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Troubles in the Rhizosphere Part 2

Dr. Alex Shigo

Part 1 Part 2 | Part 3 Part 4

Energy & Root Exudates

A mycorrhiza back-lit to show the fungus hyphae extending out from the organ. This is the world of the rhizosphere.


Microorganisms compete in the rhizosphere, an area rich in exudates from the tree. The exudates contain carbohydrates, organic acids, vitamins and many other substances essential for life. From 5 percent to 40 percent of the total dry matter production of organic carbon from photosynthesis may be released as exudates! When trees begin to decline, the amount of organic carbon released as exudates increases. Mineral deficiencies, low amounts of soil air and severe wounding are major causes for the increase. Another way to say this is that an increase in exudates would be caused by over-pruning, construction injury, planting too deeply, over-watering, compaction and planting trees in soils that have a pH too high or too low for their optimal growth.

You would think that a tree in decline would decrease not increase exudates. A possible explanation might come from the self-thinning rule of ecology, which states that when energy input into a site equals output, there will be no further growth unless some trees die. As many suppressed trees die, a much fewer number continue to grow bigger. Simple. Or, on the basis of the mass-energy ratio law, as some trees on a site get bigger, many smaller suppressed trees will die. As the suppressed trees decline, they contribute a higher percentage of their soluble carbohydrates to the rhizosphere.

The increase in exudates from a declining tree with a defense system weakened by low energy reserves would give root pathogens an advantage over other soil organisms. When the tree dies, its dead wood adds a great amount of carbon to the soil, thus benefiting all soil organisms. If this scenario is correct, then the codes for the increase of exudates as trees decline would have been set in the genes of the forest trees. Then, even after trees are taken out of their groups in forests and planted as individuals, the genetic codes for increasing exudates as the tree declines for reasons other than crowding would still be in effect.

Mycorrhizae form when mycorrhizal fungi infect newly forming non-woody roots as shown here. Note the tube-like structure of the hyphae.


A tree does not "know" why it is dying. In a crowded, young, growing forest, the self-thinning rule of ecology does benefit tree survivors and all soil organisms. But, when one or two trees in a yard, city or park start to decline, their early death may benefit only the root pathogens. And even worse, since the tree will be cut and removed from the site, there would be no benefits from added carbon to the soil.