Ecological communities of the Brunswick Valley

“Many tend to think the most significant Northern Rivers vegetation is ‘rainforest’ not recognising the ecological importance of the other diverse plant communities found in our catchment.

Within the rainforest communities, for instance, there is littoral (coastal) rainforest, sub-tropical rainforest, warm temperate rainforest, dry rainforest and the endangered ecological community of lowland sub-tropical rainforest. There are also wet and dry sclerophyll (eucalypt) forest, heathland, and mangrove forest plant communities.” (Excerpt from Flora).

Members are invited to submit for publication here descriptions of their favourite local ecosystem or habitat. Mail to: abegg100@gmail.com

The Tyagarah Heathland

by Rita de Heer

On October 1 2005, a group of about twenty of us met in the car park at the end of Gray’s Lane, for a walk-and-talk through the Tyagarah National Park led by Andy Benwell.

The vegetation here so new to me I could hardly see for looking, as the saying goes. My notes indicate that, really, if I want to be able to talk about any heath ecosystems I should just go back a couple of dozen times with a field book library in my backpack and the camera I don’t have yet, in hand.

So for this article I concentrated on underlying structures and reasons for development as is my usual habit when faced with information overload. A sort of learning by first establishing large compartments to fill with species names and details in the future.

Like a lot of people I thought dunes were highly mobile here-today-gone-tomorrow structures. Their life span measured in tens of years at the most. This opinion based on going to the beach and seeing the erosion of the most seaward dune each winter. Andy prefaced our walk with a short introduction on the recent natural history of the area and I learned that dunes are structures of considerable age formed by the interaction of biological life with climatic and geological processes.
The most seaward dune (also known as fore or primary dune) at Tyagarah, for example, is said to have formed since the last ice age and is guestimated as being between seven and eight thousand years old. 

The dunes of the Tyagarah National Park are part of an extensive system that ranges from Fraser Island in the north to Ballina in the south, formerly interrupted only by river mouths and their attendant riparian and mangrove ecosystems, and rocky headlands. Now splintered and much reduced by human activities such as agriculture, sand stripping, mineral mining and town building.

The Marine Education Society of Australia Inc (MESA) website, on its Habitat of the Month page, describes the birth of a dune as ‘a stray piece of flotsam blown high above the tide line by a winter storm …’ which then becomes a catchment for sand. Seabirds, seeking out such little heights for resting, add their droppings. A rolling seed capsule of spinifex (Spinifex hirsutus) might be caught at such a hillock, take root and begin its great endeavour: sand stabilisation. More sand is caught, hillocks link, the spinifex on the foredune is joined by primary colonisers such as eastern pig face (Carpobrotus glaucescens) and goatsfoot convolvulus (Ipomeae pes-caprae). The resulting valley between this new dune and the now secondary dune is a swale.  

At Tyagarah, according to Andy, six such dune ridges and accompanying swale systems march inland. The oldest most distant from the present day shore about two million years old. The appearance of that soft black ‘coffee’ rock scattered up and down the coast an indication of the rising sea levels since the end of the last Ice Age. Made up of
‘sand grains weakly cemented together by organic matter (plant remains). This is a reminder of a time when the sand mass stretched further out to sea – and the currently exposed coffee rock was inland …’ (EPA Qld website, Great Sandy NP page)

I imagine black rock is forming even now in the bottom of the present day swales and that those deposits will be brought to light during future rising sea levels.

The fact that black rock, and so dunes and swales, had time to form in the present day littoral zone is a kind of a proof to me of the much lower sea levels occurring as recently as during the last Ice Age. Which we know about from our general knowledge about our continent. However, my references together do not triangulate well on the effect of that time (ie the duration of the last Ice Age) on the present day dune profile at Tyagarah.  

Studying the process of dune formation, it becomes obvious that plants are the agents of the actual dune raising as well as the secret of their eventual longevity. We all know what wind and water can do to a pile of uncovered, unvegetated sand. ‘The fore dunes are held together by pioneer plants such as beach she-oak, coastal banksia and pandanus’ where bitou bush and other weeds haven’t taken over. ‘They protect wattles, hopbush, tuckeroo and stunted eucalyptus trees from harsh salt-laden winds.’ (The EPA website, Great Sandy NP).

The wallum banksia (Banksia aemula) is  present in the Tyagarah NP (Benwell, 1983) and as such a primary indicator for Tyagarah NP’s wallum heath vegetation ecosystem. With serrated leaves up to 20 cm long, this banksia has flower spikes that fade from fresh green to brown as they age. Its sweet nectar attracts many birds such as rainbow lorikeets and honeyeaters. A Bushcare Fact Sheet on Wallum Vegetation states that the term ‘wallum’ is
‘now used to refer to the vegetation as a whole, “Wallum” was originally an Aboriginal name for a species of Banksia (B. aemula) that typically grew in these areas, commonly known today as the Wallum banksia. … Aboriginal people soaked the flowers … in water to produce a sweet drink.’

A dune remains a work in progress. In the lee of the fore dune a rich diversity of life will develop to the limits of the nutrients stored in the sand. While some nutrients will remain in the dune in the detritus resulting from the growth and decay cycle, some are lost through rainfall and its tendency to leach nutrients from higher places and depositing them in lower places. Animals eating seeds and other parts of the vegetation, may not deposit those same nutrients in their droppings. Fire-winds transport nutrients in the form of ash away from the area of the fire. Creeks take away ash, droppings, whatever falls in the water. Nutrient loss results, even if it takes eons. This process illustrated by the lesser diversity of vegetation in the older dunes.

Buchanan (p131, 1989) has an interesting discussion on nutrient cycling in heath plants. Many heath plants have developed ways to improve their nutrient uptake.

1. Banksias, dryandras, some peas, and casuarinas have root clusters – bunches of hairy rootlets produced by normal roots – in the top ten percent of the soil where they have first go at any new release of nutrients.
2. Acacia, Boronia, Allocasuarina, Dillwynia and many more families of plants have a close association between their roots and particular sorts of fungi which help with water and nutrient uptake.
3. Most peas and Mimosaceae have root nodules for nitrogen fixing. Which also happens in the upper ten percent of the soil.
4. Carniverous plants such as sundew (Drosera) feed on insects to supplement their supply of nitrogen.

Underground storage of these nutrients, amongst other strategies, enables many heath species to live through fire and drought.

The first part of our walk was along a dune back from the beach, perhaps the second ridge. The startling white of the sand underfoot the result of work by the plants growing there – ‘by gleaning the fine mineral coating from the grains, turning golden beach sand white’ according to MESA. Vegetation along this ridge included common ti tree (Leptospermum polygalyfolium, formerly known as L. flavescens) along with a couple of subspecies; and heath dwelling varieties of casuarina, xanthorrhoea, banksia and melaleuca. The canopy about three metres tall, to my guess, with thick undergrowth leaving few areas of sand to be seen. At the end of the track a clearing of coarse grass with unidentifiable animal holes and tracks. Bandicoots or rats the guess.

The second part of the walk was along the fire trail in the lee of the foredune. As well as species associated with normal dune vegetation such as flowering bottlebrush grass trees (Xanthorrhoea fulva) banksias and eucalypts, we identified various elements of rainforest vegetation such as umbrella cheese tree, blue lilly pilly, brushbox, red ash and mistletoe. Andy surmised a transition happening. Reasons for which might include changing drainage patterns caused by the road and recent weather.  Buchanan (1989) describes how trees can become established on the edges of wet heaths during dry periods only to die off when conditions return to wet.

At this point in my learnings I am assuming that wet heathland surrounds the swamps in the bottom of the swales. A couple of my sources make a distinction between wet and dry heath vegetations. Based on previous experience (with Sydney sandstone-type country) it seems to me that climate variability combined with factors such as the different life spans and possible ranges as well as bushfires and/or burning regimes encourages gradation from one to the other, with a fluid borderland, rather than distinct communities.

Human visitors and the detritus they leave behind effect soil composition too. Even an apple core provides the place where it is dropped with 17.5 times the amount of phosphorus that the soil normally carries (0.02%) and heath plants are adapted to. A cigarette butt adds 16 x the phosphorus content, a banana skin 8 x. (Buchanan, 1989). Added nutrients cause some wallum plants to grow weak and die in extreme summer conditions; others such as the banksias, to suffer phosphorus toxicity, allowing non-heath plants to grab a foothold. Such as perhaps the rainforest specimens we saw establishing in the lee of the Tyagarah foredune.

Andy guessed the melaleucas to be about twenty years old, indicating no fire for that long. Some people say wallum country probably needs a seven to ten year pattern of mosaic burning to maintain optimum species diversity. The sources I have at present are not all that informative on fire and I will therefore leave its discussion for a later date. More research needed, because the most comprehensive source I could find is all about Fraser Island.

This short visit to Tyagarah NP opened my eyes to the wonders of dune formation through interaction of the biota, wind and sand. The wallum (heath) ecosystem is said to rival our subtropical rainforest in numbers of species and biodiversity. Though Tyagarah is only a small stretch compared to the amount of rainforest we have and are attempting to restore, it surely is worth cherishing – studying how that ecosystem works, educating its users, saving our dune ecology!

(C) 2006 Rita de Heer

<Return to top