Bark thickness is a key functional trait influencing tree survival, particularly in fire-prone ecosystems. However, existing research relies heavily on single-point measurements at breast height, an approach that fails to capture how trees distribute defence along the stem in response to vertically uneven threats such as surface fire, which concentrates heat near the base. To address this gap, we introduce the concept of a “bark allocation pattern”, the vertical distribution of bark thickness along the stem, and apply a simplified Kozak taper model to quantify the bark-thickness profiles of 611 trees (from a felled sample of 626) from four functionally distinct groups in the subtropical forests of southern China. These groups comprise a fast-growing conifer (Cunninghamia lanceolata (Lamb.) Hook.), a fire-tolerant conifer (Pinus massoniana Lamb.), a fire-tolerant broadleaf group (Quercus spp.), and a Soft broadleaf group. We used linear mixed-effects models (LMMs) to examine how species, tree size (diameter at breast height, DBH), and topography are associated with these profiles. We identified two contrasting patterns. P. massoniana showed a pronounced “basal reinforcement” pattern, combining thick basal bark with steep upward thinning, whereas the fast-growing C. lanceolata showed an “economic investment” pattern, with thinner, more uniform bark along the stem. The two intermediate groups diverged in how their fire ecology was expressed: Quercus spp. matched P. massoniana in total bark investment but, like the non-fire-tolerant Soft broadleaf group, tapered only moderately, indicating that its fire tolerance was expressed through the amount rather than the vertical placement of bark. DBH was the strongest predictor: basal bark thickness increased steeply with DBH (χ2 = 560.64, P < 0.001), whereas the relative rate of basal-to-apical thinning declined with DBH in all groups (χ2 = 580.71, P < 0.001), indicating a shift from strong basal reinforcement in saplings towards more even allocation in larger trees. Responses to elevation were species-specific and sometimes opposing: P. massoniana built thicker basal bark at lower elevations, whereas Quercus spp. and C. lanceolata did so at higher elevations (significant Species × Elevation interaction for basal bark thickness, χ2 = 60.73, P < 0.001). By resolving how defence is distributed along the stem, the vertical bark profile captures survival-relevant variation that breast-height measurements alone cannot, consistent with a growth–defence trade-off and with niche differentiation among the four groups.
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