The Shark and the Jellyfish: More Stories in Natural History

Tree-Squirrel Fungus

144 Tree-Squirrel Fungus The world’s predominant green province darkens broad reaches of the northern half of the northern hemisphere. A great forest there covers nearly twice the land area that the tropical forests cover in the lower latitudes . Northern conifers blanket much of Eurasia in Scots pine, Siberian spruce, and larches, while across the Bering Strait in North America, black spruce, white spruce, tamarack, and aspen grade into white pine and western hemlock. Hardwood trees mix with pines farther down the mountainous spines of western Canada and into the United States. This vast, vital arm of the biosphere has anchored itself in a circumpolar territory that routinely experiences weather of ice-age extremes. The world’s deepest winters leave the soils there perpetually soaked in snowmelt runoff. The constant running groundwater combines with the acidity of decomposing cones and needles to leach the mineral nutrients out of the earth, making this a challenging place for plants to grow. But the great conifers have conquered that challenge by entering into a complex, cooperative network with a broad assortment of other creatures . This assemblage includes many of the animals and fungi that live in the shadows beneath the soaring crowns. Great and small alike are part of a mutual commerce in sap and blood carried in the veins and the guts of organisms that insure each other’s survival. This pyramid of cooperation stands on a foundation woven of glassine fungal strands in the soil. Those microscopic tendrils colonize the roots of the trees, extending farther through the soil than the distance between the great trunks to underlap every inch of forest floor—their fine filaments fusing with one another to connect every tree with every other. Through them, all the plants in the forest are linked below ground into a single vast, symbiotic superorganism. If they have access to enough water and nutrients, the trees will never cease their growth—they do not reach maturity at a specified age and then stop but grow ever fuller. Their demands on the resources at their bases increase along with their ages and sizes, until they exhaust the essential elements within their reach. Starvation for any one of these elements leads to senescence and death. 145 But while in their prime, the trees make copious amounts of sugar out of sunshine and thin air—more than enough to fuel their growth. The extra sugar is tapped by the symbiotic root-associated fungi. In return for the sustenance, those fungi probe far and wide throughout the soil to capture water and the scarce vital salts dissolved in it—phosphate, potassium, nitrates , and other trace minerals. The fungi feed those nutrients back to the trees to sustain the barter relationship upon which the forest depends. Newly germinated pine and spruce seedlings eventually capture and recycle the store of minerals accumulated by earlier generations of trees. Those nutrients are released back into the soil fungi that infiltrate fallen trunks and rot the wood. But the new seedlings awaken to a far more vigorous pursuit of their destiny after they have made contact with the synergic fungi that colonize their roots. One of the primary components of this cohort of symbiotic root fungi is the truffle. Its subterranean network of invisible threads is welcomed by the trees’ roots—while unrecognized fungi (which may be pathogenic) encounter a rebuff from the trees’ defenses. The truffle fungi on which the trees depend are themselves dependent—during one critical phase of their cycle—on one of the animals of the forest. Though they propagate by spores, the truffles cannot disperse their spawn on the wind, unlike the many fungi that float weightless spores from beneath caps or conks pushed out into the air through soil or bark. No such levitating breezes blow where the truffles live; no bees visit underground, nor do butterflies or bats. The truffle remains buried during its entire life cycle. To entice someone to assist the spread of their spores, the truffles’ subterranean threads congeal during the summer—to metamorphose into one of the most edible delicacies of the wood. They produce seductively scented and richly flavored fruits—a choice food, if someone would just dig them up. That someone would be the northern flying squirrel. These foragers locate and feed upon more than twenty species of buried truffle fruit. They are nocturnal animals with big eyes and flat tails, with soft silver-gray fur below, cinnamon brown above. The broad, furred flight membrane—the...