Trees and People: Life in Heartwood
By Bill Pramuk, RCA #409
As an arborist, I have always considered the inner wood of trees – heartwood – as dead wood, devoid of life, and of value only for structural support. Any life in there would only be of invaders like decay fungi or borers. The sapwood, the active tissue involved in growth and movement of water and nutrients, so I thought, was virtually sterile except for living tree cells and occasional invaders—bacteria, viruses, fungi and other organisms that cause biotic diseases. Recent research explodes those assumptions, opening a deep, new world of knowledge about bacteria that live inside trees.
A research article appeared in ‘Nature’-August 2025, Wyatt Arnold et al. (I had to subscribe to Nature so I could read the study.): “A diverse and distinct microbiome inside living trees.” It describes recent research showing that “Earth’s largest biomass reservoir, the wood of living trees,” is “a harbour of biodiversity and potential key players in tree health and forest ecosystems.”
The research found that a single tree hosts about one trillion bacteria in its wood with distinct microbial communities inhabiting heartwood and sapwood.
I was aware of established arboricultural science describing tree functions in relation to organisms inhabiting the root zone: the “rhizosphere,” which is the soil zone near the roots and the “rhizoplane,” the surface of the roots. It is interesting enough, how trees have intimate interactions with external microbes in the soil. Then there are the mycorrhizae, where fungi combine with root tissues, forming distinct fungus-root organs that are essential to the uptake of water and minerals and in self-defense from disease. But this internal microbiome is a whole new ball game.
I first became aware of the term “microbiome” perhaps 25 years ago, referring to human gut health. It has become common knowledge, and a common concern, that we need to support the healthful microbes in our digestive systems. So now the question is: What is the significance of the microbiome inside of trees?
The research paper is dense with technical information, statistics, and graphs, beyond my abilities as a practical arborist, but I extracted some points of interest.
The bacteria communities are mostly inherited, not introduced via soil, and they vary by tree species. The tree species in this study included maples, oaks, birches, hickory, beech, Eastern white pine, hemlock, ash, Sassafras, Kalmia (mountain laurel) and Prunus serotina (black cherry). The bacterial communities were found to vary widely by tree species and by location, within the tree.
In the oxygen poor heartwood, is a high abundance from the anaerobic groups, e.g. Clostridia. There is a high abundance on anaerobic groups, e.g. Clostridia, in the oxygen poor heartwood. In contrast sapwood contains “obligate aerobes” like Actinobacteria, those that need oxygen. The bacterial communities in sapwood —collections, mostly of prokaryotic species- bacteria and archaea-- are pretty much the same along the height of the tree from .5 to 10 meters up, where the tree cells are alive, relatively young, and oxygen is available. In contrast, bacterial communities in heartwood seem specialized to the conditions in the wood and the communities vary greatly along the length of the trunk where oxygen is lacking and the wood can be far older than sapwood.
The study was a survey of distinct organisms within wood in over 150 trees of 16 species. Researchers found that the
wood in living trees is colonized by adapted and specialized communities that might play roles in tree health.
The research is still in its early stages and could explore how the internal microbiomes vary across different regions and climates. As the study concludes, “The wood biome is a vast and unexplored reservoir of microbial diversity with potential practical applications in biotech, ecosystem health and sustainable forestry.”
Paraphrasing the paper, studying tree microbiomes might offer new avenues toward understanding the growth, management and conservation of the world’s greatest biomass reservoir, the wood in trees.
