Flames in surface fires cause tissue necrosis to some depth within tree stems through heat transfer and tissue response mechanisms. Tissue necrosis kills stems and trees when it extends below the bark. No studies exist that provide data and physically-based equations linking flames, stem heating and tissue necrosis although parts of the problem have been investigated intensively. The rate of flame spread over a surface fuel and its fuel consumption may be used to calculate a fire intensity that in turn may be used to estimate flame velocity, height, and depth and, therefore, convective and radiative heat flux into a tree stem. Given certain boundary conditions—the convective coefficient, flame and surface temperatures, and bark moisture—temperature rise within the stem may then be modeled by computationally intensive numerical methods or by dimensional analysis of variables involved in heat and mass transfer. In this paper, heat flux and thermocouple data from surface fires, and measurements of bark moisture and tissue response, are combined with physical and dimensionless equations to elucidate the strong correlation between fire intensity and the depth of tissue necrosis in tree stems. Using known allometric relationships between tree diameter and bark thickness along with forest structural data (i.e., species composition and size distributions) our equations can be used to predict tree mortality in surface fires.