We take trees for granted as inanimate plants providing shade if standing or wood if hewn; they lack mobility and its attendant suite of sensors that our eyes and ears define as consciousness. The Ents of Tolkien’s Middle-earth and the combative apple trees that confounded Dorothy and the scarecrow on their way to Oz are at the other extreme; anthropomorphized animation. As is the case with most dichotomies of white and black, reality occupies a gray area that vacillates about a median somewhere in between. While trees don’t move from one point to another, the metaphor putting down roots is apropos, they extend their reach skyward in search of radiant energy and earthward in search of vital nutrients. Plants are the fount of photosynthesis, the miraculous chemical conversion of sunlight, water and carbon dioxide into carbohydrate compounds. All heterotrophic faunal and fungal life ultimately depends on autotrophic plants. Trees are the apex species of Kingdom Plantae, masters of the canopy that stretches over vast swaths of land. The epigeal world of plants is well known. Water and nutrients are conveyed through the xylem at the center of the trunk and the carbohydrates of photosynthesis are returned to the roots via the phloem at the circumference; a marvel of vascular engineering that extends upward through every stem to every leaf and back again. What goes on in the hypogeal realm of dirt is mysterious, and much more than meets the eye as we shall see.
Not being able to see the forest for the trees is one of the more hackneyed aphorisms used to express the inability to understand the overarching scheme due to the clutter of details. It is nonetheless an accurate appraisal of forestry until the last decade of the twentieth century. Arborists mostly deal with trees as singular and isolated, their treatment a matter of adequate nutrients and disease mitigation. The suburban manifesto lent some credence to this approach, as individual trees selected from the local garden center are buried to their root tops somewhere in the middle of a lawn of unnatural grass. With nurture in the form of watering and the addition of artificial fertilizer containing the proper amounts of nitrogen, phosphorus and potassium (the three numbers that appear with all fertilizers), the transplantation sometimes succeeds. Nature does not work that way. Plants grow from seeds dispersed at random and en masse according to the serendipity of where they happened to fall. Starting from an open field, pioneer plants that thrive in open sun (frequently considered weeds) are ultimately succeeded at a slow but inexorable pace by the plants most suited for the soil, sun and rainfall that there prevail. After many decades under the right conditions, a forest will succeed that is comprised almost entirely of only one or two tree types, a process fittingly known as old field succession. [1] But again, that is only the above ground part of the story.
There is a “wood” wide web, literally. It involves plant roots and fungi. This will surely come as a surprise to most, as it did to the world of science when it was first discovered in the 19th century. It all started with truffles, fungi noted for a redolence extolled by epicureans. Part of their mystique is that they are hypogeous; trained truffle sniffing dogs and female pigs (the smell is similar to the sex hormones of boars) are employed to root them out. This also makes them rare and accordingly expensive (a pound of white truffles currently runs for about $1,000). A German botanist named Albert Frank was commissioned by the King of Prussia to attempt to cultivate them. While this was (and still is) found to not be possible, Frank discovered that an inherent relationship existed in nature between some fungi (Greek mykēs) and plant roots (Greek rhiza) and coined the word mycorrhiza in 1885. [2] It took one hundred years for the idea that plants and fungi were inexorably twinned in the web of life to the mutual benefit of both to become mainstream. It is now universally accepted that roughly ninety percent of all plants are teamed with fungi to provide minerals from the soil, notably phosphorous, in exchange for about ten percent of the plants nutrients. [3] For almost all trees, this association is obligate; trees will not grow without their mycorrhizal partners. More on the fungi in a later chapter but for now let us return to the forest and its trees.
The fossil record is a chronicle of what lived and died in sediments painstakingly eroded from uplifted mountains. The transition of life from ocean to land about 400 million years ago is one of its more important testimonials; aquatic trilobites dominate Cambrian sediments with the first land plants appearing over 100 million years later in the Silurian. These pioneer plants faced enormous burdens in their adaptive evolution from aqueous environments with dissolved nutrients and buoyancy to bleak shorelines and the crushing gravity that land life must endure. It is hypothesized that the successful land assault of plants was made possible by the fungi that went with them as partners, dissolving the rock minerals in exchange for a share of the fruits of photosynthesis. This is supported by the fossil record; mycorrhizal fungi can be seen in the most primitive plant roots dating from their initial littoral incursions. [4] It stands to reason that plants that could not survive without the minerals needed for growth and fungi that could not survive without carbohydrates would coevolve. Lichens, which are a marriage of various fungi with several genera of algae, demonstrate the success of this association; they thrive on bleak mountain tops and frozen wastelands. If plants and fungi got started together, syllogism then suggests that the ensuing evolution of trees involved a mastering of the fungal partnership. And this is what is meant by the wood-wide web, the tree roots and their intermingled fungi.
A forest is more than a bunch of trees. It is a living, respiring and interconnected community that establishes and maintains its own ecosystem. The earth is populated at any given time by those organisms that survived against unrelenting competition for finite resources. Those that managed to eat something else, not be eaten themselves, and met and successfully mated are what remains. In many species, evolutionary success has been the result of societal collaboration. A single ant is barely a snack for a bird or a spider, but a colony of ants provides a formidable phalanx against predation. The success of Homo sapiens is surely a matter of specialization and cooperation. The plant and fungus kingdoms are, if anything, more competitive than our own. Animals have the option of fight or flight that mobility allows; teeth, claws, wings and legs the result. Plants start where their seeds landed and must somehow win out over marauding herbivores and taller plants with deeper roots; thorns, toxins, trunks and mycorrhizas the vegetative counterpart. The cooperative behavior of trees in a forest in order to enhance survival is analogous to a city of humans or a hive of bees. Trees need the forest of each other and must somehow work together to this end. How and even if they are able to do this has been a mystery.
Field experiments that began in the late 20th century provided incontrovertible proof that trees talk. Using their roots and the anastomosis of fungal hyphal connections, they not only send signals, but also nutrients when and if needed. It all started with the observation that the removal of birch trees from a Douglas fir plantation to enhance wood product growth resulted in the opposite effect; premature and systematic senescence of the fir trees. As with much of scientific research, it was the anomaly that begged the question: Were the birch trees beneficial to them? It is important to keep in mind that these are two totally different species, supposedly competing for the same limited supply of nutrients. Experimentation under controlled conditions using radioactive isotopes (which emit radiation that can be detected at its ultimate destination) found that shading the fir trees resulted in a transfer of 6 percent of the carbon from the birches. [5] This literally ground-breaking study revealed that the deep secrets of the forest were hidden from sight under the soil’s blanket cover; trees sustained each other and their individual fungal partners in a quasi-neural network. Work in this area has steadily advanced to the forest scale; a canopy crane was used to mark Norway spruce trees with carbon dioxide using the isotope carbon 13 (vice the normal carbon 12 with 6 protons and 6 neutrons). At the end of the five-year study period, 40 percent of the fine root carbon in adjacent beech, larch and pine trees was isotopic and can only have come from the spruce trees. [6] The forest-fungal ecosystem is a reality. There is much more to learn about this fundamental underpinning of all that grows naturally.
Germany and the German states that preceded its consolidation established a forest management system about 300 years ago, initially as a means of providing timber for trussing mine access tunnels and fuel to stoke iron furnaces. [7] The key to sustainability are trained and dedicated forest wardens who oversee public and private woodlands; their expertise and knowledge intertwined with the roots they manage. Peter Wohlleben was employed by the town of Hümel in the Eifel mountains of North Rhine-Westphalia to care for its communal beech forest. During his twenty-year tenure, he gradually became aware that his forest was a family and not merely trees. “When you know that trees experience pain and have memories and that tree parents live together with their children, then you can no longer just chop them down and disrupt their lives with large machines.” [8] Treebeard would assuredly concur. The forest of trees and collaborating fungi is the essence of ecology.
References:
1. Kricher, J. and Morrison, G. A Field guide to Eastern Forests of North America, Houghton Mifflin Co. Boston, 1988. Pp 6-36.
2. Stephenson, S. The Kingdom Fungi, The Biology of Mushrooms, Molds and Lichens, Timber Press, Portland Oregon, 2010. pp 88-92, 172.
3. Carlile, M., Watkinson, S, and Gooday, G. The Fungi, Second ed, Elsevier Academic Press, London, 2001, pp 394-404.
4. Money, N. Mr. Bloomfield’s Garden – The Mysterious World of Mushrooms, Molds, and Mycologists, Oxford University Press, NY, 2001, pp 60-61.
5. Simard, S. et al “Net Transfer of Carbon between Tree Species with Shared Ectomycorrhizal Fungi” 1997, Nature 288 pp 579-582.
6. Klein, T. et al “Belowground carbon trade among tall trees in a temperate forest.” Science 15 April 2016, Vol. 352, Issue 6283, pp. 342-344.
7. http://www.wood-report.de/seiten/forestrygermany.html
8. Wohlleben, P. The Hidden Life of Trees Greystone Books, Ltd, Vancouver, Canada, 2015.
Hiker's Notebook 