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Could tree roots and leaves be
Another chicken and egg perplexity?
Which came first: the flow of carbohydrates from leaf photosynthesis flowing down the inner bark of trees through phloem tissues bringing needed sugars into each of the tree’s cells from twigs to roots? Or: nitrogen and phosphorous atoms carried with water molecules defying gravity as they nudge each other up through the tree’s xylem conduits, transporting to busy, hungry leaves the beneficial solutions created by the symbiotic relationships between absorptive roots and the network of mycorrhizal fungi ubiquitous in nearly all soils?
Up and down the tree the nourishing resources go. Since trees do not have hearts to pump their life-sustaining liquid sustenance from the roots up through the xylem to the leaves, they rely on two processes to keep the nutrients flowing. The first involves the energy of the sun on a warm day. Leaves contain tiny pores on their underside called stomata where they take in carbon dioxide and give off oxygen and excess water. When the sun causes water to evaporate through the stomata of leaves, it creates a vacuum in the leaves and branches called transpiration-pull that draws water up to the leaves. Secondly, the water stays in a continuous stream due to hydrogen bonds in which negative and positive atoms of hydrogen and oxygen attract each other and hold the water molecules together. Transpiration-pull and the hydrogen bonds together create enough cohesive tension in the xylem to keep the liquid flowing all the way to the tops of the trees, even up to 380 feet in the tallest redwood tree in the world. That is quite a pull.
When the water, nitrogen, phosphorous, minerals and other elements drawn in through the roots reach the leaves, they aid them in the process of photosynthesis. In this process, the leaves take the nutrients, especially the water and minerals brought up from the tree’s roots, and combine them with photons of light from the sun and carbon dioxide from Earth’s atmosphere. This combination of above and below ground elements spark an ancient biological process that keeps all of us alive. Photosynthesis produces the sugars and other substances needed by the tree to grow and be healthy and then expels excess oxygen which we humans breathe to keep our blood flowing and our brains knowing.
Photosynthesis evolved about 3.4 billion years ago when a type of primitive single-celled bacteria became able to use sunlight to produce energy and nutrients for its own growth and well-being. The first organism to actually use photosynthesis to produce oxygen along with carbohydrates and other nutrients was cyanobacteria, an organism that has left us some of the oldest fossils on Earth. Called stromatolites, these cylindrical layers of billions of cyanobacteria can date to over 3 billion years and they are still being made today. Cyanobacteria in time produced enough oxygen to turn our atmosphere into 20% oxygen and allow the evolution of billions of species of oxygen breathing organisms, including humans, which breathe in some 2000 gallons of air per day.
The relationship between plants and animals is a symbiotic relationship on a grand scale. Plants take in the carbon dioxide that animals exhale, and animals breathe in the oxygen given off by plants. It has kept us all living for billions of years.
These photosynthetic nutrients produced in the tree’s leaves, mostly glucose and other carbohydrates, are then sent to every cell in the tree through conduits called phloem. Whereas in the xylem nutrients flow in one direction — up from the roots — in the phloem they flow both up and down, making sure every cell from leaf to root tip is reached. Scientists call the nutrients drawn from the soil and sent skyward to the leaves raw sap. The liquid produced by photosynthesis and sent around the tree is called processed sap.
A tree’s leaves elicit various emotions from us. We love the hopeful buds of spring, full of promise of leafy beauty and warm days to come. We are comforted by the many green leaves of summer and the shade provided by them that we sit beneath with our cool mint juleps and tonics. Or we enjoy exhilarating back and fourths in a swing tied to a stout branch speckled with bits of sunlight as it dapples between leaves that are busily absorbing photons from a bright summer sun.
Late summer melts day by day into autumn when the phytochrome molecules inside the leaves of deciduous trees sense the hours of darkness getting longer and release hormones that slow the flow of life-giving sap, both raw and processed. Then, deprived of its nutrients, the chlorophyll pigment that made the leaves handsomely green through the warm summer months goes through a series of molecular processes that weakens it, allowing the pigments of colors that had been hidden behind the dominant green to turn the leaves for a few weeks into splendid red, yellow, and orange harbingers, reminding us that soon autumn and cool weather will follow.
As the hours of sunlight shorten and the leaves cease their photosynthesizing, chemical stimulators line up the tree’s abscission cells between its twig and leaf stems, loosening the leaves so that any puff of wind sends them spiraling away. The deciduous tree’s leaves whose shades of green define our summer, now colorful in their senescence, float to the ground and we are left with bare trees that have their own stark beauty.
Crackly, crunchy leaves cover the soft autumn soil where saprotrophic fungi help transform them into detritus that returns to the soil the nutrients they absorbed in their spring and summer growing. Now the mother tree can reuse the nutrients she shared with her last season’s birthing of leaves and create new sun powered photosynthesis laboratories. Very little is lost. Life continues constantly cycling from one form to another as it has for millions of years.
Roots, on the other hand, snuggling safely underground, do not suffer the same weather-related vicissitudes of the seasons to the extent that leaves do. As the below-ground component of the tree, they interact with mycorrhizal fungi and a multitude of microbes and exchange nutrients such as nitrogen, phosphorous and water with their fungal companions in the dark soil. Working in symbiotic partnerships, roots and mycorrhizal fungi keep each other healthy while they send a steady supply of nutrients flowing to the tree trunk and leaves above them that they are feeding and holding up.
These leafless, chlorophyll-less mycorrhizal fungi, the unsung heroes of the netherworld, attach to the tree roots and provide them with more water and nutrients than they would be able to soak up on their own. The tree in return passes on sugars, carbon, and other savory nutrients to the fungi. The fact that there are trillions of healthy plants on Earth proves that the system works fine for both fungi and roots.
While leaves are the pop celebrities of a tree, flattered by the oohs and aahs of awed onlookers during their time of basking in summer and autumn sunlight before they shrivel and fall, roots do their unheralded and largely unappreciated work out of the spotlight, behind the scenes in the dark underground as part of the inner workings of Earth. There, living among rocks, mycorrhizal fungi, earthworms, and microbes, the small fine roots ingest the nourishment that the more exposed and looked-at parts of the tree must have, while the larger roots, anchored firmly in the soil, use their enormous strength to hold upright the tree’s heavy trunk and branches.
The fair-weather photosynthesis that takes place in leaves is absolutely essential to life, both to the leaves themselves as they put to good use the carbon dioxide and photons of light they take in, and to all of Earth’s oxygen-breathing creatures as well. But the life of the tree must also continue through the cold leafless winter when there is no photosynthesis taking place. That is when the staying power of the stalwart roots becomes critical to the life of the tree. The tree must continue to flourish even in its time of dormancy drenched in rain, covered with snow, or battered by cold winter winds that compel anything that can move to seek shelter.
While the tree must wait out the winter with its trunk and branches getting a long winter’s nap waiting to begin anew its growth and rebuild its photosynthetic systems when the warm sunlight of spring awakens it, the roots have spent the winter busily storing up nutrients and sap just waiting for the warmth of spring and longer hours of sunlight to stimulate the tree to begin its annual ritual of turning raw sap into buds and leaves. The nutrient-making, oxygen-exhaling process of photosynthesis that keeps the tree and us alive begins another productive season.
We may never solve the chicken and egg perplexity, but there is much to learn about the many other conundrums of life that tree roots and leaves can teach us. Learning about the strength of the beneficial relationships between trees, roots, and the mycorrhizal fungi that spreads through the ground from tree to tree helping them acquire the large amounts of water and nutrients they need, can be an inspiration for building strength within ourselves and cooperative relationships with others. When we all help each other, everyone benefits.
The experiences of our lives come and go as the leaves on trees, and we are emotionally up and down like the nutrient flow within trees and their roots. We all have our variations within the course of living and growing, and we know that through our myriad successes and challenges, as hour by hour we seek pleasure and shun unpleasantness, without a foundation of inner strength, without tough mental roots holding us together and upright, helping us to keep on an even keel, that like a weak rooted tree we could break and fall when too great a tempest bends our will.
Ted McCormack
November 2022
