Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Friday, 8 March 2013

Grasslands of the Darling Downs - a story of the unploughed field

One of the conclusions to be made about grasslands in temperate Australia is that they've not fared particularly well with european agricultural practices. Cultivation, cropping, addition of fertiliser (particularly superphosphate), and heavy stock grazing quickly (and substantially) degrade the native ecosystem. Indeed, in one of the first 'state and transition' models ever proposed, RM Moore highlights that the system moves from a C4 native perennial grassland to a C3 native perennial grassland with light grazing, and that sustained heavy grazing leads to the development of an annual exotic grassland. We can be certainly certain that the best examples of the original system are restricted to areas that have received the least land-use intensification: cemeteries, stock routes, railway verges.

Grassland botanists in Queensland...
more like cowboys!

I've just spent a week botanising on the Darling Downs in Queensland to see what the situation is there in relation to agriculture and native grasslands. I've never been to the Downs, but I tagged along with some terrific botanists to get a feel for the situation. Thanks to Rod Fensham, Jenny Silcock & Don Butler, I've got a great understanding for the native grasslands there, and thought I'd share my thoughts on the similarities (and differences) between grasslands in southern Queensland and southern Victoria.

Native grasslands occur on gentle, rolling country (to the west and north of Tawoomba, about 2hrs from Brisbane) and occur on rich volcanic soils (about 30 M yrs old). They are of the sub-tropical type, with lots of rain between December and April/May, then drier. They were settled early and have been extensively developed for agriculture. This is probably not surprising. The rich, very black soils are almost good enough to eat. Agriculture on the plains typically involves high input crops like cotton, sunflowers and soya beans. As a result, it's hard to imagine that any native grasslands now exist on private lands, a situation quite different to southern Australia where the history of agriculture has been more about grazing than cropping.

The only places to have escaped the plough are........you guessed it, roadside corridors, travelling stock routes, town commons and areas largely forgotten about. In some respects, the conservation status of the Downs grasslands is even more diabolical than in southern Victoria. But, there are gems to be seen. We had the great pleasure to see a plant thought extinct on the Downs until its very recent re-discovery out the front of a power transmission station - King Blue Grass (Dicanthium queenslandicum). And I took great delight seeing plants on roadsides that have been missing from grasslands in southern Australia for decades, presumably because they were highly palatable and quickly eaten out by stock - Rhaponticum australe and Picris barbarorum.

Picris barbarorum , a highlight for me. This little annual daisy was patchy
across the sites we observed, but apparently erupts in certain years
(following drought breaking rains), hinting that the year of survey probably
colours our thinking about its rarity.

While driving around the Downs, I started to compile a checklist of the similarities/differences between the grasslands I am most familar with (in Victoria) and those I was observing. There were some quite striking things to note.
1) Grasslands on the Downs are not dominated by a single species of grass but, rather, have species that are patchy. In Victorian grassland on volcanic soils, Kangaroo Grass forms monodominant stands. But on the Downs, while Silky Blue Grass (Dicanthium sericeum) is by far the most common species, it rarely dominates a site. Instead, there were patches of Mitchell Grass (Astrebla spp.), Tall Oat Grass (Themeda avenacea), Barb Wire Grass (Cymbopogon refractus), Kangaroo Grass (Themeda triandra) and Sago Grass (Paspalidium globoideum). Why this occurs is hard to know, but I suspect that small differences in microtopography are probably critical - this affects drainage and how long each grass might have to endure waterlogging. I certainly noticed that Tall Oat Grass seems to prefer the drier parts of sites that were typically under an inch of water due to recent heavy rains.

Tall Oat Grass - inflorescences extend to about 2 m
2) There were very few C3 grasses in the flora, unlike southern Australia. The great diversity of C4 grasses I observed probably is a function of the sub-tropical climate. Most of the rain falls when it is warmest on the Downs.
3) Annual exotic grasses (and forbs for that matter) are mostly absent from the flora. Instead, some of the most prominent weeds were from the genus Verbena. If nothing else, they were pretty invaders! But, there were some big C4 exotic grasses like Chloris guyana that clearly threaten the long-term viability of these small remnants.
4) Daisies are rare in the Downs grasslands. This might have something to do with prior grazing histories, or because they have 'eruptive' years followed by sparsity. Much of their role (as intertussock species) seems to have been replaced by native peas - Desmodium, Cullen, Glycine, Rincosia and Swainsona were all very common.
5) Litter build up is minimal in grasslands on the Downs, hinting that frequent fire is not necessary to remove dead thatch that smothers out intertussock species (as is the case in C4 grasslands in southern Australia). I suspect that the high humidity that abounds at this time of year, combined with warm temperatures, leads to rapid litter breakdown. The role of fire, more generally, will be interesting to spend some time examining.

Hence, my main conclusion was that (a) C4 grasslands in the subtropics are similar, yet different, to the temperate grasslands in southern Australia and (b) that the plough has overseen the destruction of the once extensive Downs grasslands rather than grazing per se. What is clear is that the Downs grasslands are in desperate need of conservation management, are highly threatened, and are very beautiful ecosystems. Just like their southern counterparts.

Sunset on the Downs native grasslands, near Oakey, Queensland

Further reading:

Fensham, R.J. (1998) The grassy vegetation of the Darling Downs, south-eastern Queensland, Australia. Floristics and grazing effects. Biological Conservation 84: 301-310.
Fensham, R.J. and Fairfax, R.J. (1997) The use of the land survey record to reconstruct pre-European vegetation patterns in the Darling Downs, Queensland, Australia.  Journal of Biogeography 24: 827-836.