Rotten to the core? How internal stem damage varies vertically in savanna trees and is influenced by tree species, traits, and external damage pressures

1. Trees are important aboveground carbon sinks in savanna ecosystems, yet consumption of internal wood by decomposers (e.g., termites and microbes) creates uncertainties in tree biomass accounting. It remains unclear whether internal stem damage is constant or variable throughout the tree, making it uncertain if a lower stem sample reflects damage in the entire tree. Furthermore, total damage and damage location are likely influenced by external damage pressures (i.e., termites, microbes, and fire), tree species and tree traits (i.e., diameter at breast height (DBH), wood density), and their interactions. 2. We sampled internal damage in the lower stem of savanna trees in North Queensland, Australia to test for vertical variation in terms of proportional and absolute amount of damage. We compared damage estimates from a single-sample method, assuming constant damage, with a multi-sample method, assuming variable damage, to test how well one sample represents the lower stem. We investigated if tree species accumulated damage differently based on their traits (i.e., DBH and wood density) or susceptibility to external damage pressures (i.e., fire scarring and termite presence). Finally, we tested if external damage pressures differentially affected tree species and if this was mediated by tree traits. 3. The absolute amount, but not proportion, of damage decreased with higher vertical position on the stem. There was no difference in total stem damage between the single-sample and multi-sample methods. Species-specific variation in internal stem damage was influenced by DBH and wood density. Total damage was greatest in large trees, particularly those with external termite presence. Finally, external termite presence, but not fire scarring, differentially affected tree species and was most likely to occur on large, dense trees. 4. Synthesis. We demonstrated that a single sample effectively captured total internal damage in the lower stem. Although species differed in total damage, damage accumulation rates with height were consistent, suggesting a general relationship. By integrating the influence of external factors and tree traits, our findings underscore the importance of considering these elements for accurately estimating carbon stored in aboveground tree biomass.