Thermally induced bark swelling in four North American tree species

Bark protects both the living phloem and the vascular cambium of trees. For some tree species the bark has been observed to swell in the radial direction when heated by nearby flames, possibly providing additional protection from thermal injury. In this study, detailed measurements of bark swelling (tumescence) are reported for four species: Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), ponderosa pine (Pinus ponderosa Dougl. ex Laws.), chestnut oak (Quercus prinus L.), and red maple (Acer rubrum L.). Tests were conducted on over 574 samples extracted from 44 separate trees. The results clearly show that bark swelling occurs in the mature bark of Douglas-fir and to a lesser degree in chestnut oak. Ponderosa pine and red maple did not exhibit statistically significant swelling, but rather a modest decrease in overall bark thickness with heating. Significant swelling in Douglas-fir bark began at approximately 125 °C and resulted in a 15%-80% increase in overall bark thickness. Swelling of chestnut oak was observed to begin at an average temperature of 225 °C and resulted in a 5%-10% increase in total bark thickness. The increase in bark thickness occurred primarily in the radial direction in mature bark.