Month: August 2021

Suzanne Simard is an academic scientist of forestry of some renown because her research has revealed that the forest is a community of plants and trees that share resources to their mutual and communal benefit.  Her recently published memoir, Finding the Mother Tree, about her 40-year career in forestry is deeply personal and informative. 

Simard grew up in the forests of British Columbia in an extended family of traditional loggers who used manual methods to selectively remove individual trees, leaving forests intact.  This is physically demanding and dangerous work, making it a predominantly male occupation. 

After her education as a forester, Simard joined the Canadian Forest Service and a profession dominated by men and committed to maximizing profit by clear cutting patches of forest with mechanized methods.  This policy requires the destruction of all vegetation in clear cuts considered potential competition for the next crop of timber.  After mechanical removal, the ground is sprayed with herbicide from helicopters before being replanted with tree seedlings.  This policy is called “free to grow,” a misnomer that was eventually revealed by Simard’s research.  The plant and tree neighbors of the seedlings are their collaborators in the enterprise of the entire forest, functioning as an ecosystem that creates a home for every life form in the community.

Suzanne Simard’s lonely professional journey in forestry

One of Simard’s first assignments as a forester was to assess the health of seedlings planted in a clear cut.  The seedlings were not doing well.  It became her mission to find out why.  A lifetime of observing healthy forests had taught her that the soil is occupied by vast networks of fungi that connect the plants and trees.  These mycorrhizal fungi transfer moisture and nutrients from the soil to the trees and plants, to their benefit.  She speculated that the destruction of all vegetation in clear cuts was eliminating that support structure and she designed experiments to test her hypothesis. 

Her experimental plots were divided into areas with varying degrees of vegetation clearance.  At one extreme, seedlings were isolated by sheets of metal buried deep into the soil that prevented development of mycorrhizal networks to support the seedlings.  Decades later, these isolated seedlings were the most likely to have died.  The seedlings that survived most often were on the perimeter of clear cuts, with access to the surrounding intact forest.

The relationships between tree and plant species and their mycorrhizal networks vary by plant and fungi species.  There are thousands of mycorrhizal fungi species associated with trees and about half are generalists that associate with most tree species.  Specialist species of fungi are confined to a narrower range of tree species, genera, or families.  There are fewer species of mycorrhizae associated with plants and most are generalists. 

The specifics of fungal associations between trees also varies, which requires that we describe a specific relationship.  Simard’s original studies focused on the fungal associations between Douglas fir and birch trees.  Birch trees were destroyed in the clear cuts that were then planted with Douglas fir seedlings that were not doing well.  Simard’s experiments eventually revealed that birch trees and firs mutually benefit one another through their fungal networks.  Carbon stored and the sugar produced by photosynthesis by firs are shared with deciduous birch during winter months while they are leafless.  In summer months when birch are foliated, they store more carbon that is shared with firs.  Birch is resistant to a root pathogen to which firs are susceptible.  In a sharing fungal relationship between birch and firs, birch confers some of that resistance to the root pathogen onto their fir neighbors.

Nitrogen is essential to plant and tree health, but not all species are capable of converting atmospheric nitrogen to soil nitrogen available to plants.  When a nitrogen-fixing plant is associated with a plant without that capability, it can share its nitrogen with its neighbor through their fungal network. 

A mature tree can store more moisture than its young seedlings without extensive root structure.  The mature tree can share its stored moisture with struggling seedlings through its fungal network.  Seedlings with access to that network are more likely to survive while establishing their own root structures.  Research of Simard’s graduate students and collaborators eventually found that such sharing of resources between mature and young trees occurs more frequently within the same species, but sharing also occurs with unrelated tree species.  The mature trees nurture their offspring, enabling their survival and the survival of the species.  They are, in effect, Mother Trees.

Herbicides used to kill vegetation in clear cuts

Another early assignment by the forest service required that Simard determine the most effective herbicide regimen to kill plants in clear cuts perceived to be potential competitors of the seedlings of the next timber crop.  Simard and her sister applied several different concentrations of herbicide to vegetation and predictably determined that the most concentrated formulation of herbicide was the most deadly.  Glyphosate is the most commonly used herbicide for this purpose.

This particular episode in Simard’s early career was disturbing in view of the fact that she eventually developed breast cancer that nearly killed her.  Simard and her sister were uncomfortable about their assignment and they suited up cautiously as best they knew how while applying herbicide.  The Simard sisters felt ill after an application and they sought medical help from whom they learned that their masks did not contain the necessary filters.  Required safety measures for herbicide applicators are only as good as the knowledge on which they are based.  That knowledge moves slowly forward and becomes more alarming as we learn more.

What has the timber industry learned from Simard’s research?

The short answer to that question is very little.  The strategy of the timber industry in both Canada and the US remains clear cuts that destroy all trees and vegetation followed by herbicide application by helicopter to kill all herbaceous vegetation before seedlings are planted.  Simard reports that concentrations of herbicide have been reduced recently.  She also says that a few large, mature trees are sometimes spared by clear cuts. 

Recent knowledge of the health effects of glyphosate is causing some concern, but few changes in policy or practice have been made.  Declining moose populations in a region of Canada led to decreased herbicide applications.  Legislators in the State of Maine recently passed a law to ban herbicide applications in timber clear cuts.  That legislation was then vetoed by the Governor of Maine.

Strangely, none of these reports of reduced herbicide use by the timber industry mention that herbicides are known to damage mycorrhizae.  Health concerns are cited as the sole reason for reducing herbicide use despite the fact that we now know the importance of mycorrhizal networks to the health and survival of forests.  While Simard opposes the use of herbicides in forests, she does not explicitly connect herbicides with the destruction of mycorrhizal networks that enable the survival of tree seedlings. 

Are these studies relevant to our urban forest?

Those who are looking for support for our urban forest in Simard’s work will be disappointed.  Her focus is on the health and preservation of native forests. In fact, she has harsh words for “exotic weed invasions:” She says they are accelerating the decline of native grassland “possibly by sending the native grasses some poisons or an infection to finish the murder.  Or starving them, taking over their energy, degrading the native prairie. Like the invasion of the body snatchers.  Or the colonization of the Americas by Europeans.”  Note that her indictment is speculative and not the conclusion of an empirical study.   

But the principles of Simard’s findings are relevant to our concerns for the destruction of our urban forests and the herbicides used for that purpose.  Mycorrhizal fungi are as essential to urban forests as they are to native forests.  Herbicides used in our urban forests are as damaging to fungal networks as they are to clear cuts of native forests. 

Trees, Truffles, and Beasts (1) was written by academic foresters in Oregon and Australia who are primarily concerned about the destructive consequences of destroying native forests and replacing them with timber plantations, often of another, faster growing species.  Ironically, in the case of old growth eucalyptus forests in Australia, the choice of replacement species is often Monterey pines.  Since some species of mycorrhizal fungi are specific to certain species or types of trees, this change of species is not successful without the inoculation of appropriate species of fungi.  For example, some of the mycorrhizal fungi that grow on the roots of conifers are not found on eucalyptus species.

I corresponded with the authors of Trees, Truffles, and Beasts to confirm that fungi are found in the eucalyptus forests of California.  Since eucalyptus was brought to California as seeds, rather than potted plants, I needed confirmation that our eucalyptus forests are also enjoying the benefits of mycorrhizal fungi.  We are grateful that the authors replied.  They report that eucalyptus forests in California are indeed populated with generalist fungi, including some species that are native to Australia.  Therefore, we can assure our readers that our description of how the forest functions as a community applies to the eucalyptus forest in California, as well as in Australia.

When eucalyptus is destroyed in California their stumps are immediately sprayed with herbicide (usually Garlon) so the tree does not resprout.  The herbicide is carried into the roots of the tree through the cambium layer that is briefly functional after the tree is destroyed.  Garlon is known to damage mycorrhizal fungi.

Herbicide is also used to destroy the non-native vegetation that thrives in the full sun after trees are destroyed.  Glyposate that is commonly used for that purpose is known to kill microbes that are essential to soil health, handicapping any replacement planting. 

Suzanne Simard’s mission

Before leaving the Canadian Forest Service, Suzanne Simard made every effort to inform her colleagues of the damage being done by the timber industry and the potential for more successful planting of a new generation of timber if policy and practice were revised to preserve soil health.  In a male-dominated profession that was committed to the methods being used, her message fell on deaf ears.  In fact, her colleagues were openly hostile to her message, making the offer of an academic position welcome relief that gave her more freedom to conduct research and deliver her message.

After recovering from a nearly fatal bout of breast cancer, Simard became more committed to bringing her research to the attention of the public.  She has delivered inspiring and wildly successful TED talks and she was immortalized as the heroine of The Overstory (2), the barely fictional account of defenders of the forest that made Simard’s research accessible to the general public.

Finding the Mother Tree, Suzanne Simard’s memoir, is a sad reminder of the difficulty of bucking conventional wisdom that is deeply rooted in the profit motive.  In the case of the timber industry, competition remains the dominant narrative that drives policy and the consequences of that approach are unnecessarily destructive.    

1. Chris Maser, Andrew W. Claridge, James M. Trappe, Trees, Truffles, and Beasts, Rutgers University Press, 2008
2. The Overstory, Richard Powers, W.W. Norton and Company, 2019.