The importance of soil microbes for forest health

Yale Environment 360, the on-line science blog, published an interesting article last week about new discoveries in forest ecology regarding the importance of microbes in the soil for forest health.  These root fungi—called mycorrhizal fungi— form a symbiotic relationship with many plants and trees, both native and non-native.  They provide water and mineral nutrients in exchange for plant carbohydrates.  Scientists have known of the existence of these microorganisms for some time, but recent advances in DNA analysis has enabled scientists to identify thousands of different species of mycorrhizae and their association with certain tree species.

Scientists at Yunnan University in China had been trying for some time to save a critically endangered tree that had dwindled to only 200 individuals.  They had been transplanting seedlings into protected areas, with little success.  Finally, they discovered that inoculating the seedlings with mycorrhizae increased survival rates from 46% to 80%

Certified arborists evaluating the Sutro Forest called it "mycorrhizal heaven."  Courtesy Save Sutro
Certified arborists evaluating the Sutro Forest called it “mycorrhizal heaven.” Courtesy Save Sutro

Root fungi and our urban forest in the Bay Area

We learned of the importance of these root fungi to our urban forest from Colin Tudge’s book, The Tree, nearly ten years ago:  “Most forest trees and many other plants too, make use of mycorrhizae; some, like oaks and pines, seem particularly reliant on them.”  And eucalypts are also dependent upon mycorrhizae:  “Many trees have mycorrhizae, but pines and eucalypts seem particularly adept.” (1)

Volume of pesticide use by San Francisco's Natural Areas Program,  Courtesy Save Sutro
Volume of pesticide use by San Francisco’s Natural Areas Program, Courtesy Save Sutro

And in 2010, we learned from the Marin Municipal Water District’s “Herbicide Risk Assessment,” that one of the most frequently used herbicides sprayed on the stumps of eucalypts when they are cut down is known to be harmful to mycorrhizal fungi in the soil.  This herbicide is also foliar sprayed on non-native vegetation such as broom, Himalayan blackberry, ivy, etc.  The active ingredient in Garlon 3A and Garlon 4 Ultra—triclopyr–is known to be toxic to microrganisms such as mycorrhizae:

 “Mycorrhizal fungi are symbionts with plants that provide water and mineral nutrients in exchange for plant carbohydrates. Cenococcum geophilum, the slowest growing fungus, was least sensitive to the effects of triclopyr, exhibiting decreased growth at 742 ppm a.e. A similar study found that triclopyr (formulation not reported) could inhibit growth in five mycorrhizal species: Hebeloma crustuliniforme, Laccaria laccata, Thelophora americana, Thelophora terrestris, and Suillus tomentosus.94Fungi were kept in liquid culture for 30 days and the reduction of biomass with increasing triclopyr concentrations was measured. A 90% reduction in biomass was observed for all species at concentrations of 720 ppm; greater than 50% reduction biomass was observed in four of the five species at 36 ppm. The most sensitive species, Thelophora americana, exhibited a 6% decrease in growth rates relative to controls at triclopyr concentrations of 0.072 ppm (this result was statistically significant). In other species, statistically significant decreases in growth were reported between 0.72 ppm and 7.2 ppm.” (2)

These studies tested this herbicide on only six species of mycorrhizal fungi.  We should probably assume that other species would also be harmed and it is likely that other herbicides would also be harmful, though no tests have apparently been conducted.  Testing of pesticides is woefully inadequate because legal requirements for testing are minimal and most testing is funded by manufacturers with little motivation for learning more bad news.

Here is one of the comments posted on the Yale Environment 360 article by an academic at University of Philippines, about presumed damage to agricultural soils by pesticide and fertilizer use:

“The article on microbes by Conniff follows what I pointed out earlier to Yale e360, that there is a group in the Philippines, of which one is a geneticist trained in the U.S. and two are foresters trained in the Philippines, who believe in fertilizer- and pesticide-free agriculture methods that do not kill off microorganisms in the soil that are much needed by the plants. They (the three happen to be brothers) applied this principle to rice and other crops and are harvesting more with less input. They have a growing following among farmers as well as a flourishing broadcasting business. They fight an uphill battle against fertilizer and pesticide multinationals and their local partners. They are slowly winning their battles and will later win the war for food security. Advances in tropical forestry will broaden horizons. Thank you Yale e360!”

Posted by Bienvenido R. Rola, PhD on 10 Oct 2013

 Implications for ecological “restorations” in the Bay Area

It seems likely that the huge amounts of herbicide that are used by local projects to eradicate non-native vegetation are damaging the microbes in the soil that are essential to forest health.  This is probably one of many explanations for the lack of success of these projects.  Here is a recap of the many reasons why these projects are rarely successful unless they are intensively planted and gardened:

  • Higher levels of CO₂ and associated climate change are promoting the growth of non-native plants. 
  • The growth of non-native annual grasses is encouraged by higher levels of nitrogen in the soil found in urban environments as a result of the burning of fossil fuels. 
  • Hundreds of species of California native plants require fire to germinate their seeds and most of the population will die within 5 years of the fire.  Prescribed burns are prohibited in San Francisco and are severely limited in most urban areas because of air quality standards and safety concerns.
  • Herbicides are damaging the soil.


(1)     Colin Tudge, The Tree, Three Rivers Press, 2005

(2)    Chapter 4, Marin Municipal Water District, “Herbicide Risk Assessment,” 2010

4 thoughts on “The importance of soil microbes for forest health”

  1. in the late 1980s, while a non-commercial, “bioregional” farmer in the westslope Central Oregon Cascades, foraging for, among other things, mushrooms as part of that lifeway living close to the land, my observations and reflections all of that had led me to the conclusion that the reason why “organic” food plant culturing led to more healthy plants and nutritious results was precisely because such culturing was more mycofriendly.

    Unbeknownst to me at that time, Paul Stamets around that time had begun a more rigorous and recorded experimental approach to documenting the benefits of fungi in that way.

    Science has been catching up to these ideas since. I think you would find that something like 95% of all vascular plants studied for it have been discovered to have obligate mycorrhizal associates

    They are not necessarily dependent on any given species of mycorrhizal fungus, but also not just any species will do. Which ones are required in which proportions or amounts is little investigated, and due to a number of factors may never be well determined. Therefore, it is prudent to avoid ANY applications of mycocides to areas where vascular plants are desired to flourish (and I would argue it is prudent to whenever possible avoid introducing them into the environment anywhere.)

  2. For some reason, reading this particular article’s mentions of “non-native” plants crystallized a portion of the realm of thought that has been swimming around in my head for some time now as I have been following this site.

    Posit that humans did not exist and “climate change” dis-favored the vigor and/or outright disappeared species of vegetation thentofore a very long time present in a given locale “A”, and that plant species more adapted to the local changes in climate then more flourished in locale “A”.

    Which species would those be? Those that had only been similarly flourishing on the other side of the globe in locale “B” with a climate similar to the now changed climate of locale “A”?


    More likely, it would be species that already had existed in pockets within locale “A” whose soil types, aspects. slopes, exposures, etc, etc. made those species more able to thrive — they would spread out from existing population sites. Or they would more slowly migrate from adjacent locales.

    This begs definitions of “non-native” and “invader”, beyond what I have seen DoaMT present. For instance, although DoaMT has brushed on this, it seems more might want to be made of the possibility that what most people affix the term “invader” species to are species that HUMANS invaded locales with, and thus the question of whether human desires ought to trump ecosystem processes deserves far more attention. “Invasion” is quite a different concept than “encroachment” — in feeling, in pace, and in tent.

    Coastal chaparral and/or Oak woodlands of species indigenous to California expanding in distribution as Bay Area climates become warmer and drier seems qualitatively quite different than Eucalyptus doing so, because humans brought the Eucalyptus when they invaded via guns, horses, and disease.

    Those that are so dead set on keeping and/or “restoring” native Bay Area ecotypes that they resort to biocides in efforts to do so are willing to be dangers to all life in pursuit of a human preference, but would DoaMT find fault with those that advocate and find non-poisonous ways to incrementally replace “invader” species with species at least indigenous to this continent (if not the same state?)

    In other words, is DoaMT infected with “antidisestablishmentarianism”, or is their objection to native vegetation crusaders merely an objection to tactics such as demonizing non-native species,enlisting poisons to kill them, and other war-ishness such as removing vast areas of vegetation in order to attempt to create a clean pallet on which to establish landscape-scale type conversions in far-less-than-ecosystem process timelines? I suspect and would like to think it is the latter

    1. If we understand your question, we believe you are asking if we make any distinction between the eradication of non-native species and the eradication of native species that have moved out of their historic ranges either in response to climate change, as a result of natural succession, or other reasons. The answer to that question is “no.” We do not see any justification for either type of eradication.

      Here are a few examples of articles on Million Trees that address the question of changing historic ranges of native species:

      The cow bird is a native bird whose range has expanded and has become a target for eradication outside its historic range:

      The native barred owl is now being shot where it is perceived to be a competitor for the spotted owl:

      Monterey pine is native just 150 miles south of the San Francisco Bay Area. There is fossil evidence that it lived in the Bay Area in the distant past. It is now rare within its historic range. Yet, it is being eradicated in the Bay Area because it does not fit with their rigid criteria of re-creating the landscape that existed here in 1769:

      The grassland that existed here in 1769 was largely the result of the frequent fires set by Native Americans. It was maintained by the grazing of domestic animals brought by Europeans in the 18th and 19th century. When both practices stopped in the 20th century, grassland began to succeed naturally to shrublands which are predominantly native shrubs. The shrubs are now being eradicated by managers of public lands who are committed to re-creating the landscape of 1769:

      This is just a selection of the many articles on Million Trees about the eradication of native species that have moved from their historic ranges for a variety of reasons.

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