Why are trees hollow? Termites, microbes and tree internal stem damage in a tropical savanna

Abstract Wood plays a vital role in the terrestrial carbon cycle, both sequestering and subsequently releasing carbon to the atmosphere via decomposition. Decomposition has largely been studied in fallen and standing deadwood; much less is known about decomposition occurring inside live trees due to hollowing by wood‐feeding termites and microbial heart rot. Internal stem damage is difficult to measure, leaving many unresolved knowledge gaps. Little is known regarding 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. We destructively harvested 63 trees, finding internal damage in 32. We intensively sampled the internal stem damage in these 32 to investigate the relative contributions of microbes and termites 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 tree‐level internal stem damage across four species. Termite and microbial damage were 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 individual cross section samples, suggesting local competitive exclusion or niche partitioning by decomposers. Tree species was a better predictor of damage than either wood specific gravity or tree size. 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 previously concealed wood decomposition dynamics occurring inside trees, with implications for accurate carbon estimation across savanna ecosystems. Read the free Plain Language Summary for this article on the Journal blog.