Experimenting with Fire

My PhD primarily revolved around the effects of fire frequency on regeneration dynamics and species coexistence in the endangered temperate grasslands of western Victoria. Here, fire plays an indirect role - frequent fire prevents competitive exclusion of the intertussock forbs from the dominant C4 tussock grasses. In this case, it was the frequency of fire, not the type of fire that seemed most important to the conservation of plant diversity.

In 2003, landscape-scale fires burnt the alpine
vegetation of Victoria. But it was clearly very patchy.
(Photo: John Morgan)

But fires can come in many guises - fires ain't fires - you only need to look at a wildfire to see that it can burn thoroughly or patchily. It's clear to me, however, that we don't understand very much about plant community responses to difference in fire "type". Rather, much of our knowledge (and research) is from the standpoint of the time-since-last fire and, perhaps, the fire frequency (and these assume that fires are much the same). Yet, it is the type of fire that might ultimately affect mortality of established plants, germination cues, and resource levels. Not to mention how much C is returned to the atmosphere.

To learn more about fire and how to measure it, I've just spent a week burning tropical savanna in the Northern Territory Wildlife Park with Dick Williams from CSIRO. This was excellent fun, but also highly informative. The Burning for Biodiversity experiment is an amazing field study examining the effect of fire frequency and timing on a variety of taxa and the dynamics of carbon. Importantly, it relates these outcomes to aspects of fire behaviour. So, what better way to learn about fires than to visit one of the few experiments in Australia that is quantifying fire!

A common measure of fire behaviour is fire intensity – the amount of energy released per unit length of fire front (kW m^-1). It is defined as the product of rate of spread (ROS), fuel load, and the heat released from the fuel during combustion. The higher the fuel load and the ROS, the higher the fire intensity.

Fuel loads are easy to calculate - the amount of fine (<6 mm diameter) fuel is sampled in quadrats pre-fire and weighed. It is the fine fuel that will rapidly combust (flamming combustion) and affect properties of the fire front. Larger diameter fuels burn more slowly (in a process called smouldering combustion), typically after the fire front has passed. These fuels are important to quantify, as they will release much more C into the atmosphere.




Harvesting fuels prior to ignition
(Photo: James Camac)

Savanna in the Northern Territory - awaiting burning
(Photo: John Morgan)



ROS is a little harder to quantify because it is much more dynamic, but plays a critical role on fire intensity. In Darwin, we used two techniques to estimate ROS from our contolled burns. First, we measured the time the fire front takes to reach pre-defined points in the landscape - using points marked with numbered poles (we used six) and a stopwatch, the average rate of spread of the fire between the points can be calculated. As a  backup to the estimates by eye (it can get quite hairy when the fire front is moving quickly), we also used specially designed automatic timers buried in the soil with a small thermocouple left exposed above-ground - these timers record the time at which the thermocouple heats to >200 deg C and, somewhat ingeniously, the residence time (the time that the temperature stayed above 200 deg C).



Lighting the fireline with a drip torch
(Photo: James Camac)

 


Timers, attached to thermocouples, are buried in the soil.

These record the time at which fire passes, and how long
flaming continues at the point. (Photo: James Camac)







Flamming combustion

(Photo: John Morgan)





Smouldering combustion
(Photo: John Morgan)



These are very simple measures that can address fundamental research questions - they should be in the toolbox of all fire ecologists because, if measured, they allow quantification of how fire intensity might affect biodiversity.

We certainly saw large differences in fire intensity in hectare-scale plots burnt on the same day! I look forward to coming back to Darwin in 2012 to observe just why these differences might matter. At this point, I'm not sure when, where, and why fire intensity affects biodiversity - there are simply too few examples in the literature to provide a coherent review. But I know that in future, I'll be quantifying fire intensity in my research (both within and across different fires) to get a better understanding of this primary aspect of fire behaviour.