Finding the Mother Tree: Uncovering the Wisdom and Intelligence of the Forest

by Suzanne Simard

The xylem transports water from roots to shoots in response to the water deficit created by transpiration—the water vapor emitted from the stomata of the needles into the air during photosynthesis. During the day, water pressure in the xylem should be low as the roots struggle to pull water from the drying soil to meet the vapor deficit created by transpiration. The xylem pressure reading at night should be higher because the stomata are shut and the taproots are still accessing the ground water, leaving the xylem saturated and not under any water stress.

Nitrogen is essential for the building of proteins, enzymes, and DNA, the stuff of leaves and photosynthesis and evolution. Without it, plants can’t grow. It is also one of the most crucial nutrients in temperate forests because it frequently goes up in smoke in wildfires.

But while the addition of new nitrogen—or, more precisely, atmospheric nitrogen that had been transformed to ammonium—stopped with the loss of the alder and its partner Frankia, there was a short-term pulse of other nutrients (phosphorus, sulfur, calcium) to the soil as the dead roots and stems decomposed. As this detritus decayed, the alder proteins and DNA were further mineralized, or broken down, into the inorganic nitrogen compounds of ammonium and nitrate. Through these processes, the nitrogen was being recycled and released as inorganic nitrogen. The inorganic compounds, dissolved in soil water, were then readily available for the pine seedlings to take up, briefly boosting their growth. After about a year, however, after the dead alder had long since decomposed and the mineralized nitrogen had been consumed by seedlings or plants or microbes, or leached through the groundwater, the total amount of nitrogen in the bare-earth treatment plummeted compared to where alder grew freely. The short-term pulse of nitrogen as ammonium and nitrates released during decomposition was quickly used up, and there was no alder left to replace or augment that nitrogen. It went missing in action. By fall of

More than 90 million critters living in each teaspoon of soil. As they ate the leaves, they made smaller and smaller bits of litter. As they consumed the litter, and one another, they excreted excess nitrogen into the soil pores, making a nutritious soup of nitrogen compounds accessible to the pine roots. But in this decomposition and mineralization process, faster-growing plants like grasses could grab the inorganic nitrogen before the pines, and this didn’t square with the great amount that ended up in the needles of the pines growing alongside the alders and grasses. One particularly gruesome study showed that the mycorrhizal fungal threads growing from the tips of roots could invade the stomachs of soil-dwelling

The fungal threads sucked the nitrogen straight out of the springtail stomachs and delivered it directly to their plant partners. The springtails, of course, suffered a horrible death. The fungi supplied one-quarter of plant nitrogen simply with the stomach contents of springtails! I wondered if there might be an even more direct route for nitrogen to transfer from alder to pine that involved the fungi. One that bypassed decomposers like springtails.

Pine got nitrogen from alder not through the soil at all but thanks to mycorrhizal fungi! As though alder were sending vitamins to pine directly, through a pipeline. After mycorrhizal fungi colonized alder roots, the fungal threads grew toward the pine roots and linked the plants.

We chattered about the implications for farms: if legumes passed nitrogen to corn, for instance, we could mix crops and stop having to pollute the soil with fertilizers and herbicides.

I explained that getting the kind of harvests we wanted was making us treat the new forests like farm fields. And the free-to-grow rules were being applied to any and all landscapes. Lots of money spread over lots of terrains, usually with reduced plant diversity. Barb and I called it “the fast-food approach to forestry.” The same broad brush applied to all sorts of forest ecosystems was like delivering identical burgers to all cultures, whether in New York or New Delhi.

Are forests structured mainly by competition, or is cooperation as or even more important?

We emphasize domination and competition in the management of trees in forests. And crops in agricultural fields. And stock animals on farms. We emphasize factions instead of coalitions. In forestry, the theory of dominance is put into practice through weeding, spacing, thinning, and other methods that promote growth of the prized individuals. In agriculture, it provides the rationale for multimillion-dollar pesticide, fertilizer, and genetic programs to promote single high-yield crops instead of diverse fields.

I wanted to know whether these birches were simply competitors—reducing the resources Douglas fir needed for survival and growth—or whether they were also collaborators, enhancing the conditions under which the whole forest could thrive. And if the leafy native plants did collaborate with their needle-leafed neighbors, I wanted to know how. To help answer these questions, I was

testing whether paper birch donated resources at the same time that it shaded and depressed fir’s ability to make food through photosynthesis. As the birches intercepted light for their own sugar production, did they make up for the reduced photosynthetic rate of understory Douglas firs by sharing their riches?

My investigation would help me figure out how in the world fir could survive and even prosper in spite of living among birch neighbors considered by foresters to be strong, unwanted competitors. And if birch did spread this bounty—the large amount of sugar it was able to produce in full light—maybe it was delivered to the shaded Douglas fir through a belowground pathway, mycorrhizal fungi...

Sir David Read, a professor at the University of Sheffield, and his students, who’d found that a pine seedling had transmitted carbon belowground to another pine. He had established pines

in the air of one of the seedlings with radioactive

If carbon did transmit between tree species, this would present an evolutionary paradox, since trees are known to evolve by competing, not cooperating. On the other hand, my theory was entirely plausible to me, because it made sense that they would have selfish interests in keeping their community thriving, so that they could get their needs met too.

imagining a birch leaf photosynthesizing—converting light energy to chemical energy (sugar) by combining carbon dioxide from the air with water from the soil. Because of their ability to photosynthesize, the leaves were the source of chemical energy, the engines of life. The sugar—carbon rings bonded with hydrogen and oxygen—would accumulate in the cells of the leaves and the sap then load into the leaf veins like blood being pumped into arteries. From the leaves, the sugar would travel into the conducting cells of the phloem—the blanket of tissue encircling the birch trunk under the bark and forming a pathway from leaves to the root tips. Once the sweet sap was in the uppermost sieve cells of the phloem, an osmotic gradient would develop between them and adjacent phloem cells. Water taken up by the roots from the soil would travel up the xylem—the innermost vascular tissue linking roots to foliage—and be loaded into the top sieve cells of the phloem by osmosis, diluting the solution to balance the concentration with the interlinking sieve cells. The increase in pressure in the cells—turgor pressure—would force the photosynthate downward through the smooth chain of sieve cells, eventually reaching the roots. The roots, as with the aboveground parts of the tree such as buds and seeds, need energy and are sinks for this sugary burst. (While leaves are the source of photosynthate, roots are sinks.) The root cells would quickly metabolize the sugar and move some into adjacent ...

The sugar train in my imagination didn’t stop at the roots. I’d read that the photosynthate was unloaded from the root tips into the mycorrhizal fungal partners, like freight unloaded off boxcars onto trucks. The fungal cells engulfing the root cells and extending from there as threads into the soil would be flooded with the sugar. Water brought up from the soil would rush into the receiving fungal cells to balance the sugar concentration with that of the neighboring fungal cells, just as it did in the leaves and phloem. The increasing pressure from the influx of water would force the sugary solution to spread through the threads of fungal cells enveloping the roots and then out through the hyphae emanating into the soil, like water flowing from a tap through a suite of conjoined hoses. Some of the sugars would fan out to help grow more hyphae through the soil, which would also help collect more water and nutrients to bring back to the roots.

cedars were glowing where the birch cast a cool shadow,

protecting their delicate chloroplasts from the high sun. Where the birch leaves could not reach, the cedars were tanned red to prevent damage to their chlorophyll. The seedlings in the threesome were so close they seemed bound in a common story—with some type of beginning, middle, and end. Barb asked why I’d included cedar next to the birch and fir.

From source birch leaves to sink fir roots. I was flushed with excitement as I scanned the columns of data. The more shade the tents cast, the steeper the source-sink gradient from birch to fir.

Paper birch and Douglas fir were trading photosynthetic carbon back and forth through the network. Even more stunning, Douglas fir received far more carbon from paper birch than it donated in return. Far

from birch being the “demon weed,” it was generously giving fir resources. The amount was staggering—it was large enough for fir to make seeds and reproduce. But what really floored me was the shading effect: the more shade that birch cast, the more carbon it donated to fir. Birch was cooperating in lockstep with fir. I re-analyzed

Birch and fir were trading carbon. They were communicating. Birch was detecting and staying attuned to the needs of fir. Not only that, I’d discovered that fir gave some carbon back to birch too. As though reciprocity was part of their everyday relationship. The trees were connected, cooperating.

The sharing of energy and resources meant they were working together like a system. An intelligent system, perceptive and responsive. Breathe. Think.

the indiscriminate removal of native plants was continuing, and the diversity of the forest was still falling victim.

shows unequivocally that considerable amounts of carbon—the energy currency of all ecosystems—can flow through the hyphae of shared fungal symbionts from tree to tree, indeed, from species to species, in a temperate forest. Because forests cover much of the land surface in the Northern Hemisphere, where they provide the main sink for atmospheric CO2, an understanding of these aspects of their carbon economy is essential.” Nature called my discovery the wood-wide web, and the floodgates opened.

I now knew that birch and fir were connected and communicated, but it didn’t make sense that birch always gave more carbon to fir than it received in return. If this were always so, fir might eventually drain the life out of birch. Were there times in its life when fir might give to birch more than it received? Perhaps when the forest was older and fir had naturally outgrown birch there was a net transfer of carbon from fir to birch.

The question of whether it was mainly competition that shaped forests was at stake, a long-held assumption based on recognizing that this was central to natural selection. The work with the arbuscular mycorrhizal plants in the English lab suggested carbon transmission through networks was irrelevant. My work, seemingly coming out of nowhere, suggested otherwise.

These experiments are now twenty to thirty years old, but the trees are still youthful, their futures mysterious. In forests, experiments are slow, and the lifespan of a scientist is far shorter. One way to see the future is to use computer models to project how the forest will grow over hundreds of years. To let us glimpse the future, imagine what it might be long after we’re gone. Don

IN SPITE OF MY BREAKTHROUGHS—that trees really are dependent on their connection to the soil and to one another—the thing I most wanted was to talk

I thought of new research on hydraulic redistribution by Douglas firs, where the deep-rooted trees lifted water to the soil surface at night and replenished shallow-rooted seedlings so they were vibrant during the day. Had anyone examined whether firs spread water through mycorrhizal networks? Perhaps they shared water to keep their community whole, replenishing their companions through times of hardship.

Subalpine firs were bent over in casts of snow and ice, and the whitebark pines were spread-eagled like bouquets of bones, dead from mountain pine beetle and rust caused by the stress of climate change.

in 1992, when winter temperatures had increased by a few degrees and the coldest months stopped dropping below minus thirty, allowing the beetle larvae to thrive in the thick phloem of the aging pines. Lodgepole pine had coevolved with the beetles in this landscape, naturally succumbing after about a century to create space for the next generation. As the trees declined, fuel accumulated as a matter of course, and wildfires were ignited by lightning or people. Flames released pine seeds from resinous cones and stimulated aspens to sprout from thousand-year-old root systems, their moist leaves reducing the flammability of the young forest. As fire fingered through the landscape, it petered out in these aspen-clad glades, leaving a mosaic of different-aged forests that was itself resistant to future fires. But in the late 1800s, European settlers disrupted this balance by burning the forest montage to the ground in search of gold, creating a vast blanket of new pine stands whose uniformity was later enhanced by fire suppression and herbicide spraying that ensured the aspens weren’t interfering with profits. As these pine trees turned one hundred and the climate warmed, the beetle populations exploded and the landscape ran red like blood flowing through water.

I had growing evidence that forests have intelligence—that they are perceptive and communicative—but I didn’t feel ready to take on the establishment. The fellows would ignore me or, worse, laugh at my talk of the sentience of plants. No, I was pregnant and needed to stay quiet to protect my child, the most precious thing in my life.

There should be a special word for the type of mourning you know is to come. In a decade, 18 million hectares of mature pine forest would be dead, representing about one-third of the forested area of British Columbia. The beetles would continue to chew their way through whitebark, western white, and ponderosa pines, through the United States from Oregon to Yellowstone, and would start infesting the jack-pine hybrids across the boreal forest of Canada, producing a total epidemic across North America in an area roughly the size of California, surpassing that of any insect outbreak in recorded history while providing fuel for devastating wildfires down the road. The beetles also infested plantations, especially those with fast-growing pines that had been weeded of their birch and aspen neighbors.

The pines clustered at the edge of this meadow were probably relatives of the same family, their genes diversified by pollen drifting in from distant fathers. These parent trees shared some of the genes of the trees around them, and sharing carbon to increase the survival of their seedlings, their own offspring, would help ensure the genes got passed to future generations. A later study would show that the roots of at least half of the pines in a stand are grafted together, and the larger trees subsidize the smaller ones with carbon. Blood runs thicker than water. This makes perfect sense from an individual-selection perspective. It’s Darwinian.

No, my trees were demonstrating that they had a lot of skin in the game. Over and over, the experiments showed that carbon moved from a source tree to a sink tree—from a rich to a poor one—and that the trees had some control over where and how much carbon moved.

I thought about the greater group of interacting species. The whole community of plants, animals, fungi, and bacteria. Individual selection might explain how the fluorescent pseudomonads interacted with the mycorrhizal fungi of birch to reduce Armillaria root disease in Douglas fir. Could selection also operate at the group level? Individual species organized into complex community structures that promoted the fitness of the whole group.

Do cooperative guilds of species—like guilds of people in societies—exist? Where multiple tree species are linked by a network for mutual aid, in the way it takes a village to raise a child, despite a risk that there might be cheaters in such guilds. But this sharing would work if our behavior was ruled by steadfast tit for tat, like the two-way transfer between birch and fir and their principle of reciprocity, changing the direction of net transfer over the course of the summer. Quid pro quo. But what about longer-term shifts in trade? Such as when fir eventually grows taller than birch. Would the quid pro quo rule of engagement change, and how might this compare to our human lives when they become more complex and our relationships transform with age?

Ecosystems are so similar to human societies—they’re built on relationships. The stronger those are, the more resilient the system. And since our world’s systems are composed of individual organisms, they have the capacity to change. We creatures adapt, our genes evolve, and we can learn from experience. A system is ever changing because its parts—the trees and fungi and people—are constantly responding to one another and to the environment. Our success in coevolution—our success as a productive society—is only as good as the strength of these bonds with other individuals and species. Out of the resulting adaptation and evolution emerge behaviors that help us survive, grow, and thrive.

Recognizing that forest ecosystems, like societies, have these elements of intelligence helps us leave behind old notions that they are inert, simple, linear, and predictable. Notions that have helped fuel the justification for rapid exploitation that has risked the future existence of creatures in the forest systems.

My students and I were tracing water and nitrogen and carbon flowing from old Douglas firs to tiny germinants nearby, helping them survive. I was finding proof for my early theories that seedlings deep in the shadows of elderly trees depended on receiving these subsidies through mycorrhizal linkages. I was discovering that the networks in the old-growth forests were far richer and more complex than I’d ever imagined, but in large clear-cuts they were simple and sparse. The larger the clear-cut, it appeared, the more

The fungus was linking the old tree and young seedling.

this old tree was connected to every one of the younger trees regenerated around it.

sequence the DNA of almost every Rhizopogon truffle and tree—and find that most of the trees were linked together by the Rhizopogon mycelium, and that the biggest, oldest trees were connected to almost all of the younger ones in their neighborhood.

One tree was linked to forty-seven others, some of them twenty meters away. One tree bound to the next, and we figured the whole fore...

This courageous root was as vulnerable as a growing bone, and it survived by emitting biochemical signals to the fungal network hidden in the earth’s mineral grains, its long threads joined to the talons of the giant trees. The mycelium of the old tree branched and signaled in response, coaxing the virgin roots to soften and grow in a herringbone and prepare for the ultimate union with it. Squatting,

The fungus delivers nutrients, supplied by the vast mycelium of the old trees, to the seedling through this Hartig net. The seedling in return provides the fungus with its tiny but essential sum of photosynthetic carbon.

The old trees, rich in living, had shipped the germinants waterborne parcels of carbon and nitrogen, subsidizing the emerging radicals and cotyledons—primordial leaves—with energy and nitrogen and water. The cost of supplying the germinants was imperceptible to the elders because of their wealth—they had plenty. The trees spoke of patience, of the slow but continuous way old and young share and endure and keep on.