km inland of the sea.
- the greatest inland extent is <480 km, in S. Qld.

D (moist, severe winter) and E (cold) climate environments are very minor.
Af - no marked dry season, e.g. Babinda, Tully, Innisfail.
Am - short dry season, e.g. Daintree-Rollingstone-Paluma.
Cwa/Cwb - temperate, dry winter, e.g. Atherton & Evelyn Tablelands, Eungella.
Csa/Csb - dry summer, e.g. W.A., S.A. (mediterranean type)
Cfb/Cfc - temperate, no marked dry season, e.g. Central and North-east Tablelands.
BS - semi-arid; BW - arid/desert

Australian rainforests can be classified according to climate (Wood & Williams 1960, in Leeper 3rd edn). Their distribution is largely determined by frosts and rainfall.

CLIMATIC TYPES

1. Tropical (with Indo-Malaysian floristic elements)

(a) Tropical monsoon rainforest (dry tropical forests)

• scattered pockets occur from the Gulf of Carpentaria, across the Northern Territory to the Kimberleys.
• Qld - found on Cape York in the Iron & McIlwraith Ra's.
• winter dry season - Köppen Aw; summer NW monsoon.
• forest (4.5-9 m) may possess dry-season deciduous spp. (e.g. Terminalia, Brachychiton australis).
• often characterised by leaves of microphyll size (<20 cm²) and the presence of lianes.

(b) Tropical humid rainforest (wet tropical forests)

• Köppen Af.
• along the 350 km coastal strip from Cooktown to Ingham.
• southernmost limit is thought to be at Sarina near Mackay (21S), or Mt Elliot (floristic?).
• includes lowland and upland (montane) forests.
• possess an evergreen upper stratum (15-40 m), from which evergreen emergents (often Hoop Pine, Araucaria cunninghamii) or occasionally semi-deciduous trees may emerge.
• often >250 tree spp ha^-1 belonging to many angiosperm families.
• most possess mesophyll (>45 cm²) or occasionally notophyll (20-45 cm²) leaves.
• characterised by robust, woody lianes, vascular epiphytes, strangling figs, and palms.
• 1160 spp seed plants (117 families, 560 genera)
  = 25% of total in 0.25% of area.
  450 spp. found nowhere else.
  43 monotypic genera, 28 found nowhere else.
• 214 fern spp. (59% of Australia's total), 90 spp. of orchids.

2. Subtropical-warm temperate (with Indo-Malaysian elements)

• evergreen closed forests often of lower stature than trop RF.
• gradual sifting out of tropical families and spp. with distance south.
• leaf sizes notophyll (20-45 cm²) to microphyll (<20 cm²).
• lianes usually present, but vascular epiphytes and plank buttresses are less common or rare.

I.E. with the change in climate, there is a change in structure, floristics and life-forms present.

(a) Subtropical rainforest

• Köppen Cfa (hot summer)
• from Tropic of Capricorn to Kiama in the lowland regions of NSW (35S).
• did occur between the Brisbane River (Brisbane) and Clarence River (Grafton) but mostly cleared.
• cover some 11,300 ha on Fraser Island.
• tall forest on rich soils, generally 3-layers.
• resembles impoverished tropical rainforest (50% of tree spp.), characterised by leaves of notophyll size (20-45 cm²) and the dominance of Araucaria spp.
• deciduous trees - Toona ciliata, Brachychiton acerifolius.

(b) Warm temperate rainforest

• Köppen Cfb (cool summer)
• occur in lowland regions of East Gippsland and on Wilson's Promontory, Vic. (the southernmost extension in Australia of the IM flora).
• with increasing latitude dominated by Ceratopetalum (Coachwood) ---> Doryphora (Northern Sassafras) ---> Acmena smithii (Lilly-Pilly).
• relatively few tree spp., palms rare (Livistona australis, Archontophoenix cunninghamiana), strangling figs absent, emergent Agathis in NQ, Araucaria in NSW.

3. Cool temperate (& submontane tropical or subtropical) regions
(containing Antarctic floristic elements of Gondwanan origin)

• Köppen Cfc (cool short summers).
• NSW - Nothofagus moorei in north
  Eucryphia moorei (Pinkwood) in south
• Vic & Tas - N. cunninghamii (Myrtle Beech), also Atherosperma moschatum (Sthn Sassafras, Monimiaceae), Pittosporum bicolor, Podocarpus, Lagarostrobus, Athrotaxis.
• Strezlecki Ranges, Central & Ne Highlands, Otway Ranges.
• lowland and highland regions of Tasmania.
• also NZ and Chile.
• evergreen closed-forests typical of high rainfall temperate regions, usually 1 tree layer with one dominant sp.

- characterised by:

(i) absence of woody lianes and plank buttresses.
(ii) understorey & lower tree layers often sparse or absent.
(iii) prominence of non-vascular epiphytes (mosses, lichens).
(iv) leaves of microphyll-nanophyll size (i.e. <20 cm²)

Similar forest occurs in sub-montane zones of highlands along the NSW/Qld border and in montane areas of tropical Papua New Guinea (Nothofagus is characteristically dominant).

Correlations between climatic factors and rainforest types are not clear cut, and are only valid for core areas.
(a) Tropical and subtropical rainforest types extend south to the Illawarra district, south of Sydney.
(b) Outliers of the temperate and sub-montane floristic elements occur northwards within the strictly tropical zone.
(c) Outliers of northern monsoon types occur southwards along the Queensland coast.

The main areas of distribution are connected by chains of "islands" of rainforest vegetation.
e.g. xeric rainforest replaces mesic types in "dry corridors" along the north-east coast (e.g. Ingham to Bowen and Sarina to Gladstone areas, as well as in the rain- shadows in the lee of the coastal ranges.)

THEREFORE, A CLIMATE-BASED CLASSIFICATION IS ONLY OF VERY GENERAL USE.

• Dick R.S. (1975) A map of the climates of Australia according to Koppen's principles of definition. Qld Geog. J. 3, 33-69.
• Jones D.L. (1986) Ornamental Rainforest Plants in Australia. Reed Books, Frenchs Forest, NSW.
• Leeper G.W. (1970) Climates. In: Leeper G.E. (ed) The Australian Environment, pp. 12-20. 4th edition. CSIRO/Melbourne University Press, Melbourne.

Page last revised Aug 2005/ Bob Congdon