Surface fires are the most frequent type of forest fire not caused by man. They play an important role in many ecosystems as they facilitate the breaking of cones, reduce parasites and fungal infections, kill competing sprouts and contribute to higher substrate turnover rates. A tree's probability to survive such a fire mainly depends on its capability to protect the cambium from lethal temperatures above 60^0 C. Thereby, the bark, the entirety of all tissues outside the cambium, serves as an insulation layer. The fire resistance, i.e. the time span until the cambium reaches a lethal temperature, of seven tree species was determined by heating bark samples in the laboratory. Thereby, the influence of bark thickness and moisture content on bark heat insulation was examined and additional bark properties such as density and bark structure were quantified. The tree species examined differ in the fire regime, for example the fire frequency, of their natural distribution area.
The fire resistance increases with increasing fire frequency of the distribution area of the tree species examined. A linear correlation between bark thickness and fire resistance became apparent for both dry and wet bark samples, where the moisture content of wet bark samples was equal to that of fresh bark. As this result is independent of the tree species examined, a tree's fire resistance can be deduced from its bark thickness alone if bark moisture content is given. Thus, a tree's fire resistance depends on its ability to build up a bark within one fire period which is thick enough to survive a surface fire. Bark density decreased with increasing fire resistance. The examination of the bark structure revealed two groups of trees: (1) tree species with little bark structure, which show a low fire resistance, and (2) tree species with pronounced bark structure, showing a high fire resistance.
Thorough analyses of the correlation between bark surface structure, internal structure and biochemistry with bark heat insulation capacity might be a source of ideas for the improvement of existing technical thermal insulation and fire-stopping materials or may even bring forth new biomimetic insulation materials. On the other hand also highly fire resistant und insulating barks, as found e.g. in Sequioadendron giganteum and Quercus suber, can be used as a raw material for the production of bio-based technical insulation and fire-stopping materials. The already existing bio-based materials could be significantly improved by including additional information found by structural and functional analyses of the bark of highly fire resistant tree species.