A year in the life of a temperate grassland - Autumn

April 2013 - the end of the dry season

This year, as part of my Blog updates, I plan to document the changes that occur in the Kangaroo Grass grasslands that dominate the western plains near my home town of Melbourne. Ecologists have long-known that grasslands are dynamic ecosystems but I've not captured this very well in my own photos, so I thought I better change that!

Following the seasonal changes that occur in ecosystems is one way to understand the dynamism of nature.

Grasslands are brilliant for illustrating such changes - they respond to the changes of the season quite dramatically. The onset of autumn rains will see a greening up of the grassland, mostly by C3 grasses (including the many exotics that now co-exist with the native flora). As spring approaches, a riot of colour etches its way into the grassland (first whites and yellow, then pink, and finally blue) - this is the flowering of the forbs that contribute most of the diversity. As temperatures rise and rainfall declines, the C4 grass Kangaroo Grass greens up and flowers, before dying back in the heat of summer. In mid to late summer, it is very likely that the grassland will get burnt as part of its ongoing management. I look forward to capturing these changes by using photos to trace the changes of the seasons.

Temperate grasslands at the end of summer look pretty lifeless.
This grassland is found on the Mt Mercer Rd at Shelford. I'll be returning to this same spot throughout the year to illustrate the changes that occur.
(Photo: John Morgan)

 Currently, the grassland is straw-coloured and dry. Kangaroo Grass leaves have died back to the very base of the tillers and no green is evident. This reflects the fact that the summer has been particularly dry this year, perhaps a sign of things to come? All forbs are now dormant, waiting for the rains to release dormant buds.

The dead grass has formed a thatch, smothering the inter-tussock spaces. Without fire, it'll be interesting to see what forbs germinate, if any, and how the biological soil crust is represented this year.

Kangaroo Grass has died back with the long summer drought. The dead grass crowds out the inter-tussock space.
(Photo: John Morgan)

On the basalt soils that are high in clay content, deep cracks have formed. These are important for two reasons. First, cracks provide refuges for animals such as Legless Lizards to avoid late summer fires. I'm guessing they are also important as refuges per se - places to avoid the effects of heat or cold. Second, the cracks are one of the reasons that trees don't grow here.  The cracking is thought to sheer their woody roots, disrupting water uptake. These cracks won't close until soils swell with the onset of autumn rains.

An example of a soil crack that has formed as the basalt soils dry over summer. I can easily put my finger in this crack.
(Photo: John Morgan)

I'll return in about 10 weeks to see just how the grassland looks. It's highly likely that drought breaking rains will have arrived, but will this have cued germination, and will any species be in flower? Till next time.

Sunset on the plains, Truganina Cemetery
(Photo: John Morgan)

The best paper of the year!!!!

I've just come across what might be the best paper I will read all year. Big call, I know.......it is only April.

Andrew MacDougall and co-authors have just published a paper in Nature (494, 86-89) that potentially has implications for understanding disturbance ecology here in Australian temperate grasslands. It's about fire impacts in degraded savanna and ecosystem stability. I've admired Andrew's work in oak savanna in British Columbia for some time. He's written about whether exotic species are 'drivers' or 'passengers' of change in degraded systems, and how to define conservation strategies using historical perspectives. It's worth checking out his Lab Homepage.

In the original oak savanna of British Columbia, a high diversity of grasses and forbs was historically maintained by frequent fire. But with fire suppression, invasions and pastoralism, present day oak savanna are species-poor, and dominated by exotic grasses. These dynamics are captured in Supplemenatry Fig 1 of the paper:

Fig S1 from MacDougall et al (2013)



Present day oak savanna. Species-poor, and dominated by exotic grasses.
(Photo: http://www.uoguelph.ca/~amacdo02/MacDougall_Lab/Home%20Page.html)

Because of fire-prevention measures since the mid-nineteenth century, oak savanna have lost many of their plant species - including the fire-tolerant species that contribute most of the diversity. Ironically, present day savanna produces a relatively stable annual biomass (from the remaining exotic grasses) and remains resilient to climate fluctuation.

MacDougall et al. conducted a 10 yr study in which the low diversity savanna plots were periodically burned after a very long period without fire. Savanna were able to recover from burning only in areas that had a relatively high diversity of native plants. (i.e. the system had maintained native species from the original flora that were capable of regenerating after fire). While these native species were rare and mostly functionally redundant, they proliferated after burning and rapidly recover the structure and function of the savanna, as well as preventing invasion by woody species. The exotic grasses, by contrast, were not well-adapted to fire and the system subsequently crashed after fire. They concluded that the study demonstrates how persistent human activity can homogenize both structure and function of an ecological system and this can weaken the diversity-related mechanisms needed to compensate for sudden disturbance.

So, why did I get excited?

Well, in a similar vein here in southern Australia, many native grasslands exist as species-poor systems due to fire exclusion and grazing. Exotic grasses are common in many grazed grasslands, many of the daisies and lilies that characterise the grasslands have been lost, and in many cases, native C4 grasses have been replaced over vast areas by C3 species.

And it is exactly these types of grasslands that are being acquired to improved the conservation status of grasslands near Melbourne and offset losses to urban expansion. What has me most interested is the idea that managers will need to manage new grassland reserves for biodiversity and one key way to do this is to remove biomass by burning. This is exactly what scientists (like me) have been saying is necessary to manage for grassland diversity.

But, if fire is introduced into a grassland that perhaps has had a century of fire exclusion, and occurs in a system where many of the fire-tolerant native herbs have been lost because of grazing, what might we expect as the outcome?

From work that Ian Lunt, Andrew Scott and others have conducted, we know there is likely to be little soil seed bank of native forbs in grasslands. Hence, fire won't promote a flush of native species to bring about a miracle cure for the grasslands. And the most fire-tolerant grasses (like Kangaroo Grass) are now absent. So, might fire actually de-stabilise grassland structure and function as a result of one hundred years of land use and the loss of native species? Might ecosystem stability be compromised by the re-introduction of fire because there is a hidden vulnerability to sudden environmental change in ecosystems that have had the buffering effects of high species diversity eliminated?

These are crucial questions for which we do not yet have answers. But it is work like that of MacDougall et al. that point to the real, perhaps even urgent, need to examine these effects, least we assume that species-poor systems function as if they were still species-rich.

Reference:
MacDougall, McCann, Gellner & Turkington (2013) Diversity loss with persistent human disturbance increases vulnerability to ecosystem collapse. Nature 494: 86-89.

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.

Why does a C4 grass fail to recolonise a C3 grassland when it is the better competitor?

This post is more a question than an insight. But it follows on from previous posts about mono-dominance in the Australian perennial C4 grass Themeda triandra (Kangaroo Grass).



Kangaroo Grass, dominating grasslands near Benambra in eastern Victoria
(Photo: John Morgan, January 2013)

Kangaroo Grass forms mono-specific swards across the basalt plains of western Victoria, and is a dominant feature of grasslands elsewhere in south-east Australia. I've mused about this before (see Monodominance in C4 Grasslands and How to Burn a Grassland) - is it because it is a superior competitor and excludes C3 grasses, or perhaps it is because it regenerates vigorously after fire whereas C3 grasses do not?

 

But what happens when there is disturbance to the soil, as opposed to disturbance to the vegetation?

Kangaroo Grass dominates on the right. But, on the left, where rocks have been removed, Wallaby Grasses (a C3 grass) dominate. It begs the question. Why?
(Photo: Tim Wills, January 2007)

In this shot, the view is rotated 180 degrees to the last one. Kangaroo Grass is now on the LHS, and the Wallaby Grasses can be seen dominating the grassland on the RHS.
(Photo: Tim Wills, January 2007)

 

The above photos highlight something that I think is pretty important.

When Kangaroo Grass grasslands are disturbed by ploughing, rock removal and heavy grazing, it declines and becomes absent. 100% cover to 0% in one or two easy steps. We know this from fenceline comparisons, and examining ploughed grasslands with intact ones. What replaces them (in areas where fertilizers are not added) are generally native grasses such as Spear Grass, Wallaby Grass and Plume Grass, plus exotic annuals. These are all C3 grasses.

But when these disturbances are removed, as far as I can tell, Kangaroo Grass seems not to re-assert it's dominance on the 'new' or 'novel' native grasslands. Why is this?

Two things come to mind (well, three if I think about it long enough).
1) Time - one reason that Themeda has not re-established is because it has not had enough time to do so. The rate of secondary succession operates at different temporal scales in ecosystems, depending on disturbance type & intensity, propagule availability and habitat productivity. Here, ploughing is a pretty severe disturbance, and it is likely that few Themeda propagules survive intensive land use. But, you'd think you would see some evidence that Themeda was recovering with increasing time-since-cultivation (assuming a linear relationship, which is dangerous to do) by examining the edges of the ploughed area. But, when you do so, you see very little evidence that Themeda is creeping into the C3 grassland
.
2) Dispersal - propagule pressure is a key attribute that contributes to the invasion of native ecosystems by exotic plant species. Indeed, the establishment of C3 native grasses after ploughing assumes that these new grasses are readily-dispersed, and there are many seeds available for colonisation. Of course, it may not be this simple. Soil disturbance may facilitate establishment of C3 grasses by breaking soil crusts, reducing above- and below-ground competition with C4 grasses, and generally increasing microsite availability.

But why would dispersal hold up recovery of Themeda after the cessation of ploughing, particularly when it is the dominant grass in the adjoining area?

Dave Tilman, Phil Grime and others have speculated that one of the tradeoffs with being an excellent competitor (e.g. high growth rate, tall stature, etc) is that this comes at a cost. That cost, for dominant grasses (and competitors more generally), is limited dispersal capacity. Many dominant plant species in ecosystems find it hard to re-occupy space because their propagules are not well-designed for long-distance dispersal. They often face an extinction-debt in small habitat fragments because of their inability to disperse through the landscape, making them vulnerable to environmental change. Is that what is limiting Themeda recovery here?

3) Competition - it seems almost unthinkable to suggest that C3 grasses that colonise after disturbance might limit C4 re-establishment because they are better competitors. But, they do occupy the site, and their maximal growth period can overlap with that of seedling emergence of C4 grasses, perhaps making the C4 seedlings vulnerable to water stress early in their life. This has not been experimentally tested (to my knowledge) so we can only speculate about the role that competition between established tussocks and seedlings play here. I suspect it would very much depend on rainfall amount each season.

I'd like to test these ideas with experiments - the most obvious is to spread some seed of Themeda around, well away from the existing plants, and see if the species can develop seedlings with/without C3 grasses in their near vicinity. This would untangle the role of competition and seed limitation for a start. I bet you (a nice bottle of red seems like a good place to start) that we'd get Themeda seedlings and tussocks within two years. And I bet we'd have more seedlings when we introduce more seeds. I think dispersal-limitation is a key here, at least for initial seedling establishment.

What do you think?

How to burn a grassland

A Themeda-dominated grassy woodland in
western Victoria.
Photo: John Morgan

Fire in temperate grasslands and grassy woodlands in southern Victoria is something close to my heart. I spent 4 yrs during my PhD trying to understand what role it might play. I focused on the intertussock flora primarily - the biodiversity. In short, fire in productive grasslands:
 * removes biomass of the dominant grasses such as Kangaroo Grass (Themeda triandra). This, importantly, improves light penetration and allows less competitive species (the intertussock forbs) to coexist with the competitive C4 grasses that contribute most of the biomass in grasslands.
* does not cue germination of (the majority of) intertussock species because most forbs have transient soil seed banks. Rather, these species primarily recover by vegetative resprouting from stolons, rhizomes, tubers and bulbs.
* promotes flowering of forbs, geophytes and grasses in the spring after burning. This pulse of flowering leads to lots of seed, which leads to delayed seedling regeneration, i.e. 12 months after an autumn fire.

More recently, I've become interested in the role that fire plays in maintaining mono-dominance in grasslands. While Kangaroo Grass dominates grasslands and some woodlands, in the long absence of fire, we have found it tends to smother itself and the tillers die. New species then move in, including exotic perennial grasses. Clearly fire removes dead thatch and allows high numbers of vigorous tillers to be maintained in each tussock.

But I think fire also is important in another key way.

I've been following mortality after fire of C4 grasses like Kangaroo Grass and comparing it to other native grasses like the C3 genera Austrostipa (Spear Grass) and Rytidosperma (Wallaby Grass). These genera are components of grasslands, but rather uncommon when Themeda dominates. This might be because they are inferior competitors to Kangaroo Grass or, and it's a hypothesis at this stage, it's because they are less effective at resprouting after fire than Kangaroo Grass. In part this may be because their tillers are rather loosely arranged and not well-protected from the heat of a fire. Hence, with every fire, if mortality is high in these species, but low in Kangaroo Grass, then fire would re-enforce dominance by Themeda.

To test this idea, I have been setting fire to grasslands and grassy woodlands these last few days. It's the height of summer here, and the C3 grasses are largely dormant, but Kanagroo Grass is growing and green(ish). I'm going to follow mortality after fire of the C3 and C4 grasses that I have introduced into these areas, and will report on the findings soon.

But I thought I'd show you how we go about burning a grassland/woodland. I've had the local rural fire brigades helping burn a site for me in western Victoria - they are more than keen to do this because it reduces fuel.  Actually, they burn this grassland almost annually, so the fire intensity is generally very low (100 - 300 kW/m) because of low fuel loads.

1. First, a mineral fire break is ploughed on one side to create a fuel-free barrier on the edge of grassland. This edge is then wet down by a fire tanker. It is critical that the fire edge can be contained, otherwise private landholders are put at risk.

Wetting down the fuel edge
Dunkeld, Jan 2013

2. On the other edge of the grassland, the downwind edge, the fire is lit using a drip torch. This is allowed to burn slowly for several metres into the wind to create an edge that has no fuel and hence, will allow us to control the downwind fire.

First, the fire is lit on the downwind side to create a burnt out break

3. Then, a second fire line is lit, half way across the grassland. This is normally lit when the first fire line is 100 - 200 m in front. This burns quickly downwind into the slowly moving fire moving upwind from the edge.

A second fireline is lit, half way across the grassland

4. A final fire line is lit on the edge . It burns quickly downwind but extinguishes as soon as it hits the burnt grassland.

A third fireline is lit from the mineral earth break

While the fire burns the grassland  thoroughly (because Kangaroo Grass is very flammable), it is clear that fire has done little more than burn the canopy. A few minutes after the fire has passed, you can see green tiller bases remain. And the soil crusts that carpet the intertussock spaces appear little singed. Hardly any heat is transferred to the soil, and even small saplings are only partly singed. This is probably a function of the low fuel loads here and low flame heights (<50 cm).

Low flame height and rapid consumption of the vegetation are typical

This photo was taken 20 mins after it was burnt. Green vegetation remains
at the base of this Kangaroo Grass tussock.

I'll be interested to see if C3 grass mortality is high here, despite the relatively 'cool' fire. I'll need to see if the C3 grasses re-sprout and then die in the summer drought. Or whether they fail to resprout altogether because fire has killed them. One thing I do expect to see is rapid regrowth by Kangaroo Grass as this is its primary growing season, and it is very deep-rooted, allowing it to tap into deep soil moisture. Perhaps it is this rapid regrowth (and ultilisation of soil moisture) that elevates C3 grass mortality?

Thanks to Anthony Watt, CFA, and the Cavendish, Hensley Park, Karabeal, Mirranatwa, Grange & Hamilton brigades