Abstract: While the fire protection function of tree bark has been extensively documented, other critical functions, including storage and mechanical support, have received less attention. In this study we examined: (1) the allometry of bark thickness (and biomass) against wood radius (and biomass) at a disc level, (2) differences in bark allocation between the ratio and the regression approaches, (3) differences between bark thickness and biomass as metrics of bark allocation, and (4) how bark allocation is associated with the evolution of wood from non-porous to diffuse-porous and ring-porous types. Thickness and biomass of bark and wood were measured using trunk discs of 88 individual trees of 36 species in a temperate forest characterized by a long fire interval. Allometric relationships of bark thickness (and biomass) against wood radius (and biomass) explained why both relative bark thickness and biomass decreased with increasing stem diameter. Variations in both among species varied by factors of 3.5 to 7.5 depending on the measurement methods. The ratio approach produced higher estimates of both relative bark thickness and biomass compared to the regression approach, while relative bark thickness was significantly lower than relative bark biomass. Ring-porous species exhibited higher bark thickness based on the ratio approach, which might reflect evolutionary adaptations where ring-porous species have developed thicker bark as protection: thermal insulation against freeze–thaw embolism coupled with carbohydrate reservoirs for hydraulic repair. The regression slope of bark allocation against wood density increased along the wood porosity gradient, demonstrating evolutionary biomechanical coordination between bark and wood. These findings highlight systematic coupling between bark and xylem multifunctionality.

Key words: Bark thickness, Stem diameter, Relative bark thickness, Relative bark biomass, Bark allocation