One thing that struck me was the number of talks I saw on fire. There was a whole symposium dedicated to fire, which was very well-attended. I also saw talks scattered through the programme that dealt with the impacts of individual fires, the regeneration biology of key species, the importance of fire return interval, why fire severity matters, etc, etc.
This is not surprising given that south-eastern Australia is one of the most fire-prone areas on the globe. It is a recurrent feature in many ecosystems, and is projected to increase in frequency because of climate warming over the coming century. Additionally, it is also a contentious issue - the management of bushfire threat in SE Oz revolves around policies that mitigate risk by hazard reduction burning.
Two important issues arise - how often should it occur in an ecosystem (both as a tolerable event and as a desirable disturbance to cue important ecological processes such as seed germination). Second, what type of fire event is likely to have positive or negative impacts on these outcomes.
I've been musing about these questions for the last few weeks, but from a different perspective. And it returns me to one of my favourite pasttimes: observations in ecology.
While fire is a recurrent event throughout much of SE Oz, there are indeed very long fire-free intervals for some parts of the landscape. We intuitively know this. In the rainforests of eastern Victoria, fire is probably fairly rare - perhaps in the order of hundreds of years apart. But what is probably less appreciated is that in the drier, less productive parts of central Victoria, perhaps fire is also equally as rare.
How can I make such a statement? What evidence do I have for one of the most fire-prone areas of the world having very long fire-free intervals. Well, that is where observation comes in. In the dry, eucalypt-dominated woodlands of north-central Victoria, around Rushworth to Wangaratta, there is a clue on the upper slopes and rocky outcrops. It's a pretty obvious clue when you think about it.
Here, there is a plant that is born to burn (or so it would seem). Grasstrees, in the genus Xanthorrhoea, are arborescent monocots, developing tall stems that allow them to grow to great heights. We know they grow very slowly. Let's focus on X. glauca. For the first 50 yrs, plants consist of leaves but no stem. The stem then emerges and height growth is about 10-25 mm per annum. My old PhD student Peter Curtis measured growth rates over 10 years, so this seems a pretty reasonable estimate. So, a plant 3 m tall might be anywhere up to 350 years old. Each year, the leaves die but are held appressed to the stem rather than shed. In unburnt grasstrees, this 'skirt' can extend all the way along the 'trunk' to the ground. When a fire occurs, the 'skirt' is a ready-made fuel source to rapidly burn the plant - a quick fire where most heat is carried rapidly away from the meristem by convection. Plants generally survive burning, although Peter's work does show that mortality of grasstrees can be quite substantial in the decade after fire. But that is another story..........
|Burnt Grasstrees, Rushworth State Forest, October 2012|
(Photo: Michele Kohout)
Well, old photos tell us so! Here are some images of grasstrees in the Warby Ranges - taken in the early 1980s by the well-known fire ecologist David Cheal - that are notable for (a) their enormous height (up to 8 m perhaps) and (b) the fact that they have grass skirts that extend all the way to the ground. This suggest that this part of the landscape has not been burnt for upwards of 600 years. Hence, we are not talking about fire suppression since european settlement. And it is not just one or two individuals, but entire slopes, suggesting that fire is very uncommon here. On recent inspection, it is hard now find examples of these great unburnt plants. Low intensity fire has seen to that.
|Grasstree in the Warby Ranges, 1983. Note the scale (approx. 1.75 m)|
and dead leaves all the way to the ground. The trees in the background
have not been burnt. Rather, they are dying from intense drought.
(Photo: David Cheal)
|Grasstrees, all with skirts that indicate long intervals between fires, across a slope at |
the Warby Ranges, 1983. Plants are approx. 2.5 m tall.
(Photo: David Cheal)
|Long unburnt Grasstrees, Rushworth State Forest, 2012. Scale approx. 40 cm.|
(Photo: Michele Kohout)
The lesson here, if there is any, is to observe patterns in nature. The simple observation that grasstrees in some parts of the range (low productivity, rocky) have not experienced frequent fire is probably very important, particularly when thinking about fire return intervals. While low intensity fires may seem ecologically benign, this needs to be put in the context of the fire history of the site.
It also challenges our notion that dry forests burn frequently simply because they occur in a part of the continent where fire is known to occur regularly!