Dry rainforests of Mt Berryman. Photo by Paul Grimshaw.
To most people, the mention of the word rainforest conjures up visions of lush green tall vegetation festooned with epiphytes (orchids mosses and ferns), numerous vines, palms and ferns. In South East Queensland (SEQ) in places such as Lamington, Springbrook, Tamborine and Mt Glorious these types of luxuriant rainforests are commonly encountered.
However there are other rainforests that are not as lush and tropical-looking as these types, one type in particular is ‘dry rainforest’. Though not as moist as other types of rainforests, dry rainforests can easily be distinguished from other surrounding vegetation such as open eucalypt forests and woodlands, due to their dark green colour and the tight, compact crown seen from a distance.
Dry rainforest is a term used to describe vegetation, where rainfall is low because of topographic conditions (sometimes referred to as rain-shadow). Other terms used to describe variants of dry rainforests are dry vine forest, dry vine scrub, softwood scrub, Brigalow scrub, bottletree scrub, Hoop Pine scrub and microphyll (small-leaved) vine forest. At the drier extreme these vegetation communities sometimes grade into what is known as semi-evergreen vine thicket, or SEVT for short. A feature of some dry rainforests is that many of their tree species have the capacity to shed most or all of their leaves as a strategy to survive long periods of drought (this is where the term semievergreen is derived from). Compared to wetter types of rainforest in higher rainfall areas the dominant canopy height of dry rainforest is considerably lower.
In most cases dry rainforest has a lower canopy layer plus an upper layer of emergent trees rising above the canopy. The lower canopy usually consists of 10-30 tree species and the upper layer consists of scattered taller trees. Common emergent trees include Hoop Pine, Lacebark (Brachychiton discolor), Rusty Fig (Ficus rubiginosa) and Crows Ash (Flindersia australis).
Due to past clearing, dry rainforest remnants are considerably diminished and the remaining patches are still under further threat from vegetation clearing, grazing, wildfires and weed invasion. In times past, dry rainforests were cleared for grazing and other types of agricultural use in the mistaken belief that the soils, that they were occurring on, were more fertile and nutritious than the soils supporting other drier types of surrounding vegetation such as eucalypt forests and woodlands. In most situations this was untrue as once the dry rainforests were cleared and utilised, for whatever agricultural activity was needed, the fertility was quickly depleted. This is because the fertility and nutrients were stored in the top few centimetres of soil and ground litter, while the dry rainforest still existed.
While some of the tree, shrub and vine species occurring in dry rainforests are confined only to dry rainforests (e.g. Narrow-leaved Bottle Tree, Brachychiton rupestris; Leopard Ash, Flindersia collina; Small-leaved Scrub Ironbark, Bridelia leichhardtii) many other species also occur in other moister rainforest ecosystems (e.g. White Cedar, Melia azedarach; Guioa, Guioa semiglauca; Hairy Birdseye, Alectryon tomentosa). These species that occur in both dry and wet rainforests often occur as pioneers (early successional) plants in the moister rainforest types.
Climate change is likely to increasingly affect our moister rainforest types in SEQ with many of the moisture dependent plants becoming stressed during long dry periods and possibly facing local extinctions. This is where many species of dry tolerant plants found in dry rainforests will be most likely to replace them and help the less drought resistant rainforests to continue to survive.
Once established, many dry rainforest plants are more likely to cope with lowering annual rainfall and warmer temperatures as a result of climate change. It is therefore suggested that the many plant species found in dry rainforest be considered for regeneration projects, or in rainforest areas where loss of moisture dependant species is already occurring.
Equally, it is extremely important to continue to protect, manage and maintain the integrity of all remnants of dry rainforests. Dry rainforests contain a good number of threatened and uncommon plant species and a large percentage of the Regional Ecosystems, in which they are represented, are listed as Endangered under Queensland legislation.
Some of the greatest threats to our SEQ dry rainforests currently are introduced weeds, wildfires, grazing and a lack of funding streams for active conservation management. Some of the worst weed threats include vines such as Madeira Vine, Cat’s Claw Vine, Climbing Asparagus and Glycine. Weedy shrubs and trees include Lantana, Chinese Celtis, Tree Pear, Ochna, Coral Berry and Privet. Weedy grasses include Green Panic, Guinea Grass, Rhodes Grass and Broad-leaved Paspalum.
Introduced grasses are a major threat to dry rainforests as the grasses grow quickly and produce huge amounts of biomass. When dry, this biomass is highly flammable. If these grasses become established on the edges of dry rainforests, when a fire comes through, the fire will kill many fire-sensitive rainforest plants. Fire will also cause the dry rainforest to contract in size, getting smaller and smaller until it is no-longer a functioning ecosystem.
Introduced grasses can also find their way deep into a dry rainforest patch. This often occurs during drought conditions when stock is allowed to enter the dry rainforest areas for shelter. Drought causes many rainforest trees to drop their leaves making the canopy more open. During these dry periods the grass seeds contained in the stock droppings, are able to germinate, and the grasses become established, thrive and eventually cure off leaving a highly combustible fuel load within the rainforests. In such scenarios, fires can encroach deep within the dry rainforest reducing the health of the ecosystem.
If you are lucky enough to have dry rainforest on your property, please do what you can to protect it from wildfires and introduced grasses.
Article by Paul Grimshaw
Land for Wildlife member
Mt Crosby, Brisbane