MICRO-ORGANISMS IN TREES

We knew that trees have lots of micro-organisms living on them but, until recently, we did not realise that trees have a very extensive microbial community inside them – a “microbiome” in fact. Living trees harbour thriving communities of microbes deep within their woody tissues. American research[1] has revealed that every living tree hosts approximately a trillion (10¹²) microbes in its woody tissue. This includes bacteria, archaea[2], and fungi that reside internally rather than only on bark, leaves, or roots. These internal communities are distinct, depending on the type of the tree’s wood (heartwood and sapwood biomes are very different) and also differ considerably from the microbial communities found in leaves, roots, bark, or soil. The American researchers examined 150 living trees of sixteen species in north-eastern U.S. forests, focusing on both heartwood and sapwood microbiomes. They found that:

• Each tree species hosted distinct internal microbial communities, consistent across species but very different between species;

• The microbiomes, both aerobic (in sapwood) and anaerobic (in heartwood), are metabolically active, including nutrient cycling;

• The results suggest these microbial communities may have coevolved with their tree hosts, adapting to species-specific traits such as density, moisture, and the availability of oxygen.

European comparative research suggests that:

• Trees with ectomycorrhizal[3] associations such as beech, oak or spruce support higher fungal biomass and growth, as well as increased bacterial biomass.
• On the other hand, endotrophic mycorrhizal[4]-associated species (e.g. ash, maple) display higher bacterial growth rates.
• These differences seem to be associated with how tree species influence soil chemistry, including pH and ratios of carbon to nitrogen.

When looking more broadly at botanical traits, such as being broadleaved rather than coniferous, mycorrhizal association and nitrogen-fixing ability significantly affect soil carbon stocks globally, partly influenced by how such traits influence microbial activity and soil chemistry. Below is a summary of the main findings in the U.S. work of microbiome differences between species: 

Inside wood

Distinct microbiome profiles for different species (e.g., maple versus. pine) which probably coevolved and are metabolically active in biological functions like gas exchange and nutrient cycling.

In soil below ground

Black walnut (Juglans nigra) increases microbial diversity and some groups of bacteria. Red oak (Quercus rubra) favours different fungi and acidophilic bacteria, with differences in microbial network structure.

More broadly, ectomycorrhizal trees sustain more fungal biomass whilst tree species associated with endotrophic mycorrhizae favour bacterial growth. Soil chemistry and global carbon dynamics are influenced by tree type, mycorrhizal association, and nitrogen-fixing traits.

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All this is important because understanding species-specific microbial interactions provides valuable data on carbon storage, nutrient turnover and ecosystem resilience. It helps us choose tree species with beneficial microbial partners that could enhance soil health, boost growth, and improve resistance to stress. Doubtless this important research will contribute much to arboreal science and woodland ecology generally.

Betts Ecology have large numbers of trees on the sites we manage and the results of this fascinating and revealing new scientific research make it even more important to care for them well.

[1] Notably at Yale – see https://shorturl.fm/KW8R0.

[2] See https://en.wikipedia.org/wiki/Archaea if you don’t know what these are.

[3] i.e indicating a symbiotic (or parasitic) association when a fungus forms a layer on the outside of the roots.

[4] i.e. when a fungus lives within the cells of the roots.