Microbial Mediators of Human–Tree Communication: From Endophytes to the Gut–Brain Axis (Perspective)

Recent work shows that living trees host vast, structured microbiomes deep inside their wood—heartwood and sapwood harbor diverse, active microbial communities on the order of ~10¹² cells per tree. In parallel, the human appendix is being re-evaluated as a long-term “safe house” for gut microbes, while the microbiota–gut–brain axis (MGBA) links peripheral microbial signals to autonomic tone and mood. We propose a testable hypothesis: microbial “keys” mediate (part of) the measured human responses to trees via volatile organic compounds (VOCs), bioaerosols, and contact-derived microbes that modulate the human microbiome and—through the MGBA—autonomic and affective state. We outline falsifiable predictions, community-deployable protocols (HRV/PPG + mood + 16S/shotgun), ethical guardrails, and low-risk, nature-positive interventions (e.g., SIGS or endophyte inoculation under regulation). We also highlight synthetic DNA nanostructure condensates—recently shown to mimic living-like molecular organization without chemical cross-linking—as controllable in vitro surrogates to probe VOC/microbe signaling safely. The goal is not mystical claims, but a biophysical, microbiome-aware account of why interacting with living trees changes how people feel—and how to measure it.