How do you measure a tree’s flammability without burning it?

Forest fires are a problem, but how do you measure a tree's flammability? New research suggests you look to the shoots, not the leaves.

It’s easy enough to see wood burns, and some wood is more straightforward to set fire to than others. Is it possible to measure and predict the flammability of a tree, or is it a matter of experience and seeing trees burn? Azharul Alam and colleagues have been looking at the trees of New Zealand and trying to see what would make a tree burn.

Forest fire
Image: Canva.

Flammability is an awkward term, as the authors say. They split it into four components

  1. ignitibility (ignition delay time or temperature required to ignite any fuel)
  2. combustibility (how much temperature the fuel emits or heat release rate)
  3. sustainability (how long the fuel continues to burn)
  4. consumability (how much of the fuel is consumed during a fire)

Setting fire to whole trees is an extreme solution. Persuading a judge that you’re doing it on a regular basis for Science is going to difficult. Ideally, you’d start from some material and then scale up. But what material?

Alam and colleagues have compared the flammability of leaves and shoots. Both seem plausible candidates to model the flammability of trees in a crown fire, a fire that has climbed up trees and is spreading through the forest canopy.

If you’d like to measure shoot flammability, there is some standard equipment for this. The leaves, in contrast, were burned in a muffle furnace. Forty-three species were tested. They were also ranked in flammability by experts with experience of fires.

Alam and colleagues found that shoot flammability was not the same as leaf flammability. “The lack of correlation between leaf- and shoot-level flammability is likely to be explained by differences in the amount and arrangement of the fuel being burned,” the authors write in their paper. “While in leaf-level studies a single leaf is burnt, the shoot-level study uses a 70 cm-long shoot as fuel, which contains multiple leaves, twigs and small branches.” It’s this structure that is the key, the authors say. “The arrangement of leaves and twigs at the shoot level likely reflects the way that fire burns through a plant canopy, with the propagation of fire from twig to twig on a shoot similar to propagation from branch to branch in a canopy.”

It’s, therefore, no surprise they found that shoot-level measurements were better correlated to expert opinion than leaf-level measurements. This result is inconvenient, as leaf-level experiments are easier to do, but Alam and colleagues say this is more likely to correlate to litter fires than crown fires.

“…[S]hoot flammability research is in its infancy, with only several hundred species (ca 300) tested so far globally…,” write the authors. Their results, however, show that there is a need for thoughtful burning of shoots to quantify flammability.