A New Full-Scale Method for Ranking Flammability of Ornamental Vegetation

Ornamental vegetation constitutes the main type of natural fuel within the wildland-urban interface. Its flammability is often the criteria that fire risk managers use to promote certain species against others, with direct repercussions in fire safety. Traditionally, flammability is assessed using four parameters (ignitability, combustibility, sustainability, and consumability), through small-scale laboratory tests. Although widely used, these techniques fail to reflect the actual burning behaviour of species when they ignite. In our study, we developed an experimental method to classify ornamental vegetation's flammability based on full-scale burning behaviour. We categorise flammability into three levels: low (resists ignition), medium (acts as ladder fuel), and high (spreads fire). To evaluate this, we model normalised weight loss rate curves as Gaussian functions (flammability bells) and use metrics from these functions (maximum weight loss per second, associated time and standard deviation) as combustibility indicators. Experimental results reveal that these metrics can effectively differentiate between low, medium and high flammability. The method was developed by burning ornamental trees from four typical Mediterranean species (Leyland cypress, Arizona cypress, Northern white-cedar, and Cherry laurel) with varying spatial configurations, physiological statuses and moisture content. Our method is straightforward, robust, and affordable for implementation in appropriately equipped fire laboratories, offering meaningful insights into vegetation flammability and fire behaviour in the wildland-urban interface. These experimental results can help reduce fire risk from ornamental vegetation in human settlements.