| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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| a closer look Fallen Oak A late-winter storm threw a massive oak to the ground in the forest below my house. Rain and wind overpowered the din of snapping branches and fracturing trunks, so all I heard through the bedroom window was a single muffled whump. The sound pierced my sleep with an intensity that lingered til daybreak, and after breakfast I wandered into the forest to find its source. The night’s rain washed down on me as I brushed past drooping branches of Douglas-firs and twigs of kit-kit-dizze, the sticky, sweet-pungent ground cover that carpets the earth in forests of the Sierra foothills. But the sight of the fallen giant was worth the dousing. It was a black oak, Quercus kelloggii, and it lay on the forest floor along with a 50-foot incense cedar and a sapling madrone, innocent bystanders swept to the ground in the trajectory of its fall. I’d been watching this oak for a decade as one branch after another had weakened and died. Yet the base of the fallen tree revealed that it had died from the bottom up, not from the top down. Its punky roots had simply broken off at the soil line. I pulled a patch of rotting bark from the trunk and examined the wood beneath looking for rhizomorphs, black tendrils characteristic of oak root fungus. I found none, nor could I find any of the fungus’s telltale musty-smelling mycelial plaques. Some other pathogen must have toppled this oak. On an April day this year, seven years after the tree fell, I went back to see how it had fared. It was a chilly morning under the tall Douglas-firs on this north slope. Flecks of sunlight slipped through the forest canopy while the reedy notes of a red-breasted nuthatch and the seesawing refrain of a Nashville warbler filled the air. Suddenly—and delightfully—the flurried scales of a scolding winter wren erupted from a nearby bush. Seven years ago the oak had landed on a clean forest floor. But now the tree lay like Sleeping Beauty, concealed in a thicket of deer brush (Ceanothus integerrimus), sprouting oaks, and small incense cedars. Shade would have suppressed these plants, but when the oak and cedar fell, they tore a hole in the forest canopy, and sunlight could now penetrate to the ground and nurture the brushy undergrowth. Two dusky-footed woodrats had moved into this perfect habitat. Their nests, long, messy piles of sticks and twigs obscured under the tangle of bushes, spilled over the branches of the downed oak. I pushed through the undergrowth and searched on my hands and knees for the larger nest’s entry hole. These openings are usually close to the ground and carefully concealed. But this rat had festooned its door with deer brush and madrone sprigs, each sporting a spray of the first leaves of spring, lime-green and diaphanous. A dead tree is like a fortified castle overflowing with food. Tunneling through the tree’s protective bark, wood-boring insects such as beetles, termites, and ants are the first to breach the walls. Some of these insects bear wood-rotting microorganisms in their guts or tucked under their wings and legs. The galleries they make are open conduits that channel air, water, and other microorganisms into the wood. Once inside the tree, the invaders ravage the most digestible tissues first, starting with the inner bark and sapwood. The outer bark is next in line, even though it’s loaded with indigestible lignin and decay-resistant chemicals that protect a living tree from attack. Exposed to the elements and breaking into small bits as it sloughs off a dead tree, outer bark furnishes plenty of surface area for attacking microorganisms. The core of the tree—the monolithic heartwood—breaks down last. Like bark, heartwood is imbued with rot-resistant chemicals. It also contains less nitrogen than any other tissue, rendering it particularly indigestible. In the warm and humid tropics, a fallen tree of similar size can decompose completely within a year. But here in the cooler, drier Sierra foothills, decomposition was just getting started on my black oak seven years after its fall. Although the tree’s inner bark was already gone and the little sapwood that remained crumbled into fibers when I rubbed it between my fingers, most of the tree’s heartwood and outer bark was still sound. Without its inner lining, though, the dark outer bark hung loosely about the tree’s branches, falling readily to the ground at the slightest touch. Growing on its fissured surface was a shaggy garden of mosses and lichens, including silver-fringed “star moss” (Tortula princeps), “fir moss” (Dendroalsia abietina), which curls and yellows when dry but unfurls in moist weather into emerald-green miniature replicas of fir trees, and the long, tangly, common oak moss with no common name, Antitrichia californica. One long stretch of the oak’s trunk was barkless. Peering close, I found the claw marks of a black bear inscribed in the heartwood. Further down the trunk, hidden beneath overhanging brush, a swath of wood had been ripped apart exposing the vestiges of a carpenter ant colony. A bear must have feasted on the insects—last fall perhaps—devouring eggs, young, and adult ants, wrapped in a crunchy crust of seasoned oak. Beneath the ant colony, great tatters of bark lay stacked in the dirt. I lifted these fragments and dug carefully with my hands into the moist, dark nexus of wood and soil below. My efforts revealed a world of pale mycelia, tiny orange millipedes, and ghostly white springtails—minute, wingless insects that feed on fungi and decaying vegetation and leap high into the air when prodded. A huge, gray millipede, thick as a pencil and nearly as long, coiled into a ball when I picked it up. In the cool soil beneath one chunk of bark I found an ensatina, a kind of salamander splashed with orange spots. As they decay, large logs can hold up to two to three times their weight in water. Especially in the parched days of summer, such oases are vital havens for all the creatures I’d discovered living in and around my log this spring. Ironic as it sounds, fallen trees are the lifeblood of a forest. In their absence, the plants and animals they sustain would disappear, and the larger ecosystem, the forest, would become but a shadow of its former self. Liese Greensfelder writes about science and the environment form her home on San Juan Ridge in Nevada County, California. |
Summer 2001
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