Burning behavior of broad leaf shrub species prevalent in the Northwest United States

Abstract

This study examines the burning behavior of three evergreen shrubs—salal, Oregon grape, and manzanita—species significant to wildfires in the Northwest United States, under varying moisture levels and convective heat fluxes. Experiments were conducted by burning individual leaves in a convective environment above a methane-fueled flat flame burner. High-speed videos revealed five distinct burning stages in living fuel from shrubs—preheating, droplet ejection, transition, flaming, and smoldering. The frequency of droplets ejected, and flaming ignition time correlated with stomatal density, though a causality requires further investigation. Ignition time increased with higher moisture content and lower heat fluxes, showing a nonlinear sensitivity near 100% moisture content. Oregon grape leaves, with moderate carbohydrates compared to the other species, achieved higher surface temperatures during the preignition stages and ignited faster at lower temperatures (≈300°C). Broadleaf shrub species required 15 to 25 seconds to ignite, longer than the 5 to 10 seconds reported for needle-like fuels.