A few years back, I enjoyed an enlightening afternoon in an ancient forest near London, discovering the crucial link between aging trees and biodiversity. My guide, Lynn Bodhis, a mycologist from Cardiff University, explained that as trees age over centuries, the inner trunk begins to decompose due to fungal activity. This phenomenon, known as “heart rot,” is a natural aspect of a tree’s life cycle, creating invaluable habitats for various species of insects, birds, and mammals. Unfortunately, as older trees diminish, the heart rot phenomenon is also waning, lacking the necessary old trees to continue its role. Bodhy and her colleagues are dedicated to preserving this essential process by promoting the early aging of younger trees.

While it didn’t strike me at the moment, heart rot is part of the tree microbiome, akin to a diverse mix of bacteria, archaea, fungi, protists, and viruses. The concept of microbiomes wasn’t commonly discussed then, but groundbreaking research published in Nature has revealed that trees possess microbiota as varied and remarkable as our own.

We’ve known for some time that the surfaces of trees (trunks, roots, leaves, etc.) harbor diverse microbiota. However, recent studies indicate that their interiors are similarly diverse. Each tree on our planet contains myriad microorganisms, many of which are unfamiliar to the scientific community.

This discovery is both significant and enlightening. It’s intuitive to consider diversity in trees, given that microbiota thrive in various natural settings, including smaller plants. Yet, this research unveils microbial ecosystems that were previously overlooked, shedding new light on trees—not merely as individual organisms but as holobionts, integral components of Earth’s ecology.

Essentially, like humans, trees are composite entities, consisting of both hosts and their associated microorganisms. If the microbiota of trees proves as vital to their biology as human microbiota are to us, these communities could play a pivotal role in efforts to sustain biodiversity and combat climate change.

The researchers involved in this study examined wood samples from the trunks, branches, and roots of 150 living trees spanning 16 species in forests in northeastern United States. They conducted what is referred to as the Microorganism Census, discovering that the interior wood of trees is teeming with microorganisms, including not just bacteria associated with heart rot but also a variety of other bacteria, fungi, and archaea. Additionally, each tree species is home to its unique microbiota.

The interior of tree trunks consists of two types of wood: outer sapwood and inner heartwood. The sapwood is alive and primarily responsible for transporting water from the roots to the leaves, while heartwood is non-living and serves mainly structural purposes (which eventually deteriorate due to heart rot). Researchers have established that the microbial communities in these two wood types differ significantly.

While only a limited number of species were sampled, it is plausible that all trees harbor similar microbiota across various wood types. The studied 16 species represent 11 genera, all of which have a global presence.

What roles do these microorganisms fulfill? It’s still largely unknown, but researchers suggest they likely contribute to tree health and the overall health of forests. They are possibly involved in the essential ecosystem services trees provide, such as offering habitats for numerous plants and animals, producing clean water, and acting as carbon sinks. Globally, wood holds approximately 60 years’ worth of current emissions according to studies over the past six decades. Trees could absorb more, helping to cap global warming to less than 2°C above pre-industrial levels, necessitating healthy, expanding forests. A deeper understanding of tree microbiota may facilitate this goal, according to researchers.

This concept of mutualism is gaining traction. While microorganisms are often viewed as adversaries of biodiversity, they are foundational to the world’s ecosystems. As primary agents of organic matter decomposition, they drive vital biogeochemical cycles that supply the biosphere with essential elements like carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Moreover, they coexist with most plant species and are integral to what many refer to as Earth’s “life support system.” However, alarming trends suggest that the overall microbiota on Earth is declining.

It’s premature to conclude whether this trend extends to tree microbiota, but now that we are aware of their existence, we must ensure they are conserved.