Abbey R Yatsko, Baptiste Wijas, Jed Calvert, Alexander W Cheeseman, Keith Cook, Paul Eggleton, Indigo Gambold, Caleb Jones, Pedro Russell-Smith, Amy E Zanne

1. Wood plays a vital role in the terrestrial carbon cycle, serving as a significant carbon store that is then released back to the atmosphere during decomposition. Decomposition has largely been studied in fallen and standing deadwood; however, decomposition can occur within living trees via hollowing by wood-feeding termites and microbial heart rot. Internal stem damage is difficult to measure, leaving many unresolved knowledge gaps, such as the location and total amount of damage done by termites and microbes, as well as whether these decomposers act in concert or separately. Furthermore, tree species, wood density, and stem size can influence fallen deadwood decomposition, but their role in living tree internal damage is largely unknown.
2. We destructively harvested 39 trees to investigate the relative contributions of microbes and termites to internal stem damage in a tropical savanna in Queensland, Australia. We tested if damage changed at different heights in the tree, quantified tree-level termite and microbial damage, and examined if termite and microbial damage co-occurred. We also tested the influence of tree species, wood specific gravity and size on internal stem damage across four tree species.
3. Termite and microbial damage was present in 45% and 33% of all trees, respectively. On average, termite damage reduced total tree biomass by 3.3% (maximum 28%, SD = 4.7%) and microbial damage by 1.8% (maximum 26%, SD = 5.3%). The amount of damage from both decomposers decreased with increasing heights up the tree. Termite and microbial damage co-occurrence was greater within trees than within cross sections, suggesting local competitive exclusion or niche partitioning. Species was a better predictor of damage than either wood specific gravity or tree size.
4. Half of the trees in our study had substantial internal stem damage, highlighting the considerable role that termites and microbes play in decomposing wood within living trees. Our findings unveil the concealed wood decomposition dynamics occurring inside trees, with implications for accurate carbon estimation across savanna ecosystems.