C.T. White Lecture for 1997
SUSTAINABLE GRAZING SYSTEMS AND AUSTRALIAN VEGETATIONAL HISTORY - THE “RETURN TO EDEN” HYPOTHESIS
J. Brian Lowry
CSIRO Tropical Agriculture, Long Pocket Laboratories, Indooroopilly, Qld 4068
The image of the tropical rangelands of northern Australia as a harsh environment for pastoral production is so strong as to be almost a cliche. But perhaps it need not be quite so harsh. Human use of fire over the last 50,000 years has drastically reduced the animal carrying capacity by favouring inedible eucalypts and acacias. Trees, shrubs and vines of much higher nutritional value may still be found in restricted habitats. The key to a more sustainable and benign form of animal production may be in the recognition and promotion of native plants that are already there.
I feel some trepidation in giving this talk tonight. Firstly, as a relative newcomer to Queensland, in featuring in an event which has such roots in this state; in the history of the Queensland Naturalists Club, and not just in the life and work of C.T. White, but through him, his grandfather and mentor F.M. Bailey the Colonial Botanist. Both were fully and very productively involved in describing the Queensland flora, and might have been impatient with the speculative nature of this talk, based though it is, in part, on their work. And as if that was not enough, I am offering you thoughts tonight that range over a vast span of time and space on this Australian continent. I would also add that although it is CSIRO that has given me the opportunity to develop them, these are my own opinions.
I want to talk about nothing less than the land, and the way we manage and see it, over much of the northern half of Australia, the tropical and subtropical woodlands. Over a vast tract of this country, beef cattle production is the only obvious productive land use. Despite the negative things that are said about agriculture, I suggest it is ecologically appropriate - as long as it is done right. This is because it surely makes sense to have large herbivores in the savanna ecosystem. Every year grass grows vigorously in the wet (if there is one) and then matures, becoming indigestible to anything but a fibre-digesting herbivore. It is much better to have some of that biomass processed by an animal than burnt, which would otherwise happen. That way, at least some of the organic matter gets to the soil, minerals are recycled, microbial activity supported. Certainly, fire is a natural part of the system. But too-frequent fires lead to loss of nutrients and biodiversity, particularly invertebrates. We can only avoid hotter or more frequent fires by having animals that digest fibre.
In economic terms the grazing animal makes sense. A self-propelled system for converting a low-value resource - rangeland biomass - into a high-value product. And there are the communities and the human culture that have developed in inland Australia, from whose loss we would be poorer. I have no problem being in favour of the pastoral industry, as long as it is managed sustainably.
However it is a cliche that in northern Australia, pastoral production takes place under harsh conditions. The soils are infertile, the climate severe, the rainfall unreliable. The fundamental limitation is nutritional. There are two over-riding biological factors here.
The tropical grasses (C4 grasses) become of low feed value in the dry
season. This happens everywhere in the seasonal tropics.
Although the country is mostly woodland, the dominant eucalypts and
sclerophyllous acacias are considered of no feed value. This is very much an Australian phenomenon.
How can one improve animal production in this situation ? Put crudely, up to now the choices have been to provide supplements, pull trees or plant stylos.
Overwhelmingly, beef producers have seen the trees as a problem. Pull trees and you get more grass. Yet increasingly it is recognised that trees are essential to prevent land degradation. Everyone is in favour of trees in theory. Overall, there seems to be a trade-off between environmental protection and increased production. When people say ‘’trees’’ they almost always mean eucalypts. Reflect for a moment how unfriendly these are to grazing animals. They provide very thin shade. The leaf structure guarantees that it will be of little feed value. And they compete with grass. Very different trees can be found, but dominance by eucalypts is very much a feature of the landscape. Do we have to accept it as inevitable as the scorching sun and drought ?
I suggest not. This is based on a simple assumption. The eucalypt dominance that we take for granted is largely a human artefact. The country can sustain different vegetation types.
This assertion is entirely based on the work of others.
Dr Peter Kershaw at Monash University and his palynologist colleagues have shown that the change from a predominant dry rainforest vegetation to eucalypt woodland took place about 50 000 years ago at a time when the climate was not dry and there was no particular climate change. Instead it coincided with increased appearance of charcoal and first indications of human habitation. Eucalypts are not only fire-tolerant but fire-promoting. He concluded that increased fire frequency, due to humans, caused eucalypt dominance.
Dr Tim Flannery of the Australian Museum has put together a masterly account of how the first colonisation by man impacted on various environments in the Austro-Pacific region. This story is told in his book ‘’The Future Eaters’’. This work is both scientifically rigorous and imaginative. It is science as literature. It is prehistory as Manning Clark might have written it. I do commend it to you.
According to this picture, when humans first crossed from Asia to Australia, they found a continent with a mosaic of forest and woodland but much more of various rain forest types than we have today. These mesic forests and woodlands provided much more feed that sclerophyll woodland and sustained a diverse fauna. This included large herbivores (diprotodons and giant kangaroos) that were totally unadapted to an intelligent predator such as man. They were, literally, easy meat. They were gradually hunted to extinction. Not only did man make use of fire, but, without these large herbivores, uneaten grass accumulated in the dry season, fires became more intense, and the eucalypts were on the march.
We seem to have two paths for the same result:
Kershaw: Increased, purposeful burning reduced productive habitat, increased eucalypt woodland and animals failed to survive in a more adverse habitat.
Flannery: Hunting of large herbivores allowed accumulation of dry-season grasses, resulting in hotter and more frequent fires, resulting in eucalypt dominance.
I want to take a moment to face head on the implication that talking about extinctions caused by early man somehow adversely reflects on aboriginal skills and culture today. Firstly, these findings are not really controversial but consistent with elsewhere in the world. The tendency for man to over-exploit each new environment into which he moves is very much a common human trait. It is , sadly, an expression of our common humanity. Further, one can only point to the absurdity of associating current societies with things that were done 50 000 years ago. Or the impossibility in a hunter-gatherer, or any human culture, of being aware of changes that would have taken place over thousands of years. Modern man, with all his mastery of information, can destroy resources in just a few years and still not react in time. One can only look at the aboriginal peoples who occupied the Australian continent when the First Fleet arrived, respect their skills, understanding, and use of the environment as it then was, and wonder at the triumphs and disasters, joys and suffering, discoveries and adventures, that had gone on in that vast path through time from when the continent was first settled.
So much for a picture of Australia long ago. How can it be related to sustainable land management today ? I now want to take up another theme and talk about a few findings from my own research
First, let me tell you about a remarkable tree, a legume, the siris tree, Albizia lebbeck. This common shade tree of northern Australia can grow very well in most of the semi-arid woodlands, and could benefit pastoral production in a remarkable variety of ways. It can be browsed, but once established as a large tree with the canopy out of reach it continues to feed the animals. At various times in the dry season it drops leaf, flower and pod which are all good feeds. And below its canopy the grass grows better and stays green longer. A property stocked with these trees would be a remarkably more productive grazer-friendly environment.
The second step was to realise that siris, which is usually regarded as an introduced shade tree, is also found as a native tree in parts of the north. If this remarkable tree is part of the local flora, what else of value might be there ?
Now, it is well known that there are a number of native ‘’browse shrubs’’. It was a well-known former member, Dr Selwyn Everist, who put together an account of them. As I got interested I was surprised to come across quite a few species that he had not mentioned. Also, for Everist and most of us, the concept of ‘’browse plants’’ meant trees or shrubs from which animals could feed directly on the green leaf. The revelation for me was to find that there was more to the feed potential of woody plants than this. Some examples.
There is a small native tree, the cocky apple (Planchonia careya), which, during the late dry season, drops large amounts of flower that is picked up by cattle. Rough measurements indicate that there is enough of this, and enough trees, for it to be making a useful supplementary contribution in some areas. However the trees are swept away with all the rest in any tree clearing.
In the tropical rain forest there is a prominent plant life form about which people are often curiously unaware. You can find accounts of the trees, herbs, epiphytes, epiphylls, but rarely of the large woody lianes. This applies also in the dry tropics even though they give their name to habitats such as “vine scrubs” and “vine forests”. However I am happy to say they have certainly not been ignored by one of our members, Dr Elwyn Hegarty, who made them the subject of her Ph.D. research. I did not think of lianes having pastoral significance until I encountered this giant liane with no real common name - maybe "Burdekin creeper" or "snake vine" (Argyreia nervosa) - which can grow rampantly in the dry tropics yet is highly palatable to cattle. It would be bizarre to consider it as a pasture plant yet there seems no reason why it could not be managed to make a real contribution. However if noticed at all, it is seen as weed, and it is likely to be wiped out incidentally in the attack on the truly noxious rubber vine. Objective consideration suggests several reasons why large woody lianes could be useful for large herbivores. There are of course plenty of legumes in this category.
I used to think production of young leaf in a pink flush like this Syzygium photographed on a Club trip in Belthorpe State Forest was a feature of tropical or subtropical rain forest trees. However it is also shown by many trees in the dry tropics as with this Pongamia pinnata in a dry creek bed near Townsville. The interesting thing is that this leaf flush often occurs at the end of the dry season, before any rains come. For the animal, the new leaf has other features besides colour. It has about half the fibre and twice the protein content of mature leaf - even a small amount would be a valuable supplement to that dry-season grass. I thought this was only theoretical but recently I saw a study showing that this brief leaf flush in certain trees was very important to rangeland cattle in the Sahelian savanna. I can only conclude it can play a role here too, but no one has documented it yet.
The next case is more general. Almost all the non-sclerophyll trees are deciduous in the dry season. No one has previously suggested fallen tree leaf as feed for grazing animals. However remember that the dry-season grass is of very low digestibility. It turns out from work I did in Townsville that fallen leaf from many native deciduous trees could be a better feed than the dried-off grasses, simply because these are of such low quality. It is a concept that can only arise from a study of the plants and the animals in the dry tropic environment. There are important differences between the two materials. The leaf breaks up more readily, and although fermentation is limited, it occurs very quickly. The animal can eat more and extract the available nutrient as it passes through.
The materials are so different that we tried rumen-simulation experiments where we mixed them together and found digestion went better than with either separately. This is a new result that, if real, has profound implications. An animal in deciduous woodland could do better on a mixture of grass and fallen leaf than on either separately.
So much for my research. How does this relate to vegetational history ?
Simply that almost all of these woody plants that might improve animal nutrition tend to occur in discrete patches of vegetation. We visited one such patch on the Club trip to Minto Crags. We call them dry rain forests, vine-scrubs, softwood scrubs, gallery forests, vine thickets. The pioneering rainforest ecologists, Len Webb and Geoff Tracey classified them in some detail. In our eucalypt-dominated landscape they are a minor element, in terms of area. No wonder their plants have been little studied in terms of grazing animal nutrition, although they are known to have important wildlife values. But these are the remnants of the vegetation that once supported those large herbivores. In a recent talk about rainforest remnants in the Brookfield area, Tim Low pointed out that the arrangement of thorns in dry rainforest shrubs, which have no adaptive significance against native herbivores today, are living evidence of those long-gone big browsers. Evolutionary ghosts - the term coined by Jared Diamond. As we have seen, vegetation like this once covered much of the land.
As for their limited occurrence, the tendency today is to assume they are found where they are because of a little extra moisture, or better soil, and can’t grow anywhere else. But this is not so.
Now to the point of this talk. All is not lost. Not only have elements of the mesic vegetation that supported the megafauna persisted for 50 000 years, there is evidence that they can recolonise the eucalypt woodland, given half a chance. It is this concept that I call the ‘’Return to Eden’’ hypothesis.
What is the evidence ?
First of all, lets look at my favourite siris tree. Today its natural habitat is at the closed forest open woodland ecotone in the Kimberleys and Northern Territory. But when planted it can thrive over a vast area of northern Australia, even in Julia Creek. Its limited distribution as a native tree is certainly not because of soil and climate limitations; it seems more likely to be the result of millenia of past fire regimes. Surely, it was part of that vegetation that supported the diprotodons.
A close relative, Albizia procera, is regarded as a native rain forest timber, but the tree will grow quite well in eucalypt woodland. Anyone who grows native plants knows that many of our rain forest species can grow outside the rain forest.
We know that native mesic species such as Macaranga, Umbrella tree, Pittosporum umbellatum can vigorously invade eucalypt woodland in the absence of fire. The vigour with which some exotic mesic species invade native woodland suggests to me that the “natural’’ state would be a more closed forest; the most opportunistic species are simply trying to make it happen. Recent studies on “dry rainforests’’ in Queensland indicate that in many cases their present boundaries are not determined by nutrients or rainfall but by past fire history.
But the best evidence is the spontaneous advance of rain forest into wet sclerophyll forests when fire is suppressed, which have been documented in several places. This is so obvious in North Queensland national parks that the Department of Environment carries out deliberate burning to prevent "rainforest encroachment" into eucalypt forest. It is a major question as to whether we should actively manage national parks to keep them as they were 200 years ago or let them go to the way they really were. An article by my CSIRO colleague Graham Harrington in ‘’Wildlife Australia’’ pushes this view of the threat to sclerophyll habitats. I must say my feeling is, if the rainforest can advance, let it.
Back to pastoral production.
The ‘’Return to Eden’’ hypothesis is that what happens quite obviously in the more humid zone can be made to happen more widely. That we can find ways for producers to manage the land to let some of these dry rainforest species reclaim at least some of the habitat that they once occupied.
Here I probably would differ from Flannery, who sees the productivity of the Australian landscape as having being irretrievably degraded. I think it can be regenerated.
What would we have ? It would be a landscape with many different trees, a mosaic of types from open woodland to dry rain forests. There are fewer eucalypts, and more trees with greener, thicker crowns. There is a diversity of shrubs and lianes. Some trees foster greener grass underneath. Weed invasions are less marked. In this environment cattle are less heat stressed, do less walking, have lower calving losses, get more feed and better dry season feed. Even in drought some feed is available. Production is not only increased but of higher value. Native wildlife is more diverse and prolific, due to habitat and more diverse feed resources.
When the storm rains come, runoff is moderated, less soil is eroded, and it holds moisture longer. Plant growth rebounds faster. Streams flood less and flow longer, pools last longer..
Perhaps the strangest thing about this vision is that it is optimistic. There is so much bad news on the environmental front, and the prospect of remedying some of the degradation so remote, that it seems quite anomalous to be proposing a message of hope. Surely it is worth a try.
Key Sources of Inspiration
EVERIST, S.L. (1986). Use of Fodder Trees and Shrubs. Queensland Department of Primary
Industries, Information series, QI85015.
FLANNERY, T. (1994). The Future eaters: An Ecological History of the Australasian Lands and
People. Reed Books, Port Melbourne.
KERSHAW, A. P. (1986). The last two glacial-interglacial cycles from north- eastern Australia:
Implications for climate change and aboriginal burning. Nature 822, 47-49 .