Effects of moisture on ignition behavior of moist California chaparral and Utah leaves

Individual cuttings from eight plant species native to California chaparral or Utah were burned in a well-controlled, well-instrumented facility. Gas temperatures above a flat-flame burner were controlled at 987 +/- 12 degrees C and 10 +/- 0.5 mol% O-2, resulting in a heat flux at the leaf surface varying from 80-140 kW/m(2). High moisture leaves were observed to burst due to the rapid escape of vapor from the leaf interior. Bubbles in or on the leaf surface were observed for leaves with moderate moisture contents. A large number of leaf temperature measurements were made, along with measurements of the ignition time and temperature, flame height, and flame duration. Average ignition temperatures were species dependent, ranging from 227 degrees C to 453 degrees C, with a large degree of scatter from leaf to leaf. Correlations of time to ignition and ignition temperature were made, but showed only a weak dependence on leaf thickness and almost no dependence on mass of moisture in the leaf. Leaf samples with similar mass showed that Utah juniper took longer to burn than the other species, and that the Utah broadleaf species burned more rapidly than all the other species.