In this Blog, I return to one of the big unknowns about fire in grasslands. Despite it's importance, there is almost no data on fire behaviour in temperate grasslands.
Fire behaviour describes the fire event: the fire intensity, the thoroughness, the extent, the duration of heating. While there has been much research examining the effects of historic fire regimes (1 versus 3 versus 5 year burning frequencies), managers have almost no information about how individual fires vary and what the implication of these differences in fire events might mean for mortality processes and rates of regrowth.
To address this information gap, my Honours student Karina Salmon and I have been quantifying Fire Intensity and Residence Time for a number of grassland fires this summer. These two measures tell us something about (a) amount of energy released by a fire (I = fuel load * rate of fire spread * constant) and (b) duration of heating (above 200 deg C). Clearly, Intensity will vary with fuel and fire weather conditions on the day; faster moving fires are generally more intense. Fast moving fires probably also have lower residence times at a point, although this is not well-established in the literature for mesic grasslands.
Below is a link to a video that shows how grassland managers burn grasslands (in this case, a local Council near Sunbury). I want you to observe a couple of things here: rate of spread and flame height. While watching this, observe the lighting patterns and weather conditions. This has a tremendous effect on the way the grassland burns.
A couple of questions to ask yourself: Is this an "intense" fire? What is the ecological impact of this fire?
Let's answer the first question. Is this an intense fire?
How long is a piece of string?? Fire intensity is a relative measure in many respects. Forest fires can exceed 100,000 kW/m in intensity and hence, no grassland fire will ever be this intense as the fuel loads do not accumulate to the same degree. Hence, it is better to ask: is this an intense fire relative to other fires you get in grassy ecosystems. Even then, that answer is not straightforward. In tropical savannah, fire intensities up to 18,000 kW/m have been recorded, but fuel loads can be enormous - annual grasses take advantage of the wet season and put on several metres of growth each year. So we need to narrow this question even further. Is this an intense fire in the native grasslands in southern Australia?
We characterised this fire at about 500 kW/m, which isn't very high.
This is a one of the 'middle of the road' fire intensities that have been recorded in native grasslands. But it clearly 'looks' like a decent burn that you may have initially thought was 'high intensity'. It goes to show that just observing fires isn't very informative. All fires are hot, to some extent, so value judgements like 'low' intensity, or 'cool' burn aren't very helpful.
Of great interest to me is another (almost completely ignored) aspect of fire behaviour: Residence Time.
This fire had a temperature above 200 deg C for an average of 29 seconds (at the point of recording by a Type-K thermocouple). This heating, from a relatively low intensity fire, is what plants and animals actually have to put up with. Burning for half a minute is the real challenge here when thinking about probability of survival. Interestingly, at very low Fire Intensity, we found that Residence Time can be quite long, more than a minute in some cases. When fires are slow moving (as low fire intensity fires usually are), heating above 200 deg C is prolonged - the flames are not moving away from a point quickly. Hence, an (unintended) outcome of 'low fire intensity fires', by slow back burning of the grassland is that they heat for longer durations than higher intensity fires, and they burn very thoroughly. Almost no fuel goes unburnt in some cases.
Hence, it is clear we still have much to learn about ignition patterns, fire intensity and residence time, and how these interact to affect plant and animal population dynamics. I'm keen to explore these ideas next summer, so if you're planning on burning a grassland, I'd be keen to set up some dataloggers and quantify what's actually going on.