Tuesday, 3 February 2015

Wildfires and the burnig of Pine needles

In our recent paper published in Fire and Materials, we use laboratory experiments to investigate the differences in fire dynamics between live and dead pine needles. This is important because limited research has been conducted on the burning characteristics of live fuels, which are commonly assumed to behave like moist dead fuels.

The high flammability of conifer forests in the Mediterranean and Boreal biomes is due mostly to the presence of needles in very large amounts. Needles are fine fuels that ignite and spread flames faster than coarse woody fuels and represent an important portion of the total fuel consumption in wildfires. Needles are found both in the tree canopies and on the ground. Live needles (green colour) are part of the foliage and typically burn in crown fires. Dead pine needles (red colour) are on the ground, accumulating gradually on the litter and humus layers, and burn both in surface and ground fires.

Samples of Pinus halepensis needles used in the experiments (from left to right): live, aged and dead.

Our fire calorimetry results show good repeatability and demonstrate that the difference in burning dynamics of live and dead pine needles is significant and can be quantified and understood. Using a series of 10 flammability parameters extracted from the experiments, we show that the most flammable samples are fresh dead needles, followed by dry dead and dry live needles. The least flammable is fresh live needles. Live needles ignite about four times slower, and burn with ~60% lower power and ~50% lower heat of combustion than dead needles. The results confirm the importance of moisture content in the burning behaviour of pine needles, but the differences between live and dead samples cannot be explained solely in terms of moisture but require consideration of plant chemistry and sample drying.

(Left) Time to ignition and (Right) flaming time.

The results show that there are fundamental differences in the physics and chemistry of the flames of these fuels and that fire dynamics does not follow a simple trend from live to aged and to dead fuels.

Our results also defy the common assumption that oven drying only affects the water content of samples, or that the drying conditions are not important. Data suggest that observed fire behavior is substantially affected by the drying process in the oven, which induces chemical and structural changes (eg, loss of volatile organic compounds inside the oven). The fact that oven drying is widely used in wildfire laboratory studies merits more research.

- F Jervis, G Rein, Experimental study on the burning behavior of Pinus halepensis needles using small-scale fire calorimetry of live, aged and dead samples, Fire and Materials (in press) 2015.  http://dx.doi.org/10.1002/fam.2293  (open access)