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| A Themeda-dominated grassy woodland in western Victoria. Photo: John Morgan |
* 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.
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| 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.
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| 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.
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| 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.
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| 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).
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| Low flame height and rapid consumption of the vegetation are typical |
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| 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